DESCRIPTION OF SURFRAD DAILY DATA FILES FILENAME INFORMATION: The files in this directory contain daily data from established SURFRAD stations, and are written in ASCII. The file naming convention is sssyyjjj.dat, where sss is a three-letter station identifier, yy represents the last two digits of the year (i.e., 95 for 1995), and jjj is the Julian day. Julian days less than 100 are preceded by one or two zeros, e. g., day 75 would appear as 075, day 2 would appear as 002. "bon" is the station identifier for Bondville, Illinois "fpk" is the station identifier for Fort Peck, Montana "gwn" is the station identifier for Goodwin Creek, Mississippi "tbl" is the station identifier for Table Mountain, Colorado The extension ".dat" is used because both radiation and meteorological data are included. The file "bon95099.dat" contains all of the radiation and meteorological data for Bondville on day 99 of 1995. DATA STRUCTURE: The data files are written in ASCII. They may be read with the following FORTRAN code. The format was specified in such a way to ensure that all entries are separated by at least one space so that the files may also be read with free format. Quality control parameters follow each measurement or calculated value. SURFRAD follows the quality control (QC) philosophy of the BSRN. Instead of deleting questionable data, they are flagged. A QC flag of zero indicates that the corresponding data point is good, having passed all QC checks. A value greater than 0 indicates that the data failed one level of QC. For example, a QC value of 1 means that the recorded value is beyond the physically possible range for that measurement. A value of 2 indicates that the data value failed the second level QC check, indicating that it is physically improbable and should be used with scrutiny, and so on. Missing values are indicated by -9999.9. Small negative values of irradiance at night are not flagged. Experiments have shown that instruments rarely register a zero signal at night. The source of the small spurious voltages that are seen is unknown. These errors are greatest for the PSPs. Nighttime values between 0 and -10 Wm^-2 are typical for these instruments. This may represent a bias for the instrument, the sensor cooling to space, noise on the data logging system, grounding problems, etc. Because this behavior is very common, we flag only night time signals that drop below the common range. If a reported data value is -9999.9 (missing), its corresponding QC value should be set to 1. If it is not, the entry should be still considered missing. The following FORTRAN code may be used to read daily SURFRAD files: parameter (nlines=480) character*80 station_name c integer year,month,day,jday,elevation,version integer min(nlines),hour(nlines) c real latitude,longitude,dt(nlines),zen(nlines),direct(nlines) real netsolar(nlines),netir(nlines),dw_psp(nlines),uw_psp(nlines) real diffuse(nlines),dw_pir(nlines),dw_casetemp(nlines) real dw_dometemp(nlines),uw_pir(nlines),uw_casetemp(nlines) real uw_dometemp(nlines),uvb(nlines),par(nlines) real netsolar(nlines),netir(nlines),totalnet(nlines),temp(nlines) real rh(nlines),windspd(nlines),winddir(nlines),pressure(nlines) c integer qc_direct(nlines),qc_netsolar(nlines),qc_netir(nlines) integer qc_dwpsp(nlines),qc_uwpsp(nlines),qc_diffuse(nlines) integer qc_dwpir(nlines),qc_dwcasetemp(nlines) integer qc_dwdometemp(nlines),qc_uwpir(nlines) integer qc_uwcasetemp(nlines),qc_uwdometemp(nlines), integer qc_uvb(nlines),qc_par(nlines),qc_netsolar(nlines) integer qc_netir(nlines),qc_totalnet(nlines),qc_temp(nlines) integer qc_rh(nlines),qc_windspd(nlines),qc_winddir(nlines) integer qc_pressure(nlines) c read (lun_out,10) station_name 10 format(1x,a) read(lun_out,20) latitude, longitude, elevation,version 20 format(1x,f7.2,1x,f7.2,1x,i4,11x,i3) c icount = 0 do i = 1,nlines c read(lun_out,30,end=40) year,jday,month,day,hour(i),min(i),dt(i), 1 zen(i),dw_psp(i),qc_dwpsp(i),uw_psp(i),qc_uwpsp(i),direct(i), 2 qc_direct(i),diffuse(i),qc_diffuse(i),dw_pir(i), 3 qc_dwpir(i),dw_casetemp(i),qc_dwcasetemp(i),dw_dometemp(i), 4 qc_dwdometemp(i),uw_pir(i),qc_uwpir(i),uw_casetemp(i), 5 qc_uwcasetemp(i),uw_dometemp(i),qc_uwdometemp(i),uvb(i), 6 qc_uvb(i),par(i),qc_par(i),netsolar(i),qc_netsolar(i),netir(i), 7 qc_netir(i),totalnet(i),qc_totalnet(i),temp(i),qc_temp(i),rh(i), 8 qc_rh(i),windspd(i),qc_windspd(i),winddir(i),qc_winddir(i), 9 pressure(i),qc_pressure(i) c icount = icount + 1 30 format(1x,i4,1x,i3,4(1x,i2),1x,f6.3,1x,f6.2,1x,20(1x,f7.1,1x,i1)) c enddo 40 type *,'end of file reached, ',icount,' records read' The file structure includes two header records; the first has the name of the station, and the second gives the station's latitude, longitude, elevation above mean sea level in meters, and the version number of the file. These are followed by at most, 480 lines of data, nominally from 0000 to 2357 UTC. The files are organized in Universal Coordinated Time (UTC). The date and time parameters and solar zenith angle are given on every line. Data are reported as 3-minute averages of one second samples. Reported times are the end times of the 3-min. averaging periods, i.e., the data given for 0000 UTC are averaged over the period from 2357 (of the previous UTC day) to 0000. The solar zenith angle is reported in degrees on each line of data. It is computed from the latitude and longitude of the station, the date, and the central time of the averaging period of the sampled data, i. e., 1.5 minutes before the reported time on the line. Missing-data periods within the files are not filled in with missing values, therefore, a file with missing periods of data will have fewer than 480 data records. Radiation values are reported to the tenths place. Although this is beyond the accuracy of the instruments, data are reported in this manner in order to maintain the capability of backing out the raw voltages at the accuracy that they were originally reported. Also, if ever, the accuracies of the instruments improve to a fraction of a Wm^-2, the data format will not have to be changed. The variables, their data type, and description are given below: station_name character station name, e. g., Goodwin Creek latitude real latitude in decimal degrees (e. g., 40.80) longitude real longitude in decimal degrees (e. g., 105.12) elevation integer elevation above sea level in meters year integer year, i.e., 1995 jday integer Julian day (1 through 365 [or 366]) month integer number of the month (1-12) day integer day of the month(1-31) hour integer hour of the day (0-23) min integer minute of the hour (0-59) dt real decimal time (hour.decimalminutes) e. g., 23.5 = 2330 zen real solar zenith angle (degrees) dw_psp real downwelling global solar (Watts m^-2) uw_psp real upwelling global solar (Watts m^-2) direct real direct solar (Watts m^-2) diffuse real downwelling diffuse solar (Watts m^-2) dw_pir real downwelling thermal infrared (Watts m^-2) dw_casetemp real downwelling PIR case temp. (¡K) dw_dometemp real downwelling PIR dome temp. (¡K) uw_pir real upwelling thermal infrared (Watts m^-2) uw_casetemp real upwelling PIR case temp. (¡K) uw_dometemp real upwelling PIR dome temp. (¡K) uvb real global UVB (milliWatts m^-2) par real photosynthetically active radiation (Watts m^-2) netsolar real net solar (dw_psp - uw_psp) (Watts m^-2) netir real net infrared (dw_pir - uw_pir) (Watts m^-2) totalnet real net radiation (netsolar+netir) (Watts m^-2) temp real 10-meter air temperature (¡C) rh real relative humidity (%) windspd real wind speed (ms^-1) winddir real wind direction (degrees, clockwise from north) pressure real station pressure (mb) As of July 1995, diffuse solar measurements are available only at the Table Mountain station. Barometers were added to Table Mountain, Bondville, and Fort Peck in July, August, and October, 1995. All radiation parameters, except UVB, are reported in units of Watts m^-2, UVB is reported in milliWatts m^-2. The UVB flux is given as the total measured surface UVB flux convoluted with the Diffey erythemal action spectrum, i.e., that part of the UVB flux responsible for sun burns on human skin (erythema) and DNA damage. It is reported in this way because the response function of the UVB instrument approximates the Diffey action spectrum; thus the reported value is most representative of what the instrument is actually measuring. The Diffey UVB irradiance reported in SURFRAD data files is computed (and is valid) for a solar zenith angles from 45 to 50 degrees, i. e., we multiply all output signals (in mV) throughout the day by 0.136 (see below). However, since the Diffey action curve changes with solar zenith angle, only the reported UVB values reported for zenith angles from 45 to 50 degrees, inclusive, are accurate. If you want more precision in the 3-minute UVB data over the course of the day, you may apply the following factors for the various solar zenith angle, interpolating where necessary: Solar Diffey zenith calibration angle coefficient (W m^-2 / V) 21.8 0.144 25.0 0.143 30.0 0.141 35.0 0.139 40.0 0.138 45.0 0.136 50.0 0.136 55.0 0.137 60.0 0.140 65.0 0.145 70.0 0.145 For example, to compute a more accurate Diffey irradiance for a reported UVB value at 30 degrees solar zenith angle, divide the reported value by 0.136 (to back out the original output signal), then multiply the result by 0.141. (The above table was supplied by the manufacturer of the UVB Photometer.) In order to be consistent with other reported radiometric data in the file, values for the PAR are reported in units of Wm^-2. The factory calibration does not transform raw output from the instrument to these units, rather, it converts the sensor output to mmoles (of photons) m^-2 s^-1. These units are converted to Wm^-2 by dividing mmoles m^-2 s^-1 by 4.6, which is the factor derived for the solar spectrum. To convert the reported value back to the original units, simply multiply our reported values times 4.6. The procedure to convert mmoles (of photons) m^-2 s^-1 to Wm^-2 for various light sources (including the sun) is described in Proceedings of the NATO Advanced Study Institute on Advanced Agricultural Instrumentation, 1984. W. G. Gensler (ed.), Martinus Nijnoff Publishers, Dordrect, The Netherlands. QUALITY CONTROL AND QUALITY ASSURANCE These data are provisional. SRRB has attempted to produce the best data set possible, however the data quality is constrained by measurement accuracies of the instruments and the quality of the calibrations. Regardless, SRRB has attempted to ensure the best quality possible through quality assurance and quality control. The data are subjected to quality control procedures before the daily files are processed. At present, they are subjected only to a first-level check, however, as quality control procedures become more refined, they will be applied, and new versions of the data files will be generated. Quality assurance methods are in place to ensure against premature equipment failure in the field and post deployment data problems. For example, all instruments at each station are exchanged for newly calibrated instruments on an annual basis. Calibrations are performed by world- recognized organizations. For example, pyranometers and pyrheliometers are calibrated at the National Renewable Energy Laboratory (NREL). There is no equivalent center to NREL for the calibration of pyrgeometers, however, SURFRAD pyrgeometers are checked against three standards maintained at SRRB's Field Test and Calibration Facility at Table Mountain near Boulder, CO, and their calibrations are traceable to a standard instrument in Davos, Switzerland. In general, all of the standards the SRRB and NREL use are traceable to NIST or its equivalent. Calibration factors for the UVB instrument comes from the manufacturer, however, they will be calibrated at SRRB's National UV Calibration Facility in Boulder, when it becomes operational. Finally, in order to maintain continuity between the returned instruments and their replacements, all instruments are gauged against three standard instruments or an absolute instrument before and after field deployment. THE INSTRUMENTS: 1. The Yankee UVB Photometer The calibration applied to the UVB data is valid for the solar spectral band from 280 to 320 nm and has a response function that approximates the Diffey action spectrum. The calibration applied varies as a function of the solar zenith angle. In these files, we apply the coefficient valid for solar zenith angles between 45 and 50 degrees, inclusive. 2. The LI-COR Quantum (PAR) Sensor The LI-COR Quantum (Photosynthetically Active Radiation or PAR) sensor measures radiation in the band from 400 to 700 nm, which is the part of the solar spectrum that activates photosynthesis in plants. The PAR sits on the main platform at SURFRAD stations and collects downwelling global radiation in the photosynthetically active band. 3. The Normal Incidence Pryheliometer (NIP) The NIP measures direct solar radiation in the broadband spectral range from 280 to 3000 nm. Those used at SURFRAD stations are calibrated nominally on a yearly basis at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. 4. Precision Spectral Pyranometer (PSP) Two PSPs measure downwelling and upwelling global solar irradiance at SURFRAD stations. These instruments are sensitive to the same broadband spectral range as the NIP, 280 to 3000 nm. They are also calibrated nominally on a yearly basis at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. 5. Precision Infrared Radiometer (PIR) Two PIRs measure upwelling and downwelling thermal infrared irradiance. They are sensitive to the spectral range from 3000 to 50,000 nm. For calibration of the PIRs, SRRB maintains three standard instruments, the calibrations of which are traceable to standards at Eppley Laboratory and the World Radiation Center in Davos, Switzerland.