W W O R D WO OR RLLLD D M M E E O R O O G A ME ETTTE EO OR RO OLLLO OG GIIC IC CA ALLL O O R G A N A N OR RG GA AN NIIIZZZA ATTTIIO IO ON N INTERNATIONAL OCEANOGRAPHIC COMMISSION OF UNESCO WORLD CLIMATE RESEARCH PROGRAMME BASELINE SURFACE RADIATION NETWORK (BSRN) Operations Manual Version 2.1 L.J.B. McArthur APRIL 2005 WCRP-121 WMO/TD-No.
Acknowledgements The efforts required in creating any docum ent far exceed the capabilities of any one person. This m anual has been no exception. I would particularly like to thank the W orld Meteorological Organization for financial support during the initial drafting of this report and the Atm ospheric Environm ent Service for providing m e with the necessary tim e away from m y regular duties to research the m anual.
Preface to the First Edition Like all aspects of the Baseline Surface Radiation Network, this m anual is in its infancy. The ideas contained within m ay be new to m any, but have been applied successfully at various locations throughout the globe. On the one hand this indicates that these concepts should be considered seriously before being rejected, but on the other hand there m ay be som e that are unworkable because of various clim atic or operational factors.
Preface to the Second Edition The W orld Clim ate Research Program m e (W CRP) Baseline Surface Radiation Network (BSRN) has been operating as a network of surface radiation m onitoring observatories for over 10 years. During this tim e period significant progress has been m ade in the m easurem ent of various radiation quantities. Others have not progressed as rapidly. Observations of other quantities are now being requested by the user com m unity.
Table of Contents Acknowledgem ents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Preface to the First Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II Preface to the Second Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III 1.0 Introduction .
4.2.3 Mechanical installation of shaded sensors (pyranom eters and pyrgeom eters) ............................................................... Installation of instrum ents for the m easurem ent of direct beam radiation . . . . . . . . . . . 4.3.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Pre-installation checks and service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Mechanical Installation . . . . .
9.3.2 9.4 Procedures for specific fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 9.3.2.1 Direct, diffuse and global . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Data Subm ission to the BSRN Archive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Annex A A.1 Site Description Docum entation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C 3. C 4. Annex D D 1. D 2. Annex 2 to the Diffuse Geom etry W G Report: Optim ization of Diffusom eters to Pyrheliom eters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C 3.2 Basic considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C 3.2.1 Sky functions . .
List of Figures Figure 1.1. Map of BSRN sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 3.1. Diagram indicating appropriate distances from an obstruction m eteorological instrum entation (from AES Guidlines for Co-operative Clim atological Autostations, Version 2.0). . . . . . . 25 Figure 3.2. Sim ple post m ount in concrete base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure D 1.1. The sky functions used in this calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Figure D 1.2. The contribution of the solar disk to the irradiance of pyrheliom etric sensors depending on the pointing error. Case of m ountain aerosol and 60 degrees solar elevation. . . . . . . . 138 Figure D 1.3. Sam e as Fig. D 1.2 except the case is for continental background aerosol and 20 degrees solar elevation. . . . . . . . . . . . . . . . . . . . . . . . . . .
List of Tables Table 1.1. BSRN Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Table 1.2. List of site evaluation criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 2.2. Recom m ended m easurem ent requirem ents for ancillary m eteorological variables . . . . . . . . . 14 Table 3.1. Topography types used in archive site identification . . . .
Baseline Surface Radiation Network Operations Manual (Version 2.1) 1.0 Introduction The determ ination of a global clim atology of the radiation budget at the surface of the Earth is fundam ental to understanding the Earth’s clim ate system , clim ate variability and clim ate change resulting from hum an influence.
im plem entation docum entation. W hether a site is new or has been in operation for m any years, operators and scientists can learn from each other to im prove the m easurem ent of surface radiation budget param eters at there own observatories. The purpose of this m anual is to provide a standardized guide to m easurem ent techniques for all stations involved in the program m e based on the experiences gained from a variety of researchers and site scientists.
Location of Operating and Planned BSRN Stations Symbol Station Name Sponsor Latitude Longitude Status CAM Camborne Great Britain 50/ 13' N 5 / 19' W Operational BUD Budapest-Lorinc Hungary 47/ 50' N 19/ 05' E Pending SBO Sede Boqer Israel 30/ 52' N 34/ 46' E Operational TAT Tateno Japan 36/ 03' N 140/ 08' E Operational SYO Syowa, Antarctica Japan 69/ 00' S 39/ 35' E Operational MAL Maldives Maldives/United States 5/ N 73/ E Candidate ILO Ilorin Nigeria/United Stat
• how will the data be quality controlled and archived? In the BSRN, standards of m easurem ent accuracy and archiving have been clearly defined, but the exact m anner in which these standards can be achieved is left to national experts responsible for carrying out the m easurem ents.
• experts intending to obtain the necessary resources to establish a BSRN station • technologists involved in the construction and operation of a BSRN station. For experts, it is hoped that the m anual will provide: • the necessary inform ation required to obtain resources, • the docum entation required to support the establishm ent of a BSRN site • inform ation on types and m anufacturers of instrum ents that can be used within the BSRN and that m eet the guidelines on accuracy.
• Extended-Surface Reflectance and In Situ Measurem ents: developm ent of m ethods for m easuring surface reflectance over a larger area (e.g., 20 X 20 km ) by using a tower or sm all aircraft, special aircraft and balloon experim ents to collect in situ inform ation to validate the rem ote sensing m easurem ents.
upgrading older networks can also benefit from results of the ongoing research conducted specifically to im prove the m easurem ent of solar and terrestrial fluxes using com m ercially available instrum entation. The quality control procedures outlined later in this m anual and the archiving procedures presented elsewhere, can be used with little m odification for m any other radiation networks.
2.0 Sampling Frequency and Accuracy Requirements for BSRN Stations 2.1 Sampling Frequency 2.1.1 Sam pling Frequency of Radiation Measurem ents The BSRN requires that all radiation variables be sam pled at 1 HZ with an averaging tim e of one m inute. The final output for each variable should consist of the one-m inute m ean, m inim um , m axim um and standard deviation.
2.1.2 Sam pling Frequency of Ancillary Measurem ents At stations where the ancillary m easurem ents are under the control of an independent agency, such as a national weather service, the frequency of the various m easurem ents cannot often be altered. The higher the frequency the greater the usefulness of the data, up to the sam pling rate of the radiation m easurem ents.
BSRN Measurement Uncertainty Quantity 1991* 1997 Target** 2004 Target† 1% or 2 W m -2 0.5% or 1.5 W m -2 1. Direct Solar Irradiance 2. Diffuse Radiation 10 W m -2 4% or 5 W m -2 2% or 3 W m -2 3. Global Radiation 15 W m -2 2% or 5 W m -2 2% or 5 W m -2 4. Reflected Solar Radiation 15 W m -2 5% 3% 5. Downwelling Infrared Radiation 30 W m -2 5% or 10 W m -2 2% or 3 W m -2 6.
A solar tracker with an accuracy of ±0.10° or better, is needed to accom m odate the pyrheliom eter, the ACR and, during calibrations, a second ACR. It is recom m ended that the tracker pointing be m onitored using a four-quadrant sensor because pointing accuracy is im portant in determ ining the quality of the direct beam m easurem ent. The sam pling rate should be the sam e as that of the instrum ents attached to the tracker.
2.2.1.3 Global Radiation BSRN target uncertainty is 2% (5 W m -2 ). Although the global radiation m ay be determ ined as a sum of direct and diffuse irradiance, a direct m easurem ent will be m ade with a ventilated pyranom eter (the sam e instrum ent type as for diffuse radiation) to provide a basis for quality control; including instrum ent characterisation and calibration (see Section 8.3 - Calibration procedures).
A recent com parison 8 indicates that the uncertainty associated with infrared m easurem ents approxim ate the BSRN target values when the instrum ent responsivities used are those of the independent calibration laboratories. W hen using field calibrations, one-hour m ean irradiances are found to com pare to approxim ately 1 W m -2 for night conditions and 2 W m -2 during daylight hours.
better than one second, this tim e accuracy was relaxed to one second at the BSRN Science and Review W orkshop (Boulder, Colorado, USA, 12-16 August, 1996).
com m unications are m ade via satellite links (long-distance services), but again, correction can be applied. Obtaining a true tim e via the internet is m ore difficult than with m odem s because of the increased variability in response tim es of the service. To overcom e som e of the variability associated with these delays, the Network Tim e Protocol (NTP) was developed. The advantage of N T P is its ease of use and its ability to be used on m ost com puter platform s.
3.0 The BSRN Site 3.1 Geographic Location of Site 3.1.1 General Considerations In selecting sites for the Baseline Surface Radiation N etwork, the objective is to choose a site which is representative of a relatively large area (greater than 100 km 2 ) with com m on features. The site location should be consistent with the intended purpose for which the observations are being m ade.
(6) near vehicle parking areas; and (7) where heat is exhausted by vehicles or buildings. Conversely, BSRN stations m ust be located where facilities exist, preferably on a full-tim e 24-hour basis. Ideally, the site should be co-located with a synoptic station and within 50 km of an upper air station. 3.1.2 Horizon The ideal site for the m easurem ent of solar and terrestrial radiation for m eteorological purposes is one that has a com pletely flat horizon.
In locations where a site is presently located, this inform ation should be present with the required accuracy. Global Positioning System (GPS) technology can provide the site location to within 30 m without correction and to better than 5 m with corrected system s. Elevation can also be accurately determ ined from GPS. For a new site, this technology m ay be the easiest and m ost accurate m eans of determ ining its location. 3.1.
The description consists of 11 sections broken down into three m ain areas: General Description, Site Description and Station Description; m uch of this inform ation is required for the Archive, but it is set up as an inform ation package for prospective data users. A description of the inform ation required to com plete the package follows. A blank docum ent is included in Annex A. 3.2.1 General Description (1) Inform ation on whom the scientific authority is for the site.
Data in relation surface type Value Major Surface Type Descriptor 1 glacier accum ulation area 2 glacier ablation area 3 iceshelf - 4 sea ice - 5 water river 6 water ocean 7 water ocean 8 desert rock 9 desert sand 10 desert gravel 11 concrete - 12 asphalt - 13 cultivated - 14 tundra - 15 grass - 16 shrub - 17 forest evergreen 18 forest deciduous 19 forest m ixed 20 rock - 21 sand - Table 3.2. Surface types used in archive site identification.
3.3 Instrument Exposure To obtain data on the radiative field with respect to the surroundings, it is necessary to m ap horizon of the instrum ent. W ith few exceptions this actual horizon will be different from theoretical horizon because of buildings, trees or landform s. In som e cases other instrum ents create reflecting surfaces from which additional radiation will be incident on the receiver of sensor of interest.
power available. This can be accom plished by obtaining inform ation on the power supply from the local power authority. The m inim um suggested protection on all crucial equipm ent (e.g., com puters, trackers, line powered data acquisition system s) is an uninterruptible power supply (UPS) capable of m aintaining the system during outages caused by electrical storm s, increased com m ercial dem and (brownouts) and autom atic switching of grid loads due to equipm ent failures.
The W ide Area N etwork is sim ilar in nature to a LAN but is designed to connect geographically distant locations. W ithin m any countries national or regional governm ents operate W AN’s for internal use (e.g., transfer of m eteorological data from observing stations to the central forecast office). If these can be accessed to transfer data over long distances, significant operating cost m ay be saved, albeit at the expense of slower data transfer rates.
data. W hile it is im possible to have com plete defence against loss, the need for security m ust be balanced against the cost of its im plem entation. Distance: The physical distance between the data collection location and the data archive location will often determ ine the m ethods of com m unication that are available. For exam ple, the only viable solutions for long distances m ay be com m on carriers or even satellite com m unications, while shorter distances (e.g.
radiation are separate, the diffuse and infrared (if shaded) m easurem ent should be the furthest poleward and slightly elevated, while the direct instrum ent should be closest to the equator and at the lowest height. The global instrum ent should be centred between these two instrum ents and higher than the direct instrum ent.
Figure 3.2. Sim ple post m ount in concrete base . least affects the data. In the case of a wind m ast, the m ast should be placed where the obstruction alters the wind field of non-prevailing winds. Distances from growing vegetation should be increased to account for any future growth. 3.5.2 Instrum ent platform s Instrum ent stands can be as sim ple as a vertical post holding a single pyranom eter or as com plex as a raised platform that can hold a large num ber of individual instrum ents and trackers.
provided. This can vary from a perm anent deck structure to a sim ple step ladder, rem em bering that the easier the access to the instrum ent the m ore likely the instrum ent will be well m aintained. If the instrum ent is to be m ounted on the roof of a building care m ust be taken to guarantee that the instrum ent will not be blown off during high winds. The secure anchoring of the instrum ent stand should be done in consultation with the building m anager or engineer.
A num ber of dataloggers are capable of withstanding harsh environm ents, including hot and cold temperatures and high relative humidity. Such data collecting platform s should be considered as an alternative to transm itting sm all analog signals through long cables back to a central facility. Once the data is collected it can be transferred m uch m ore reliably as a digital signal.
Figure 3.4 Generalized schem atic of the interface between radiation sensors (RF) and a data acquisition unit showing lightning protection and cable grounding.
4.0 Installation of Radiation Instruments 4.1 General The installation of pyranometers, pyrheliom eters and pyrgeom eters is relatively sim ple (Annex B provides inform ation on som e of the instrum ents that m ay be suitable for use at BSRN stations), but nevertheless requires care and attention to detail. Originally, the BSRN recom m ended that the m anufacturer and type of instrum ent used for the m easurem ent of global radiation should also be used for the m easurem ent of diffuse radiation.
(iv) the directional responsivity of the instrum ent (cosine and azim uthal response of the instrum ent) for pyranom eters (v) the deviation of the tem perature com pensation circuit of the instrum ent over the tem perature range (-10° to +40° of range) or if not com pensated, the required tem perature correction of the instrum ent. In clim ates where the tem perature range is greater than that specified, instrum entation should be selected to m eet the tem perature regim e.
Spring loaded bolting devices for m ounting the instrum ent are also an excellent m eans of guaranteeing the instrum ent will rem ain fixed while providing the added ability of levelling the instrum ent without requiring the bolts being loosened. (3) The instrum ent should be levelled using the supplied three levelling feet. By first adjusting the foot closest to the bubble, the instrum ent should be adjusted until the bubble is centred within the inner circle of the supplied bubble.
Figure 4.2. Ventilator with the m otor located beneath the instrum ent. Note the extra ventilation holes near the top of the housing used to reduce snow accum ulation (Davos, Switzerland). The two recom m ended styles of ventilated housing are: (1) W here the ventilator fan is situated beside the instrum ent and the pyranom eter is com pletely enclosed so that the air flows evenly around the dom e. Figure 4.1 illustrates this type of blower as used by the Deutscher W etterdienst.
Figure 4.3. An one-axis tracker used in shading a pyranom eter. Note the use of two fine wires to m aintain the stability of the shading disk. (Developed by Deutscher W etterdienst). 4.2.3 Mechanical installation of shaded sensors (pyranom eters and pyrgeom eters) The general installation of shaded sensors follows the guidelines set out in 4.2.2, but includes the added com plexity of aligning the shading device with the instrum ent.
Figure 4.4. View of two Swiss oversize tracking disks. Note how the pyranom eter is physically separated from the m otor and the shade device. The increased width of the arm holding the shade disk elim inates the need for stablilizing wires, but increases the am ount of sky obscurred. The slot along the arm is for the m ovem ent of the shade disk. On the instrum ent to the right of the photograph, the vertical wires are used to deter birds perching on the instrum ent.
designed system shading both a pyranom eter and pyrgeom eter along with m easuring the norm al incident direct beam (see Section 4.4 for details on two-axes trackers). To use the tracker as a platform for both the shading of a pyranom eter and the pointing of a pyrheliom eter, the elevation drive m ust be m echanically translated so that it is horizontal and at the sam e height as the signal transducer of the instrum ent to be shaded.
Figure 4.5. Australian active tracker used for both diffuse and direct beam m easurem ents. This tracker is shading a single pyranom eter, and pointing two norm al incidence pyrheliom eters and a GAW PFR sunphotom eter on one side of the tracker and another pyrheliom eter on the far side of the tracker. An active-eye is also situated on the far side of the tracker.
Figure 4.6. Canadian com puter-controlled, friction-drive tracker used for m easuring direct beam , diffuse and infrared radiation using a shaded pyrgeom eter. The pyrheliom eter m ounting block is capable of holding three instrum ents, including an active cavity radiom eter. A second m ounting place is m ounted on the opposite side of the tracker. 4.2.
This m ethod works well if the instrum ent is on a vertical post attached to the boom extending from the tower. The pyranom eter is levelled while the post is vertical in an upright position. The m easurem ent of the angle of the post can be accom plished to within 0.1° using a high quality carpenter’s level. (2) The second procedure requires the construction of a levelling jig.
(iii) the deviation of the tem perature com pensation circuit of the instrum ent over the tem perature range (-10° to +10° of the local range in temperature) or if not compensated the required tem perature correction of the instrum ent (iv) the opening angle and the slope angle of the instrum ent (2) Checks should be m ade of all wiring to ensure that there are no nicks in the sheathing nor stress on the connections.
Figure 4.7. The contribution of the solar disk to the irradiance of pyrheliom etric sensors depending on the pointing error. (A) Case of m ountain aerosol and 60° solar elevation. (B) Case of continental aerosol and a 20° solar elevation. (Calculations and graph courtesy of G. Major) instructions for each of these devices. A broad overview, however, is im portant because of the significance solar tracking plays in the m easurem ent of direct and diffuse radiation.
Figure 4.8. A single-axis synchronous m otor tracker. This m odel is an Eppley Model ST-1 Equatorial Mount. (3) The base on which the tracker is to be placed m ust be stable. W hile active trackers and som e passive trackers are able to correct for a non-level surface, all trackers perform better if they are m ounted such that the instrum ent base is level.
on an unlevelled tracker. (4) The tracker needs to be aligned in the north-south direction. Depending on the type of tracker the accuracy of this alignm ent varies. Equatorial trackers need to be precisely aligned, while m ost two-axes passive and active trackers have correction algorithm s built into the software to allow alignm ent to be less precise. However, the greater the accuracy in aligning the tracker, the easier it will be to initiate accurate tracking.
5.0 Data Acquisition 5.1 Introduction Installing and m aintaining the network data acquisition system (s) is crucial if consistent high quality radiation data is to be sent to the archive. W ithin this m anual data acquisition system (DAS) m eans those electronic devices (including the controlling software) and the connectors, which carry out the process of m easuring the signals em anating from the radiation and ancillary m easurem ent devices (transducers).
Of secondary im portance in the selection of the DAS is its programm ability. While the minim um requirement for the DAS is to m easure a set of signals with a 0.01% accuracy at 1 Hz, the output to be archived is the one minute mean, minim um , maxim um , and the standard deviation. Thus one can store the second data and post process the results or use the features associated with the DAS. In overall storage requirem ents and operator ease, the program m able DAS is the m ore attractive option. 5.
to a fault in the system by perform ing the sam e zero test with the resistor attached directly to the input term inal of the unit. Servicing by authorized personnel is required if the data acquisition unit fails. If the unit does require servicing it is also a good opportunity to have the unit calibrated, a procedure that should be repeated every two years. If the problem is not found in the data acquisition unit, it m ust be assum ed that local conditions are causing electrical interference.
6.0 Maintenance 6.1 Introduction High quality, consistent on-site m aintenance is crucial if accurate long-term records are to be obtained. Not only does the individual have to care for the instrum ents, they m ust also carefully docum ent any work that they do on those instrum ents. It is not good enough to assum e that instrum ents are cleaned regularly; this activity m ust be properly docum ented. To help in this docum entation, sam ple log sheets are reproduced in Annex G.
the radiometer dome by sand or by hyrdometeorites such as hail. If the dom e is dam aged, it should be replaced with one m ade of the sam e optical m aterial. The change should be docum ented and the dom e kept for future reference. Although dom es do not normally change the overall calibration of the system , the instrum ent with the new dom e should be m onitored for any differences, particularly changes in directional responsivity.
(ii) Two-axes passive solar tracker Passive trackers use either internal or external com puters to calculate the position of the solar disk. Following the initial setup of the system , tracking of the solar spot by the shading disk or the instrum ent attached to the tracker should not norm ally vary except when the power is rem oved from the tracker and/or the com puter operating the tracker, or when the tim e used to calculate the solar position is incorrect.
- for friction-driven drives check for slippage of the drive disks (see tracker operating m anual for the proper procedure). - if slippage occurs on gear driven trackers, the gears should be inspected for m issing teeth and the gear alignm ent tested (see tracker operating manual for proper procedures). - check to ensure that the tracker has not changed its physical position, either in level or location (e.g., the tracker has not been bum ped accidentally).
6.3 Weekly Maintenance The m inim um weekly requirem ents for m aintaining a BSRN radiation station are as follows (in addition to the daily m aintenance): (1) Check the desiccant in each sensor. Desiccant should norm ally last several m onths, but is dependent upon atm ospheric water vapour, the quality of radiom eter seals, the size of the desiccant cham ber and the quality of the desiccant.
6.4.2 Annual m aintenance Ideally, the annual m aintenance should take less than one day to com plete if a team of workers is present. Although unlikely, it would be best done while the sun is below the horizon. (1) Calibration of the cavity radiom eter (see Section 8.2.1). (2) All field support assem blies should be checked for level and structural integrity. (3) All bolts should be loosened, lubricated and tightened.
7.0 Measurement of Aerosol Optical Depth 7.1 Introduction The m onitoring of aerosol optical depth (AOD) has been considered an im portant, but difficult, observation that is necessary if there is to be an increase in understanding of the surface radiation budget.
AOD values obtained from the archive would continue to be based solely on the subm itted transm ission data. 7.2 Instrument and Wavelength Specifications 7.2.1 Instrum ent Specifications Historically, sunphotom eters have been designed as either single detector instruments that use a rotating wheel to place a filter between the opening aperture and the detector, or instrum ents that m easure each bandwidth by m atching a detector and an interference filter in its own optical fram e.
Table 7.1 lists the BSRN wavelengths, m axim um displacem ent from the nom inal wavelength and the m axim um waveband (Full W idth at Half Maxim um ) in order of priority. Nominal Wavelength (nm) Maximum Displacement (nm) Maximum Bandw idth FWHM (nm) Wavelength Mandatory Absorption 412 2 6 Yes NO 2 862 4 6 Yes 500 3 6 Yes O 3 , NO 2 (m inor) 368 2 6 Yes NO 2 778 2 11 675 3 11 O3 610 2 11 O3 Table 7.1.
original filters and from the sam e m anufacturing lot. In this m anner, the waveband characteristics can be m aintained over longer periods of tim e. 7.3 Data Acquisition 7.3.1 Sam pling Observations are to be m ade at a frequency of once per m inute. Unlike other irradiance observations that obtain one-m inute averages, the m easurem ent of direct spectral irradiance is to be an instantaneous observation at a given tim e (t).
(2) A series of 20 or m ore ‘Langley’ type calibrations at a high transm ission site over a period of three m onths or less. (3) An absolute calibration of the radiom eter using a set of calibrated lam ps traceable to a national standards’ laboratory.
7.4.2.1 Quality Assurance Procedures for Langley Calibration The acceptance of high values of the coefficient of determ ination (r 2 ) obtained from a regression analysis has been shown to lead to erroneous values of V 0 . Therefore, several quality assurance procedures can be used to better determ ine the quality of the intercept.
which dom inates the extinction. Forgan18 using this observation has developed the ratio-Langley technique to reduce the extrapolation error associated with norm al Langley calibrations. For a wavelength pair 8 2 < 8 1 , where neither wavelength occurs in a region of strong absorption, and the signals can be given as: and where i represents the ith atm ospheric attenuator.
7.4.2.3 Objective Algorithm The objective algorithm described by Harrison and Michalsky5 provides a means to rem ove observations that m ay contam inate the Langley calibration m ethod using a quantitative approach. The m ethodology is used on airm ass between 2 and 6 where airm ass changes are rapid, but the problem of atm ospheric refraction increasing the uncertainty of the analyses is avoided.
The use of a standard lam p either as a calibration source or as an irradiance source for use with a detector standard, requires precision measurements and optical alignment. A specially designed calibration assem bly or an optical bench is essential to obtain a high quality calibration. The following m ust be taken into consideration: (1) The distance between the lam p filam ent and the first instrum ent optic m ust be precisely m easured to determ ine the spectral irradiance at the instrum ent.
Devices that use diffusers should also be cleaned daily by gently brushing debris from the diffuser m aterial. If the diffuser is extrem ely dirty, distilled water can be used on m ost diffusers, but other cleaning chem icals should be avoided. The m anufacturer should be consulted on the m ost appropriate cleaning m ethods. The half-angle of spectral radiom eters is generally less than half that of pyrheliom eters.
Field Param eter Description Explanation 1 Number of Instruments numeric value of number of instruments supplying data more than one instrument may be supplying the transm ission data 2 Instrument #1 Type numeric key defining type of instrument direct pointing (1), cosine derived (2) 3 Instrument Description Text description of instrument Manufacturer, model, serial number (if applicable), general information on method of observation, data collection, etc.
8.0 Radiometer Calibration 8.1 Introduction W ell defined and docum ented, system atic procedures m ust be carefully followed to ensure accurate and reproducible instrum ent responsivities. Calibrations must be routine, internally consistent and traceable if the BSRN is to provide the quality of data required for the calibration and developm ent of satellite algorithm s and the m easurem ent of variations in radiation fluxes that m ay be responsible for clim ate change.
to guard against perform ance degradation between international com parisons. One means of m onitoring perform ance is the use of the reference instrum ent in W MO Regional Pyrheliom eter Intercom parisons. It should be cautioned that an instrum ent’s calibration coefficients should not be changed unless a confirm ed shift in the instrum ent properties has occurred. The reference instrum ent will be used to m onitor the responsivity of the field instrum ent on an ongoing basis depending upon weather.
(7) An averaging period where the difference between the reference instrum ent and the field instrum ents is greater than 1%, and is greater than 3 standard deviations away from the m ean difference should be discarded. (8) A m inim um of 25 acceptable series m ust be com pleted for each com parison. (9) All changes in the ratio between the reference and the field instrum ent(s) m ust be recorded. Changes of less than 0.1%/year (norm alized) need not be reported to the archive. Changes greater than 0.
Secondly, it alleviates the potential of therm al shock to the instrum ent which occurs first when the instrum ent is exposed to direct beam radiation and then again when the instrum ent is shaded. The actual extent of such shock has not been m easured for all instrum ents, but m ay be significant. Thirdly, the pair of instrum ents being used to m easure diffuse and global (the redundant m easurem ent) solar radiation are calibrated sim ultaneously.
To maintain the traceability of pyrgeom eter m easurements the following procedure has been established: (1) Each BSRN station requires a m inim um of two pyrgeom eters, initially calibrated at the W RC. One of these instrum ents is to be declared a site reference instrum ent and used only during tim es of com parison. The other instrum ent(s) will be classified as the field instrum ent(s).
Using these values, the difference between the m easured tem perature at a given resistance and the calculation at that resistance is no m ore than 0.02 °C through the tem perature range -60 °C to 50 °C. Figure 8.1 illustrates the effect of a positive deviation in the determ ination of the case tem perature on the calculated ‘case flux’ from the correct value. This difference m ay be due to sam pling errors, an incorrect therm istor reading (e.g.
9.0 Radiation Data Reduction and Quality Assurance Procedures 9.1 Introduction To be certain that the quality of the data obtained is of a high standard, care m ust be taken from the initial site set-up through the selection of the instrum entation and DAS to the daily m aintenance of the radiom eters. Once a voltage or resistance m easurem ent is taken, nothing can be done to im prove the quality of that m easurem ent.
changes by greater than 0.5% over the operating tem perature range of the instrum ent, a responsivity correction factor should be applied. The zero-offset due to therm al emittance should be corrected, but presently no agreement on the correction m ethodology has been reached.
= pyrgeom eter body tem perature (K) = pyrgeom eter dom e tem perature (K) = = the electrical output from the therm opile a correction factor for infrared irradiance on unshaded dom es. Details are given in Philipona et al. (1995).
considered in these cases. The first is the norm al range of the instrum ent, for exam ple a pyranom eter range m ay be -0.1 to 12 m V, while the second is the absolute range such as 0 - 5 V for a pressure transducer. In setting bounds checks on the form er, one can sim ply be observing an unusual phenom enon, while on the latter, if the lim it is exceeded, an instrum ent problem has occurred. 9.3.1.
9.3.2.2 Downwelling infrared radiation The downwelling infrared signal should be com pared against the effective infrared irradiance derived from the air tem perature at the sam e location: Leff = F T4 where F = Ta = = L eff effective radiation signal in W m -2 Stefan-Boltzm ann constant air tem perature (K) W ith the exceptions of isotherm al fog and strong inversions over cold surfaces, the effective irradiance should be greater than that m easured by the pyrgeom eter.
Annex A Site Description Documentation Tem plates for use with the site description docum entation that is found in Section 3.2.
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A.1 Example of Site Description Documentation The following pages provide sample pages of the Site Description Docum entation for the Bratt’s Lake Observatory. The regional and local m aps of the area, the instrum ent field of view surveys, and the docum entation concerning the site location, operator, contacts etc. are m andatory.
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Annex B B 1. Selected Instrumentation Instrument Specifications B 1.1 Introduction The inform ation found in this annex is based upon the use of particular instrum ents within the BSRN network. When used following the instructions given within the m anual, it is believed that these instrum ents can m eet the accuracy requirem ents specified by the BSRN. Other instrum entation m ay m eet these accuracies but have not been used within the program at the tim e of publication of this m anual.
Pyranometer Specification List Class of Pyranom eter 27 Specification Secondary Standard First Class Second Class < 15 s < 30 s < 60 s Zero off-set: (a) Response to 200 W m -2 net therm al radiation (ventilated) + 7 W m -2 + 15 W m -2 +30 W m -2 (b) response to 5 K h -1 change in am bient tem perature ± 2 W m -2 ± 4 W m -2 ± 8 W m -2 Resolution (sm allest detectable change) ± 1 W m -2 ± 5 W m -2 ± 10 W m -2 Stability: percentage change in responsivity per year ± 0.8 % ± 1.
Pyrheliometer Specification List Specification Class of Pyrheliom eter Secondary Standard First Class Second Class < 15 s < 20 s < 30 s ± 2 W m -2 ± 4 W m -2 ± 8 W m -2 ± 0.5 W m -2 ± 1 W m -2 ± 5 W m -2 ± 0.5 % ±1% ±2% ± 0.2 % ± 0.5 % ±2% Spectral selectivity: percentage deviation of the product of the spectral absorptance and the spectral transm ittance from the corresponding m ean within 0.3 :m and 3.0 :m ± 0.
B 2. Pyranometers B 2.1 Eppley Laboratory Model PSP Pyranom eter The Precision Spectral Pyranom eter is designed for the m easurem ent of sun and sky radiation totally or in defined broad wavelength bands. It com prises a circular m ulti-junction wire-wound Eppley therm opile. The therm opile has the ability to withstand severe m echanical vibration and shock. Its receiver is coated with Parson's black lacquer (non-wavelength selective absorption).
Zero off-set a)response to 200 W m -2 net therm al radiation (ventilated) b)response to 5 K h -1 change in am bient tem perature + 7 W m -2 ± 2 W m -2 Non-stability percentage change responsivity per year ± 0.5 % Non-linearity percentage deviation from the responsivity at 500 W m -2 due to the change in irradiance within 100 W m -2 to 1000 W m -2 ± 0.
(bubble half out of the ring) Coincident with base of the instrum ent. Detector surface and base are coplanar within 0.1° Materials Anodized alum inium case. Stainless steel screws in stainless steel bushes. W hite plastic screen of ASA Drying cartridge PMMA W eight 830 g Cable length 10 m (standard) Dim ensions 91.5 m m total height, 150 m m diam eter, 25 m m dom e height, 50 m m dom e diam eter B 2.
Spectral selectivity percentage deviation of the product of spectral absorptance and spectral transm ittance from the corresponding m ean within 0.35 :m and 1.5 :m ± 2% Tem perature response percentage deviation due to change in am bient tem perature within an interval of -20 to +50 °C, relative to 20 °C. ± 1% Tilt response percentage deviation from the responsivity at 00 tilt (horizontal) due to change in tilt from 0° to 90° at 1000 W m -2 irradiance 0.
Directional response for beam radiation 5 W m -2 Quartz dom es Infrasil II B 2.5 Carter-Scott Middleton EP09 Pyranom eter The EP09 sensor has an upwards facing black receiver disk with a radial heat conduction path for rapid response. An identical (reference) disk faces into the instrum ent body. The tem perature difference between the disks is a direct function of the intensity of radiation absorbed by the receiver disk.
Signal output (responsivity) 1.00 m V/W m -2 Signal resolution < 1.0 W m -2 Zero point ( at 20 o C ) ± 1.5 W m -2 Zero point tem perature coefficient < ± 0.05 W m -2 / o C Calibration accuracy ± 2 % (factory certificate); traceable NATA Certificate available as extra cost option Operating tem perature -35 to +60 o C Power supply requirem ent 5.5 to 14.5 VDC, 10 m A Measurem ent instrum ent requirem ent -0.05 to +2.0 VDC, > 1MS Standby m ode Shutdown input: 2 to 14.
B 2.7 Eppley Black and W hite Pyranom eter (Model 8-48) The Black and W hite Pyranometer has a detector consisting of a differential therm opile with the hot-junction receivers blackened and the cold-junction receivers whitened. The receiver is of radial wire-wound plated construction with the black segm ents coated with a flat black coating and the white with Barium Sulfate.
Linearity < 0.5 % in the range 0.5 to 1330 W m -2 Response tim e < 25 sec. (95%), < 45 sec. (99%) W eight 1.
B 3. Cavity Radiometers and Pyrheliometers B 3.1 Eppley Laboratory HF/AHF Cavity Radiom eter The self-calibrating Absolute Cavity Pyrheliom eter has been a reference standard device for m any years. The sensor consists of a balanced cavity receiver pair attached to a circular wirewound and plated therm opile.
CONTROL BOX: Size: 7 in. high x 17 in. wide x 16 in. deep W eight: 23 lb (approx) Power requirem ent: 115 VAC 60 @ or 230 VAC 50 Hz selectable B 3.2 PMOD/PMO6 PMO-6 Absolute radiom eter (excerpted from Applied Optics, Vol. 25, Page 4173, Novem ber 15, 1986) The PMO6 radiom eter is based on the m easurem ent of a heat flux using an electrically calibrated heat flux transducer.
Receiver Cavity with inverted cone shaped bottom , coated with specular black paint cavity (absorptance : >.9998). Detector Electrically calibrated differential heat flux transducer with resistance thermom eters as sensors Accuracy Measurem ent uncertainty (referred to SI-Units) Precision < ± 0.25% ± 0.01% Mechanical Dim ensions Diam eter Length W eight, approx. Field of view (full angle) Slope angle Receiver aperture diam eter (nom inal) 75 m m 200 m m 2.
Linearity ±5% from 0 to 1400 W m -2 Response tim e 1 second (1/e signal) Mechanical vibration tested up to 20 g’s without dam age Calibration reference Eppley prim ary standard group of pyrheliom eters Size 11 inches long W eight 5 pounds B 3.4 Kipp and Zonen Delft BV CH1 T he pyrheliom eter CH1 is designed to m easure direct solar irradiance at norm al incidence.
Full opening angle 5° ± 0.2° Slope Angle 1° ± 0.2° Sight accuracy +0.2° from optical axis Materials Anodised alum inum case, stainless steel screws W indow m aterial Infrasil 1-301 W eight 700 gram s Desiccant Silica gel Cable length 10 m (Standard) Absorber coating Kipp & Zonen carbon black B 3.
Com pact size and light weight W indow is optical sapphire for chem ical and scratch resistance Marine-grade alum inium , hard anodised, for corrosion rsistance The DN5 pyrheliom eter has a twin-therm opile sensor em bedded in a therm al-m ass that is isolated from the instrument body. Optical geom etry, and baffling, is set by four precisely located apertures. The instrum ent is easy to dism antle, and the window is sim ple to replace.
Standby current draw: 0.1 Ma Startup settling tim e: 1.5s Tem perature output YSI 44031 therm istor (10kS @ 25 o C) W indow m aterial Optical sapphire, 2m m thick Body construction Marine grade alum inium , hard anodised Fasteners Stainless steel Desiccant Silica gel Lead 6m W eight 0.75 kg (excluding lead) An optional FW 01 filterwheel is available for the DN5/D N 5-E. T he five position FW 01 has three glass filters (Schott OG530, RG630, RG695), open position, and blocked position.
B 4. Pyrgeometers B 4.1 Eppley Precision Infrared Radiom eter (PIR) This pyrgeom eter is a developm ent of the Eppley Precision Spectral Pyranom eter. It is intended for unidirectional operation in the m easurem ent, separately, of incom ing or outgoing terrestrial radiation as distinct from net long-wave flux. This instrum ent com prises the sam e type of wirewound-plated therm opile detector and cast bronze desiccated case as the PSP.
A specially coated silicon dom e transm its incom ing radiation with wavelength of m ore than 3 m icron, by cutting off shorter wavelengths. The output of the therm opile sensor is added autom atically to the output of a built-in tem perature com pensation circuit that incorporates a therm istor to produce the correct electrical signal corresponding to the incident infrared radiation.
B 5. Sunphotometers and Spectral Radiometers B 5.1 Kipp and Zonen POM-01L Sky Radiom eter The POM-01L Sky Radiom eter is a research instrum ent intended for the analysis and determ ination of optical aerosol depth and particle size distribution. The POM-01L is capable of perform ing high accuracy angular and spectral scans for both direct and diffuse sky solar radiation, across seven different spectral bandwidths.
Mechanical: Instrum ent dim ension Instrum ent m ass Control box dim ensions Control box m ass Cable length Electrical: Power requirem ent Serial data link B 5.3 i x L: 89 x 390m m 3 kg H x L x W : 300 x 250 x 160 m m 8.250 kg + cables 10 m to instrum ent, <30 m to PC and m ains 85 ... 264 VAC, 40..
B 5.5 CIMEL Electronique Autom atic Sun Tracking Photom eter CE 318 The CE 318 autom atic sun tracking photom eter has been designed and realized to be a very accurate sun photom eter with all the qualities of a field instrum ent: motorized, portable, autonom ous (solar powered) and autom atic.
Cavity size; CW L tolerance 3-cavity, Ø25 m m ; ±2 nm Side-band blocking OD4, UV to 1200 nm Detector type; active area UV si-photodiode; 33 m m 2 Sensitivity gain setting x 4 channels high/low by jum per; trim via m ulti-turn pot Output signal x 4 channels -0.05 to 4.5 VDC m ax. Resolution <0.005OD (Langley m ethod) Response tim e 0.2 s to 99% Operating tem perature -30 °C to +70 °C Power supply requirem ent 5.5 to 14.5 VDC, 20 m A Tem perature output 10 m V °C -1 (0.
Detector tem perature control selection: 30°C, 40°C, 50°C (by jum per on circuit board); stability ±0.
Annex C The Geometry and Measurement of Diffuse Radiation This annex provides the report of the BSRN W orking Group on Diffuse Measurem ents that describes in detail the geom etry and uncertainties associated with the m easurem ent of diffuse radiation with tracking shade instrum entation.
C 1. Final Report of the Working Group on Solar Diffuse Shading Geometry Prepared by: G. Major and A. Ohmura C 1.1 Terms of reference The Baseline Surface Radiation N etwork (BSRN) is a subprogram of the W orld Clim ate Research Program m e (W CRP).
of integration of the radiation falling on the receiver of the instrum ent: Ohm ura integrates first the direction of the rays, Major integrates first along the surface of the radiation receiver. The num erical solutions of the equations would result som e difference between the two type of calculation.
C 2. Annex 1 to Diffuse Geometry WG Report: The effect of diffusom eter shading geom etry Prepared by G. Major, Z. Nagy and M. Putsay for the BSRN Meeting, May 1-5, 2000, Melbourne C 2.1 Introduction The diffuse radiation is the solar radiation received by the horizontal surface from the above 2p solid angle except the solid angle of solar disk. To exclude the solar disk from the whole sky som e kind of shading device should be used.
Country Australia I Australia II Austria R mm 34.8 34.8 45 r mm 10 5.64 16 L mm 795 795 500 Slope angle 1.79 2.10 3.32 Limit angle 3.23 2.91 6.96 Opening angle 2.51 2.51 5.14 Germ any I 25 10 298 2.88 6.70 4.79 Germ any II 30 10 603 1.90 3.80 2.85 Germ any III 30 10 687 1.67 3.33 2.50 Hungary I 25.4 10 577 1.53 3.51 2.
Figure C 2.1. Penam bra functions of diffusom eters for 45 degrees solar elevation. Com paring the calculated and m easured sky functions it is seen, that the m easured ones are m uch larger then the calculated ones. This is illustrated by Figure C 2.2 where the m easuredlargest, sm allest and m ean function is seen altogether with ones calculated for different aerosol m odels, for 45 degrees solar elevation. All calculated functions are below the m easured mean one.
Sum m arizing the above m entioned results it should be stated, that the circum solar radiation is a very variable param eter and it is alm ost independent of other characteristics (direct or diffuse radiation, optical depth) of the solar radiation field. C 2.2.3 Circum solar diffuse irradiances Let us suppose that the diffusom eters listed in Table C 2.1 are at the sam e place and m easure the diffuse radiation.
Diffusometer Diffuse radiation from 0.5 0 Circum 1 0.5 0 – 1.0 0 Circum 2 1.0 0 - Z l “Measured” value Australia I 160 -3.11 -8.47 148.4 Australia II 160 -3.11 -8.51 148.4 Austria 160 -3.11 -16.90 140.0 Germ any I 160 -3.11 -15.96 141.0 Germ any II 160 -3.11 -9.90 147.0 Germ any III 160 -3.11 -8.46 148.3 Hungary I 160 -3.11 -8.53 148.4 Hungary II 160 -3.11 -10.41 146.5 Hungary III 160 -3.11 -11.92 145.8 USA 160 -3.11 -10.03 146.9 -2 Table C 2.2a.
Pyrheliometer Direct radiation Circum 1 Circum 2 “Measured” CRO3 770 2.92 7.74 780.7 ABS 770 3.05 8.04 781.1 KIPP 770 3.11 8.65 781.8 NIP 770 3.11 10.92 784.0 Table C 2.2b. Pyrheliom eter irradiances in W m -2 CRO3 is the absolute pyrheliometer designed by D. Crommelynck. ABS is a group of absolute pyrheliometers (HF, PMO5, etc), KIPP and NIP are thermoelectric station pyrheliometers Instrum ent Austria Germ any I Germ any III Hungary II Hungary III USA Table C 2.3.
C 2.4 Reduction of measurements to standard geometry The em pirical form ulae in section C 2.4.2 are valid for HunIII. Since RATIO Hu n III = HunI/HunIII ~ Standard/HunIII, therefore the reduction of m easurem ents of HunIII to standard geom etry can be m ade by using any of the two form ulae. The correction of other diffusom eters are connected to that of HunIII in Table C 2.3 by the norm alized deficit.
(4) Using the em pirical form ulae mentioned in (1) and (2) the diffuse radiation measured by a diffusometer could be corrected to standard geom etry. (5) Since the circum solar sky radiation has high variability and it is alm ost independent from other radiation param eters, only the m onthly (or at least several days) m ean corrected diffuse radiation values could be expected as reliable. C 2.
C 3. Annex 2 to the Diffuse Geometry WG Report: Optimization of Diffusometers to Pyrheliometers Prepared by G. Major and M. Putsay,Hungarian Meteorological Service C 3.1 Introduction The instrum ent m ost com m only used to m easure global radiation is the pyranom eter. The sensitivity of pyranom eters depend on several factors.
m easurem ent, nam ely the atm ospheric conditions (scattering) and the solar elevation. For a given pyrheliom eter and given circum stances several such diffusom eters could be com posed that would result in zero DE. For a given pyrheliom eter + diffusom eter system , DE varies with varying circum stances. To obtain sm all DE values in each condition, the two penum bra functions should be as close to each other as possible.
C 3.2.2 The pyrheliom eters and pyranom eters For several pyrheliom eters the basic geom etric data can be found in (Major 1995). From these, three instrum ents have been selected that: (1) can operate continuously at radiation stations, (2) cover a relatively wide range of slope angles. The Hickey-Frieden pyrheliom eter (H-F) is a cavity type with a slope angle of 0.78 deg. The Kipp and Zonen CH-1 has a flat therm opile sensor. Its slope angle is 1 deg. The Eppley Inc.
the diffusom eter has been fitted to the pyrheliom eter. Table C 3.4 provides inform ation on the original length of the diffusom eter arm and the optim al arm -length fitted for the pyrheliom eters used for these calculations. The m easures of fit are: (1) the standard deviation (2) and the m axim al absolute value of DE.
Pyranom eter Radius of shading Arm length Arm length Arm length disk/sphere to fit to HF to fit to CH-1 to fit to NIP CM 11 or 21 25.4 630 603 505 EPPLEY PSP 25.4 635 605 510 EPPLEY 8-48 30 726 703 574 SCHENK Star 34 840 815 668 T able C 3.6. Optim al geom etric param eters (m m ) of diffusom eter for the considered pyranom eter- pyrheliom eter pairs. C 3.3 References Major, G.
C 4. Annex 3 to Diffuse Geometry WG Report: Examination of shading mechanisms for diffuse sky irradiance measurement for use in the BSRN Prepared by: Atsum u Ohm ura, Institute for Atm osphere and Clim atic Science, Swiss Federal Institute of Technology (ETH) C 4.
C 4.2 Irradiance in the instrument: Numerical solutions Since the relationship between the radiation and the sensor continuously changes with the solar zenith angle, exact com parability happens only when the sun is at zenith. This rare occasion nevertheless is useful to develop the general case for which m ost m easurem ents are carried out. C 4.3 Sun at zenith The geometrical condition between radiance and an instrum ent is illustrated in Figure C 4.2 for the sun at zenith.
Figure C 4.3a. Relationship between the shadow disc and the sensor of a pyranom eter. C 4.4 Figure C 4.3b. Detail of the sensor projection on to the norm al plane parallel to the shadow disc. Sun at an arbitrary zenith angle The relationship between a pyranom eter sensor and the shading disc is illustrated in Figure C 4.3a. The details on the geom etrical situation on and near the sensor is presented in Figure C 4.3b.
Pyrheliometer and Diffuse Geometry Configurations AC R or Kipp & Zonen C H1 Kipp & Zonen 2AP T racker with C M series pyranometer Eppley SD K BSR N with Eppley 848 B/W with similiarity to AC R BSR N with Kipp & Zonen C M Series pyarnometer with 2000 design Instrument Characterisitics R (mm) 8 25.4 30 28 28 r (mm) 4.8 10.4 15.8 15.8 10.4 L (mm) 183 577 600 641 750 z l (°) 4 3.5 4.4 3.9 2.9 z o (°) 2.5 2.5 2.9 2.5 2.1 z s (°) 1 1.5 1.4 1.1 1.
The integration of Fr' for the entire sensor surface gives, Then, Likewise for the sun at an arbitrary zenith angle q, we obtain the followings: where Then further, For the unit area, 133
Since the irradiance on the surface by a pyrheliom eter adjusted for the horizontal surface F p h is the following relation m ust be kept, where the subscript d stands for the variable for the diffusom eter. This last equation shows two im portant aspects concerning the geom etry of the diffusom eter. C 4.
Annex D D 1. Pyrheliometers and Pointing On the Pointing Error of Pyrheliometers Prepared by G. Major for the BSRN discussion held in Davos, Switzerland, in October of 1995 D 1.1 Introduction The direct radiation is the solar radiation com ing from the solid angle determ ined by the solar disk. The pyrheliom eters are designed to m easure the direct radiation. T heir view lim iting angles (slope, opening and lim it angle) are larger than the visible radius of the solar disk.
If the optical axis of the pyrheliom eter is not directed to the solar centre, than the angle m easured from the solar centre (z1) differs from the angle m easured from the optical axis (z). The transform ation: cos(z1) = cos(d) cos(z) + sin(d) sin(z) cos(n), where d n D 1.2.1 = the deviation between the solar centre and the optical axis, that is the pointing error, = an azim uth angle m easured in the plane of the receiver, it is zero if the radiance com es from the solar centre.
(2) D 1.5 If the pointing error of a pyrheliom eter is larger than its slope angle, the irradiance of the pyrheliometric sensor decreases rapidly with increasing mispointing. The value can be estim ated using Fig. C1.2 and Fig. 1.3. References Allen, C.W . 1985: Astrophysical Quantities. The Athlone Press, London. Major, G. 1995: Circum solar Correction for Pyrheliom eters and Diffusom eters. W CRP, W MO/TDNo.635 Putsay, M. 1995: Circum solar Radiation Calculated for Various Aerosol Models.
Figure D 1.1. The sky functions used in this calculation. Figure D 1.2. The contribution of the solar disk to the irradiance of pyrheliom etric sensors depending on the pointing error. Case of m ountain aerosol and 60 degrees solar elevation.
Figure D 1.3. Sam e as Fig. D 1.2 except the case is for continental background aerosol and 20 degrees solar elevation. Figure D 1.4. The contribution of the circum solar sky to the irradiance of pyrheliom etric sensors. The upper 3 curves belong to the case of continental background aerosol and 20 degrees solar elevation, the lower 3 curves belong to the case of m ountain aerosol and 60 degrees solar elevation. In both group of curves the instrum ents are from the top to down: NIP, KIPP and ABS.
D 2. Effect of Clouds on the Pyrheliometric Measurements Prepared by G. Major for the BSRN W orkshop held in Boulder, Co, 12-16 Aug. 1996 D 2.1 Introduction In the last BSRN Meeting (Davos, October 1995) the question aroused: how large could be the effect of variable clouds around the Sun on the pyrheliom etric m easurem ents? In this report som e results are presented. The basic difficulty of m aking m odel calculations is the lack of proper radiance distributions around the Sun for cloudy situations.
Using the data of Figure D 2.1 where the constant D 2.2.2 has to be determ ined to calculate absolute radiance values. Cloud side reflectance Figure D 2.3 shows a cloud in the right side of the Sun.
The cloud radiances have been tuned to the m easured ones, while the cloudless parts are the sam e as calculated for the atm ospheric colum n containing the nam ed m odel aerosol. This latter does not agree with the m easurem ents. D 2.5 The pyrheliometers Geometrical differences can be found even am ongst the newly developed pyrheliometers. The calculations were m ade for 4 pyrheliom eter geom etry (see Fig. D 2.9). ABS represents PMO2, PMO5, Pacrad and HF instrum ents.
Figure D 2.1. Surface Irradiance: the shadow of the m odel cloud. Figure D 2.2. The geom etry of cloud edge scattering.
Figure D 2.3. The geom etry of cloud side reflectance.
Figure D 2.4. Measured radiance functions: exam ple for the cloud side reflectance (upper curve) as well the clearest cases for high and low solar elevation. Figure D 2.5. Measured radiance functions: various unidentified exam ples for cloud edge scattering and the clearest case (cloudless high sun (x)). In all cases the solar elevations is around 60 degress.
Figure D 2.6. Model radiances of clear sky at 60° solar elevation with m ountain aerosol, clear sky at 20° solar elevation with continental background aerosol and the sam e conditions with cloud at 1, 2 and 3° from the solar centre. Figure D 2.7. Model radiances for the cloud edge scattering and for the clear sky, m ountain aerosol, h=60 degrees.
Figure D 2.8. Model radiances for the cloud edge scattering and for the clear sky, background aerosol, h=20 degrees. Figure D 2.9. The basic geom etrical characteristics of the pyrheliom eters involved into the calculation.
Figure D 2.10. Cloud effect for the ABS pyrheliom eter group. Figure D 2.11. Cloud effect for the Crom m elynck 3L pyrheliom eter.
Figure D 2.12. Cloud effect for the KIPP pyrheliom eter. Figure D 2.13. Cloud effect for the NIP pyrheliom eter.
Annex E Suppliers of Solar Tracking Instruments (Partial Listing) Brusag Chapfwiesenstrasse 14 CH-8712 Stäfa Switzerland http://www.brusag.ch/ Eppley Laboratories 12 Sheffield Avenue PO Box 419 Newport, Rhode Island 02840 USA http://www.eppleylab.com / Kipp & Zonen B.V. Röntgenweg 1, 2624 BD Delft P.O. Box 507, 2600 AM Delft The Netherlands http://www.kippzonen.com / Middleton Instrum ents Carter-Scott Design 16 W ilson Avenue Brunswick, Victoria 3056 Australia http://www.carterscott.com .au/default.
Annex F F 1. Suppliers of Data Acquisition Systems (Partial Listing) Data Acquisition Types Although the requirem ents for observing the basic radiation quantities associated with the BSRN are relatively sim ple in principle, the need for high accuracy, high resolution observations to be obtained once per second will tax m any data acquisition system s if m ore than a few channels are to be sam pled and the BSRN uncertainty requirem ent in irradiance observations of 0.01% of the reading or ±1 :V.
Specification Type 1 Type 2 Type 3 Analog-to-digital converter type Integrating Integrating, Sigm adelta or Successive approxim ation Successive approxim ation, Flash converters Resolution (bits) 16 - 28 16 - 24 12 - 16 Max. Voltage Resolution (:V) <0.1 <1 20 (~ 2 with averaging) One-year relative uncertainty (:V) for 10 m V reading ±0.
Data Translation Inc. 100 Locke Drive Marlboro, MA 01752-1192 USA http://www.datx.com / EIS Pty Ltd P.O. Box 281 Roseville, NSW 2069 Australia http://www.eis.com .au/DT/DT.htm Type 3 Type 2 Intelligent Instrumentation, Inc. 3000 E. Valencia Road, Suite 100 Tucson, AZ 85706 USA http://www.instrum ent.com / John Fluke Manufacturing Co., Inc. Fluke Corporation P.O. Box 9090 6920 Seaway Blvd. Everett, W ashington, 98206-9090 http://www.fluke.
Annex G Sample log sheets The prim ary reason for keeping a log of the activities about the station is to help in determ ining the quality of the data. Until recently such logs were kept either by filling out form s on a daily basis or writing inform ation into a station log book. The form er has a tendency to encourage the observer/technician to record only those activities that are required by the sheet, while the latter is often used only for extraordinary occurrences or events (e.g.
Figure G 1. Sam ple log sheet from the University of Calgary.
Figure G 2. Sam ple log sheet from the NREL HBCU solar radiation network.
Figure G 3. Sam ple log sheet from the Canadian BSRN site.
Annex H Common Terms and Formulas used in Uncertainty Determinations The term s and definitions reproduced below are based on those in Guide to the Expression of Uncertainty in Measurement (1995), International Organization for Standards.
viii. ix. approxim ations and assum ptions incorporated in the m easurem ent m ethod and procedure variations in repeated observations of the m easurand under apparently identical conditions Standard Uncertainty Uncertainty of the result of a m easurem ent expressed as a standard deviation. Type A evaluation (of uncertainty) Method of evaluation of uncertainty by the statistical analysis of series of observations.
Notes: a. The value of a quantity m ay be positive, negative or zero. b. The value of a quantity m ay be expressed in m ore than one way. c. The values of quantities of dim ension one are generally expressed as pure num bers. d. A quantity that cannot be expressed as a unit of m easurem ent m ultiplied by a num ber m ay be expressed by reference to a conventional reference scale or to a m easurem ent procedure or to both.
Influence quantity Quantity that is not the m easurand but that affects the result of the m easurem ent. Result of Measurement Value attributed to a m easurand, obtained by m easurem ent. Notes: a. b. W hen a result is given, it should be m ade clear whether is refers to: i. the indication ii. the uncorrected result iii. the corrected result and whether several values are averaged A com plete statem ent of the result of a m easurem ent includes inform ation about the uncertainty.
Experimental standard deviation For a series of n m easurem ents of the same measurand, the quantity characterizing the dispersion of the results and given by the form ula: being the result of the measurem ent and being the arithm etic m ean of the n results considered. Notes: a. Considering the series of n values as a sam ple of a distribution, the m ean , and is an unbiased estim ate of the variance is an unbiased estim ate of , of that distribution. b.
b. Because only a finite num ber of m easurem ents can be m ade, it is possible to determ ine only an estim ate of random error. Systematic error Mean that would result from an infinite num ber of m easurem ents of the sam e m easurand carried out under repeatability conditions m inus a true value of the m easurand. Notes: a. System atic error is equal to error m inus random error. b. Like true value, system atic error and its causes cannot be com pletely known.
H 2. Common Formulas H 2.1 Type A Evaluation If the num ber of m easurem ents is is the i th m easurem ent then: , and Mean Variance Standard Deviation Experimental Standard Deviation of the Mean Standard Uncertainty, Type A Evaluation H 2.
Rectangular Distribution If the sem i-range is , then the standard uncertainty, , is given by: The degrees of freedom (v) for a rectangular distribution are infinite if the sem i-range represents absolute lim its.
Annex I Solar Position Algorithm An algorithm is provided for the calculation of astronom ical param eters in QuickBasic. The subroutine is based upon the publication of Michalsky (1988) and uses the approxim ation form ulae found in the Astronom ical Alm anac. C-code based upon Michalsky (1988) can be found at http://rredc.nrel.gov/solar/codes_algs/solpos/#refs. Inform ation on the use of the code and a m anual is also available at the site.
Subroutine Solar: Equations based upon the paper of Michalsky (1988) and the approxim ate equations given in the Astronom ical Alm anac. Note: Subroutine call is to be a single line SUB AstroAlm (year, jd, GMT, Lat, Lon, StnHeight, Az, El, EOT, SolarTim e$, Decdegrees, Airm ass$, HaDegrees) ' =========================================================================== ' The following subroutine calculates the approxim ate solar position and is ' based on the following paper: ' Joseph J.
' ' ' ' MOD(X,Y) = X (MOD Y) = X - INT(X / Y) * Y The INT function in Fortran is identical to that in QuickBasic; they both return the sign of x tim es the greatest integer <= ABS(x). ' ========================================================================== ' W ork with real double precision variables and define som e constants, ' including one to change between degrees and radians. DEFDBL A-Z Zero = 0# Point02 = .02# PointFifteen = .
HC1 = .0001184# ' Constant for the calculation of airm ass AC1 = -1.253# ' Get the current julian date (actually add 2,400,000 for JD). Delta = year - basedate Leap = INT(Delta / 4) JulianDy = baseday + Delta * ThreeSixtyFive + Leap + jd + GMT / Twentyfour ' First num ber is m id. 0 jan 1949 m inus 2.4e6; Leap = Leap days since 1949. ' Calculate ecliptic coordinates. Tim e = JulianDy - FiveOneFiveFourFive ' 51545.0 + 2.4e6 = noon 1 jan 2000. ' Force m ean longitude between 0 and 360 degrees.
' Calculate hour angle in radians between -Pi and Pi. Ha = LMST - Ra IF Ha < -pi THEN Ha = Ha + TwoPi IF Ha > pi THEN Ha = Ha - TwoPi ' Hour angle in degrees, 0 North HaDegrees = Ha * ToDegrees + OneEighty ' Local Apparent Tim e or True Solar Tim e in hours. TST = (Twelve + Ha / pi * Twelve) ' Change latitude to radians. Lat = Lat * ToRad ' Calculate azim uth and elevation.
SolarMn$ = RIGHT$(STR$(SMn), 2) IF ABS(SMn) < Ten THEN SolarMn$ = "0" + RIGHT$(STR$(SMn), 1) SolarSc$ = RIGHT$(STR$(SSc), 2) IF ABS(SSc) < Ten THEN SolarSc$ = "0" + RIGHT$(STR$(SSc), 1) SolarTime$ = SolarHr$ + ":" + SolarMn$ + ":" + SolarSc$ ' Solar zenith angle in degrees. Zenith = (Ninety - El) ' Station pressure in millibars. StnPress = stdPress * EXP(-HC1 * StnHeight) ' Calculate the relative optical air mass.
Annex J BSRN Data Management This annex contains an outline of the BSRN data m anagem ent. A com prehensive description is given in (Gilgen et al. 1995). The relationships between the BSRN stations and the W RMC are shown in Figure J1 which is a sim plified version of Figure 2.1 in (Gilgen et al. 1995). The observations are m ade at the BSRN stations. The data are accum ulated during a m onth and their quality is checked by the station scientist.
All data in the BSRN database are consistent. The radiation data however m ay be afflicted with error, though their quality was controlled by the station scientists. Therefore at the W RMC, autom ated quality control procedures are applied to the radiative flux data to detect erroneous values which subsequently are flagged. The radiation data flagged at the W RMC as being afflicted with error and the reason for the flagging are reported to the station concerned.
Index Aerosol Optical Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . airm ass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . extraterrestrial constant . . . . . . . . . . . . . . . . . . . . . Langley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ozone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ratio-Langley technique . . . . . . . . . . . . . . . . . . . . . Rayleigh . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instrum ent Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 24, 26, 28, 31 Instrum ents absolute cavity radiom eter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 64, 89 active cavity radiom eter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 40 bubble level . . . . . . . . . . . . . . . . . . . . . . . . .
Shade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 25, 33-35, 37, 66, 68, 113 Signal Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 27 Steinhart and Hart equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 68, 72 Surface . . . .
