Meteorological data from ground stations
General description
The processing of observed station weather into the MCYFS involves four steps:
Data acquisition from weather stations
The selection of stations is limited to those stations that regularly collect data and can supply data in near real time. Relevant meta data of stations includes station number, station name, latitude, longitude and altitude. This data is available in the object STATIONS.
Currently, data acquisition and processing applies to two regional windows: Europe and China. However in the documentation mainly examples for Europe are shown.
Some of the historic meteorological data were purchased directly from National Meteorological Services. Others were acquired via the GTS. As data are obtained from a variety of different sources, considerable preprocessing was necessary to convert them into a standard format. Around 1992 the historic meteorological data represented approximately 380 stations in the EU, Switzerland, Poland and Slovenia with data from 1949 to 1991 (Burrill and Vossen, 1992). Later the historic sets have been extended with stations in Eastern Europe, western Russia, Maghreb and Turkey. The historic data were converted into consistent units and checked on realistic values. The database was also scanned for inconsistencies, such as successive days with the same value for a variable, or minimum temperatures higher than maximum temperatures (Burrill and Vossen, 1992).
From 1991 to present, meteorological data are received in nearrealtime from sources like the GTS network for different hours within one day. The data is preprocessed and quality checked using the AMDAC software package (MeteoConsult, 1991) which extracts, decodes and processes the observations. Since 2014, more and more National Meteorological Services (NMI) migrate to the new encoding format BUFR, owned by the WMO. For the encoding of BUFR additional software, i.e. FMdecode by MetWatch, is applied.
In recent years, the earlier archives (19752004) of Scandinavia and eastern Europe have been enriched. In 2016 data from around 300 Chinese stations have been acquired starting a new service for this region.
Available stations
The stations, stored in object, STATIONS holds over 9377 stations distributed over 40 countries in Europe and neighboring countries. Over 4500 of these stations provide weather data in near real time. All weather data is stored in the stations weather database (object WEATHER_OBS_STATION and object RAIN_OBS_STATION) that currently counts over 48 million records.
Raw station data are collected from various sources:
 GTS (essential data and data licensed by ECOMET restrictions)
 NOAA (USA)
 European National Meteorological Institutes (NMI) (licensed)
For transmission and international exchange, the station reports are encoded in formats standardized and maintained by WMO and International Civil Aviation Organizaton (ICAO)
Observations which are provided directly by National Meteorological Institutes or regional authorities come from secondary networks and are provided in proprietary formats.
Meteorological stations selected in priority are those located in the agricultural zones and equally distributed over the mainland (instead of over islands  for Portugal, Spain or Greece in particular). In particular, for western Russia (western of Urals) the main areas covered are the agricultural districts.
In the case of China a little bit less than 300 stations were selected meeting the following criteria:
 Near real time delivery
 A 20years archive
 Located in the main agricultural areas
 Covering the elements: precipitation, minimum and maximum temperature, humidity and wind speed
The raw station data for China is collected from GTS.
Basic indicators
The basic indicators that are received from weather stations include:
 Sum of precipitation
 2m air temperature
 Maximum of 2m air temperature
 Minimum of 2m air temperature
 Downward directed solar radiation measured at earth's surface (global radiation)
 Duration of sunshine
 Total cloud cover
 Water vapour pressure
 Relative humidity
 10m mean wind speed
 Snow depth
For SYNOP FM12 and BUFR bulletins, WMO defines regional regulations to consider time zones and national coding practices. The extent of reported parameters and frequency differs per country and is for ECOMET member countries affected as well by license restrictions. The METAR code is standardized through the ICAO. In Europe and China, the WMOmaintained codes SYNOP FM12 and BUFR provide higher accuracy for the various parameters and more detail. In these regions, METAR provides only temperature, dew point, visibility, cloud amount and wind speed and is reported in coarser increments for the various parameters. Nevertheless, METAR reports are used as well, mostly to fill spatial gaps in areas with less WMO stations.
The following table summarizes basic information on the availability and reporting regulations from the various observing station data sources:
Parameter  Reference periods of reports as defined by WMO  SYNOP FM12/BUFR (*)  METAR (**) 

Sum of precipitation  24hourly sum, 12hourly sums, 6hourly sums reported, depending on region WMOregion and local regulations  Europe: 06 UTC: past 24 hours / 00 UTC and 12 UTC: past 6 hours / 06 UTC and 18 UTC: past 12 hours  China: Reports 00 UTC for past 24 hours, some stations report 21 UTC for previous 24 hours (***)  Not reported in Europe and China 
2m air temperature  Instantaneous value  Reported with 0.1°C accuracy  Reported as full degrees 
Maximum 2m air temperature  Maximum of continuous measurement during reference period (****)  Europe and China: reported 18 UTC  Not reported in Europe and China 
Minimum 2m air temperature  Minimum of continuous measurement during reference period (****)  Europe and China: reported 06 UTC  Not reported in Europe and China 
Downward directed surface solar radiation (global radiation)  Sum accumulated over past 24 hours  Available for some European countries at 00 UTC  Not reported in Europe and China 
Duration of sunshine  Sum accumulated over past 24 hours  Most European countries report at 06 UTC  Not reported in Europe and China 
Total cloud cover  Instantaneous value  Octas 08  5 stages, only clouds up to a height of 5000 feet over ground reported 
Measures for the humidity of the air at 2m above ground: dew point, water vapour pressure and relative humidity  Instantaneous value of dew point temperature reported (*****)  Reported with 0.1°C accuracy  Reported as full degrees 
10m mean wind speed  Mean over past 10 minutes  Meters per second  Mostly full knots, occasionally less accuracy during low wind situations 
Snow depth  Instantaneous value, increasing automatization of measurement  When a station reports snow depth, it is done in Europe by 06 UTC, in China by 00 UTC  not reported 
(*) Main synoptic hours are 00, 06, 12, 18 UTC. Intermediate synoptic hours are 03, 09, 15, 21 UTC.
For most European countries up to hourly data is used, for China 3hourly reports are used.
(**) Up to hourly reports, depending on the local airport schedule. Frequency of reports can change over daytime, weekday and season.
(***) In BUFR, many countries erroneously do not report the reference period during dry conditions. In this case, it is assumed that the WMO definitions for the reference period are applied.
(****) Europe: Covers past 12 hours. China: Covers past 24 hours.
(*****) Other thermodynamical measures for the humidity of air can be calculated from dew point and air temperature.
Data quality check
The software package Actual Meteorological Database Construction (AMDAC) is the main processing tool for completing and quality evaluation of actual meteorological data which is used as input for agrometeorological models. The chain of data processing and quality control can be described as follows:
Near realtime preprocessing (hourly reports with extended information at intermediate and main synoptic hours)
 Decode SYNOP FM12, BUFR and METAR reports from weather stations as available on the GTS and at NOAA with external decoding software (FMDecode). As far as implemented in the external decoder, semistandardized formats from National Meteorological Institutes are decoded as well this way.
 Additionally, AMDAC decodes proprietary formats provided by National Meteorological Institutes for secondary networks.
 Extract or calculate and store the meteorological parameters in a separate database;
 Check the quality of the observed elements in the received weather reports by performing data range and time consistency checks. The latter is done by comparing the values of reported parameters with those previously or subsequently reported from the same station. For a number of weather elements, the observed values are compared with shortterm (<12h) forecast values that serve as reference values. These forecasts are obtained through a technique called MOS (Model Output Statistics). Meteorological forecast models, e.g. the ECMWF model, compute the physical status of the atmosphere on a grid, and the results represent the expected situation per grid box. A MOS uses statistical relationships between the observations of a particular station and historic model forecasts for surrounding grid points. Each observing location has its own statistics, which are applied onto the grid point results of one or more physical models of the atmosphere. That way, the very local situation on an observing station can be mimicked. AMDAC uses these individual location forecasts to define time and locationdependent thresholds for the trustworthiness of station reports, for the elements air temperature (including minimum and maximum), dew point (applies to all derived measures for the humidity of the air), precipitation and wind speed, respectively. That way, the thresholds consider season, climatology and even the actual weather pattern. A welcome side effect is the high spatial consistency of the statistical MOS approach and therefore of the thresholds. Individual MOS forecasts is used for almost all stations (4600, state June 2016). For temperature, humidity measures and wind speed consecutive reporting of a value, e.g. due to broken equipment or data encoding issues, is usually detected by the daily checks. This does not apply for precipitation, i.e. for consecutive reports of 0 mm. This rather typical reporting bug is not found when quality checks are applied on to the data of the very day. Due to the mostly “patchy” pattern of precipitation events quality checks accept dry stations in between. To find stations that report consecutively 0 mm several weeks of history need to be considered, see retrospective checks.
 Correct automatically obvious errors detected while performing these checks;
 Automatically fill gaps in the database through interpolation based on time consistency criteria;
 Flag dubious observations which cannot be corrected automatically;
 Write all automatic corrections and flagged dubious observations to a log file;
 Have the flagged observations checked and, if necessary, corrected by a trained meteorologist; when a correction is done, the derived parameters are recalculated and the data are written back to the database.
Dedicated trained and qualified meteorologists go through the dubious observation values that are flagged as such by the AMDAC automatic prechecking program. An interactive system for the visualization of meteorological data is used to graphically visualize and analyze additional information such as:
 Station observation data
 Satellite images
 Precipitation Radar data
 Analysis and short range forecasts computed by physical models of the atmosphere
 Short range forecasts for weather station locations
This additional data is used by the analyst to decide on either confirmation or rejection of the observed values.
Conversion to daily values
Once the database has been filled following the method described above, data are aggregated to daily values. This includes the indicators as summarized in the following table:
Parameter  Aggregation  Reference period Europe  Reference period China 

Total cloud cover (N)  Daily mean  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC  18 UTC prev., 21 UTC prev., 00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC 
Duration of sunshine (Msun)  24hourly sum  00–24 UTC  Not available 
Downward directed surface solar radiation (global radiation) (Mrad)  24hourly sum  0024 UTC  Not available 
Minimum 2m air temperature (Tn)  Lowest value of continuous reference period (*)  18 previous day 06 UTC  06 UTC previous day – 06 UTC 
Maximum 2m air temperature (Tx)  Highest value of continuous reference period (**)  0618 UTC  18 UTC previous day – 18 UTC 
Water vapour pressure (e)  Daily mean  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC  18 UTC prev., 21 UTC prev., 00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC 
10m mean wind speed (ff10)  Daily mean  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC  18 UTC prev., 21 UTC prev., 00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC 
Sum of precipitation (RRR)  24hourly sum  Mostly 06 UTC until 06 UTC next morning  Mostly 00 UTC – 00 UTC next day (indicator 2). For some stations 21 UTC previous day – 21 UTC (indicator 6) 
2m air temperature (TT)  03hourly instantaneous values during daytime  06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC 
Relative humidity (RH)  03hourly instantaneous values during daytime  06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC 
State of soil  Instantaneous value (***)  00 UTC following day  
Water vapour pressure deficit (vpd)  Daily mean  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC  18 UTC prev., 21 UTC prev., 00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC 
Slope of saturation vapour pressure vs. temperature curve slope  Daily mean  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC, 21 UTC  18 UTC prev., 21 UTC prev., 00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC, 15 UTC 
Total cloud cover (N)  Daytime mean  06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC 
Low or (when no low clouds) medium clouds (Nh)  Daytime mean  06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC  Not available 
Calculated sunshine duration (Csun)  24hourly sum  Calculated by AMDAC, 024 UTC of the day specified  Calculated by AMDAC, 18 UTC previous day  18 UTC of the day specified 
Highest possible global radiation at clear sky (Crad)  24hourly sum  Calculated by AMDAC, 024 UTC of the day specified  Calculated by AMDAC, 18 UTC previous day  18 UTC of the day specified 
Potential evapotranspiration (ETP)  24hourly sum  Calculated by AMDAC, 024 UTC of the day specified  Calculated by AMDAC, 18 UTC previous day  18 UTC of the day specified 
Visibility (VV)  Daytime mean  06 UTC, 09 UTC, 12 UTC, 15 UTC, 18 UTC  00 UTC, 03 UTC, 06 UTC, 09 UTC, 12 UTC 
Snow depth  Instantaneous value  06 UTC  00 UTC 
(*)When no minimum is reported but hourly instantaneous temperatures AMDAC estimates the minimum from the hourly local early morning values, see AMDAC documentation.
(**)When no maximum is reported but hourly instantaneous temperatures AMDAC estimates the maximum from the hourly local afternoon values, see AMDAC documentation.
(***)Code, for translation see AMDAC documentation.
A final check is then performed on these daily values before an output file is created for further processing. This automated quality check consists in verifying the data according to the table below. If errors are found, the meteorologist will check the data again and make modifications if relevant.
Parameter  Constraint 

Daily mean of total cloud cover : N  0 to 8 octas 
Measured sunshine duration: MeaSun  0 to 24 hours 
Measured radiation: RadMea  0  36 MJ/m2 
Minimum temperature: Tn  35 to 35°C depending on region 
Maximum temperature: Tx  20 to 50°C depending on region 
Maximum temperature  Minimum temperature  0< TxTn <30°C 
Daily mean vapour pressure: e  0 to 35 hPa depending on region 
Daily mean wind speed at 10 metres: ff10  0 to 15 m/s 
Amount of precipitation from 6 UTC6 UTC: RRR  0 to 140 mm depending on region 
Air temperature: TT  35 to 50°C depending on region 
Relative humidity: RH  5 to 100% depending on region 
Daily mean vapour pressure deficit: vpd  0 to 60 hPa depending on region 
Daily mean slope of saturation vapour pressure vs. temperature curve: slope  0 to 3 hPa/°C 
Daytime mean of total cloud cover: N  0 to 8 octas 
Penman evaporation: ETP  0 to 25 mm/day depending on region 
Snow cover: SNOW  (Tn+Tx)/2 < 10°C required do allow any snow cover 
Information on the way the daily element values are constructed/defined is stored in the tables WEATHER_OBS_STATION_INFO and RAIN_OBS_STATION_INFO. Currently, the table WEATHER_OBS_STATION_INFO is only used to store information on rainfall e.g. period definition of the daily rainfall sum.
Finally, the meta data of all stations is checked once a year.
Retrospective checks and blacklisting of suspect stations
Some suspicious station reports are only detectable by checking time series of several weeks. Continuous reports of 0 mm precipitation (instead of a "precipitation not observed" flag) do not stick out in daily rainfall sums, but only by investigating the station's reports over a longer period. Global radiation and cloud cover have a high spatial volatility, and continuous observation or encoding errors at a certain station become more explicit when looking into several weeks of station reports.
For all European stations, the AMDAC output of the past 40 days is inspected each week through time series checks. Provided a station reported on more than half of the tested days, the reports are checked, consulting ECMWF model analysis and short range forecasts for model grid points surrounding the station of request.
The checks are set up as follows:
Precipitation
A station is flagged as suspicious for precipitation when suspicious consecutive zero rainfall reports are detection. Criteria are
 The observed precipitation sum is 0 mm whan aggregated over the whole checked period.
AND
 the ECMWF model near real time forecasts during the checked period included at least 10 wet days. A day is considered being wet when more than 0.5 mm precipitation is forecasted by the model's near real time foreast.
Radiation
For each day of the investigated period, the station’s maximum possible daily solar radiation sum is calculated, based on its latitude, the time of year, and using a standard atmospheric optical depth. A station is flagged as being suspicious for radiation when:
 There are at least 10 days with observed solar radiation exceeding 110 % of the maximum possible amount of solar radiation.
OR
 There are at least 10 days on which the observed solar radiation remained below 10 % of the maximum possible daily sum of solar radiation.
OR
 There are at least 10 days with observed radiation of 0 MJ m2 day1 whilst the ECMWF shortrange forecast analysed solar radiation exceeding 0 MJ m2 day1.
OR
 The total sum of observed solar radiation is less than 25 % of the maximum possible radiation sum for the period, whilst the sum of the model's shortrange forecasts for the parameter exceeded 25 % of the maximum possible daily sum of solar radiation. Naturally, the maximum possible radiation period's is only summed up from days with observations being availble.
Mean daytime cloudiness
A station is flagged as being suspicious for cloudiness when:
 A difference of more than 2.5 octa between the daily mean of observed total cloud cover and the daily mean of ECMWF model analysis and shortrange forecast for total cloud is found for all days of the investigated period.
OR
 The reported instanteaneous cloudiness was always higher than 4.0 octa whilst the model analysis and shortrange forecasted for at least three time steps (hours) in the period a total cloud cover of less then 3.0 octa.
OR
 For all time steps in the period more than 5 octa total cloud cover was reported.
When the process flags stations as suspect a final manual inspection by a meteorologist follows. If the time series of the station are found to be wrong the following actions are executed:
 The station is added to a black list: the station is immediately excluded from the operational station list.
 The erroneous time series are deleted from RAIN_OBS_STATION and WEATHER_OBS_STATION. The erroneous values are saved in separate tables (WEATHER_OBS_STATION_ERRORS and RAIN_OBS_STATION_ERRORS).
 All affected grid cells (WEATHER_OBS_GRID) and regions (WEATHER_OBS_REGIONCOVER) are reprocessed. In case these erroneous data were also used in the crop simulation these data sets are also reprocessed.
 Before mirroring the data to the analysts, they are informed to secure an optimal analysis environment.
Once a year each station on the blacklist is verified. Afterwards it is decided if stations can return to the operational work flow. Falsely blocked data is backordered, added and reprocessed.
Station data availability
Each month an overview is created showing the delivered number of stations per country. Information is also added on sudden changes and followup actions. Similar listings are made on a daily basis for internal use.
Example daily overview (pdf)
The following maps illustrate the available stations (red 020%  green 80100%) for the main elements in a recent year 2017. The main elements (maximum temperature, minimum temperature, precipitation, sun shine, cloud cover, wind speed and vapor pressure) have a good spatial spread over Europe with a relative high spatial density in western and central Europe. Data availability for the Ukraine a bit lower due to technical problems that occurred second half of 2017. This will be repaired in the near future. Availability of measured radiation is mainly limited to western and central Europe.
maximum temperature  minimum temperature  precipitation 
global radiation  sunshine  cloud cover 
wind speed 10m  vapor pressure  snow depth 
The following graph shows the increase of observations for the main elements between 1975 and 2017. Most elements have at least 600,000 annual observations which equals over more than 1600 stations in case they would have a complete temporal coverage. However, most stations have temporal gaps and therefore the number of reporting stations is much higher. Since 2004 the number of observations increased uptill a level of around 1,500,000 reported by more than 4500 stations. During the recent years also observations of radiation related elements increased drastically. This is especially true for cloud cover and sunshine. Prior to 1995 these elements have a relative low number of observations meaning that the global radiation of these years, required in MCYFS, is mainly based on the daily temperature range, see Calculation of advanced parameters.
In general the station density and available data in the monitored areas is considered sufficiently high for the purpose of the project.
Ingestion into the database
After the station weather data passed all checks, daily weather data is exported to a fixed formatted ASCII file (sfile) containing the data of a single day that can be imported in the table WEATHER_OBS_STATION. In the near real time situation a sfile is delivered one day later. For example in the afternoon of day 31 March 2016 the following file is generated: s20160330.dat.
Format ASCII sfile (daily station weather)  

* Codes for state of soil: 0 = surface of ground dry, without cracks or appreciable amount of dust or loose sand, 1 = surface of ground moist 
The 6hourly rainfall data is exported to a plain ASCII file (rrrfile) containing the data of one 6hourly time step within one single day. This data can be imported in the table RAIN_OBS_STATION. In the near real time service each day 4 rrrfiles are generated at once containing data of 4 6hourly time steps: 12 UTC (0612 UTC of previous day), 18 UTC (1218 UTC of previous day), 00 UTC (1800 UTC of previous day) and 06 UTC (0006 UTC of present day). For example in the afternoon of day 31 March 2016 the following files are generated: rrr_2016033012.txt, rrr_2016033018.txt, rrr_2016033100.txt and rrr_2016033106.txt.
Format ASCII rrrfile (6hourly station rainfall)  


Calculation of advanced parameters
Global radiation
Global radiation is the daily sum of incoming solar radiation that reaches the earth surface. It is mainly composed of wavelengths between 0.3 μm and 3 μm. Approximately half of the incoming radiation with wavelengths between 0.4 and 0.7 μm is Photosynthetically Active Radiation (PAR). Global radiation is the driving variable in the growthdetermining CO2 assimilation process and thus crop growth models are sensitive to radiation data (van Diepen, 1992).
A major problem is the scarcity of measured global radiation. In cases where no direct observations are available it must be derived from sunshine duration, cloud cover and/or temperature, on the basis of statistical relationships. If measured global radiation is missing, it is based on one of three formulae (ÅngströmPrescott, SupitVan Kappel, and Hargreaves), depending on the availability of meteorological parameters. An important component in these formulae is the amount of Angot radiation which is the extraterrestrial radiation integrated over the day at certain latitude on a certain day. The calculation of the Angot radiation and the three different formulae are described by Supit et al. (1994) and van der Goot (1998a).
Angot radiation
The principle component of all three formulae is the extraterrestrial radiation, or Angot radiation. In fact, all of the three formulae estimate the fraction of Angot radiation actually received at the earth surface. The Angot radiation is calculated as:
The following hierarchical method is used to calculate global radiation for each station (Supit and van Kappel, 1998) in case measured global radiation is missing:
ÅngströmPrescott formula
If sunshine duration is available, global radiation is calculated using the equation postulated by Ångström (1924) and modified by Prescott (1940). The two constants in this equation depend on the geographic location.
SupitVan Kappel formula
When neither measured radiation nor sunshine duration are available, but minimum and maximum temperature and daytime cloud cover are known, the SupitVan Kappel formula is used. This is an extension of the Hargreaves formula (Supit, 1994). The regression coefficients depend on the geographic location.
where:

Hargreaves formula
When only the minimum and maximum temperatures are known the equation of Hargreaves et al. (1985) is used. The regression coefficients depend on the geographic location.
where:

Any one of the above three methods has an additional upper limit. The maximum calculated global radiation is set to Angot radiation, corrected for atmospheric transmissivity, by multiplying the Angot value with the sum of the Angstrom A and B coefficients.
Deriving ÅngströmPrescott, SupitVan Kappel, and Hargreaves regression constants
The main problem with the application of the ÅngströmPrescott, SupitVan Kappel, and Hargreaves formulae is the quality of the regression constants. Studies by Supit (1994), Supit and van Kappel (1998) and van Kappel and Supit (1998) showed no relationship between latitude and the coefficients for Europe, although such a relation is frequently used to estimate these regression constants. Initially in MCYFS regression constants of Supit and van Kappel (1998) and van Kappel and Supit (1998) for Europe were used. They obtained sets of regression constants for the formulae for as many weather stations as possible, with a geographic distribution that corresponds to the area of interest for the MCYFS. As a result, a set of 256 reference stations was identified for which a relevant set of measured radiation data and other parameters in the formulae existed. For these stations regression constants were calculated based on measured radiation data for the three formulae mentioned above.
In 2012 the regression coefficients of these solar radiation models for Europe were updated using a new set of weather station data (temperature, sunshine and cloudcover) and an alternative training data set: 6 years (20052010) of the downwelling surface shortwave radiation flux (DSSF) 30minutes product derived from Meteosat Second Generation satellite data by the Land Surface Analysis Satellite Applications Facility (LSA SAF) (Bojanowski et al.,2013). For each solar radiation model a set of weather stations was selected having sufficient observations of either sunshine duration, or cloud cover/temperature or only temperature (minimum and maximum) to perform a regression analysis. Results are stored in object STATION_REFERENCE_COEFFICIENTS.
Station archive data for China did not include measured radiation nor sunshine. Therefore radiation was derived from other observed elements namely cloud cover and minimum and maximum temperature. The Hargreaves and SupitVanKappel models have been trained using modelled radiation by Tang et al., 2013. The 50yrRad database of Tang et al., 2013 containing ‘modelled’ radiation data for 716 CMA stations, has demonstrated its superior performance over previous estimates of locally calibrated AngstromPrescott models. While radiation is based on the Hargreaves or SupitVanKappel models, coefficients of the Angstrom method are still required to calculate net outgoing long wave radiation within the potential evapotranspiration calculation (xxx). For determining Angstrom coefficients only the 50yrRad archive was used. Since no sunshine duration data is available, an alternative was sought. Transmissivity was derived by dividing the measured solar radiation at the ground by the solar radiation at the top of the atmosphere. By selecting only the period between day of year 150 and 200 (during midsummer) the transmissivity is almost constant and can be linked to the Angstrom coefficients.
The program SupitConstants uses this set of data (via the view SUPIT_REFERENCE_STATIONS), consisting of latitude, longitude, altitude and calculated regression constants, to derive the regression constants for all stations in the MCYFS. Interpolation of the regression constants of the reference stations to other stations is based on a distance weighted average of the three nearest stations. This process is carried out once, unless the set of reference stations changes or when new stations are added or when meta data of stations change.
Interpolation of regression constants 

Data of the reference stations, consisting of latitude, longitude, altitude and the regression constants, is being used for the derivation of the regression constants for the set of stations used for the interpolation of the daily meteorological data. This is a process that only has to be carried out once, unless the set of reference stations changes or when new stations are added or when meta data of stations change. Once the regression constants have been established for the operational set of stations, the global radiation estimation can proceed using any one of the formulae.
The interpolation of the regression constants is based on a simple distance weighted average of the three nearest stations. For each of the three sets of constants (ÅngströmPrescott, SupitVan Kappel, and Hargreaves) a subset is created from the complete set of reference stations, by selecting only those stations that have the regression coefficients for the desired method. This subset of stations is then sorted based on distance to the station for which the regression coefficients are being calculated. This sorting process is also subject to an altitude threshold test i.e. if the altitude difference between the target station and a reference station is greater than a set threshold the reference station is rejected in favour of the next nearest reference station. Depending on a distance threshold, the nearest one, two or three stations are then used to calculate the regression constants. If the threshold tests exclude all stations, the nearest station will be used, regardless of the distance. The altitude threshold value is 200 m; the distance threshold is 200 km. The distance weighted average method used, is based on the relative distance of the reference stations to the station of interest. Assume the distances d0, d1 and d2 to be the distances to the three nearest reference stations, and w0, w1 and w2 the weights to be used in the calculation. As an example, assume that d1 is 2*d0, then w1 will be w0/2. More general, w1 = w0*d0/d1. Similarly, w2 = w0*d0/d2. Furthermore, the sum of the weights should be 1, so w0+w1+w2 = 1. From the above, the following relation can be established:

Interpolated regression constants are written in the temporary table SUPIT_CONSTANTS and copied to table STATIONS. After the regression constants have been established for all stations, global radiation can be calculated by using any one of the above formulae. Finally, the derived daily global radiation of each station is written into table WEATHER_OBS_STATION_CALCULATED (see flowchart).
Evapotranspiration
Daily meteorological station data collected from stations does not contain potential evapotranspiration by crop, wet soils and open water. Potential crop evapotranspiration (ET0) is calculated by the PenmanMonteith equation while potential evapotranspiration of wet soils (ES0) and open water (E0) is calculated by the Penman equation.
Calculation of potential evapotranspiration 

++ PenmanMonteith ++ Daily meteorological station data collected from stations does not contain potential crop evapotranspiration. This parameter is calculated by the PenmanMonteith equation (Allen et all., 1998). In general, the evapotranspiration from a reference surface, the socalled reference crop evapotranspiration or reference evapotranspiration can be described by the FAO‑PenmanMonteith:
Next, the different components of this formula are calculated. As the magnitude of the day or tenday soil heat flux (G) beneath the grass reference surface is relatively small, it is ignored. The net radiation (Rn) is the difference between the incoming net shortwave radiation (Rns) and the outgoing net longwave radiation (Rnl). The net shortwave radiation (Rns) is calculated as follows:
The outgoing net longwave radiation (Rnl) is calculated as follows. First clearsky radiation (Rso) is derived:
Then, the outgoing net longwave radiation (Rnl) is calculated:
The psychrometric constant is corrected for atmospheric pressure:
Next, saturatedvapourpressure is calculated for both the minimum and maximum temperature and averaged afterwards:
Finally, the slope of the saturation vapour pressure curve is determined (first the minimum and maximum temperature are averaged to obtain the average temperature):
The PenmanMonteith algorithm is valid only for a reference canopy (ET0) and therefore it is not used to calculate the reference values for bare soil and open water (ES0, E0). The background is that the PenmanMonteith model is basically a surface energy balance where the net solar radiation is partitioned over latent and sensible heat fluxes (ignoring the soil heat flux). To estimate this partitioning, the method links between the surface and air temperature. However, the assumptions underlying the model are valid only when the surface where this partitioning takes place is the same for the latent and sensible heat fluxes. For a crop canopy this assumption is valid because the leaves of the canopy form the surface where both latent heat flux (through stomata) and sensible heat flux (through leaf temperature) are partitioned. For a soil, this principle does not work because when the soil is drying the evaporation front will quickly disappear below the surface and therefore the assumption that the partitioning surface is the same does not hold anymore. For water surfaces, the assumptions underlying PenmanMonteith do not hold because there is no direct relationship between the temperature of the water surface and the net incoming radiation as radiation is absorbed by the water column and the temperature of the water surface is codetermined by other factors (mixing, etc.). Only for a very shallow layer of water (1 cm) the PenmanMonteith methodology could be applied. For bare soil and open water the Penman model is preferred. Although it partially suffers from the same problems, it is calibrated somewhat better for open water and bare soil based on its empirical wind function. Finally, in crop simulation models the open water evaporation and bare soil evaporation only play a minor role (presowing conditions and flooded rice at early stages), it is not worth investing much effort in improved estimates of the reference values. Evapotranspiration from a wet bare soil surface (ES0) and from a water surface (E0) is calculated with the Penman formula (Penman, 1948). Only the albedo and surface roughness differs for these two types of evapotranspiration as explained below:
The net absorbed radiation depends on incoming global radiation, net outgoing longwave radiation, the latent heat and the reflection coefficient of the considered surface (albedo). For ES0 and ET0 albedo values of 0.15 and 0.20 are used respectively. The evaporative demand is determined by humidity, wind speed and surface roughness. For a free water surface and for the wet bare soil (E0, ES0) a surface roughness value of 0.5 is used. For a more detailed description of the underlying formulae we refer to Supit et al. (1994) and van der Goot (1997).

Calculated E0, ES0, and ET0 are stored in table WEATHER_OBS_STATION_CALCULATED.
Messages to the Project Management Board
Information on successfull completion of the various processing steps is sent to the Project Management Board (PMB).
List of signals communicated to the Project Management Board (PMB) in connection to the processing of observations from ground weather stations.  

