Appendix VI

 

The use of meteorological data in CGMS

Introduction
Daily meteorological data from a number of weather stations covering the area of interest constitutes the most important set of input data for the system. This data is interpolated to grid level to provide input for crop growth simulation. The interpolation method is described in Section 4.3.

Some meteorological parameters needed (i.e. global radiation and potential evaporation) are not available on a daily basis, and are therefore estimated by the system. Not all weather stations provide all data, and the system takes account of this by labelling each station according to the sort of data it can provide on a regular basis. A station can be labelled as RAIN, TEMP and REST, meaning that it provides either rainfall data, temperature data or global radiation and potential evaporation data. The interpolation algorithm selects the most suitable stations for each grid from the set of available stations. Station information is stored in table WEATHER_STATION, which contains the following columns:
 

WMO_NO station reference number (possibly WMO) [-]
WMO_NAME station name (only for identification, not used) [-]
LATITUDE latitude [decimal degrees]
LONGITUDE longitude in decimal degrees [decimal degrees]
ALTITUDE altitude [m]
CLIMATE_BARRIER_NO code to indicate the presence of a climate barrier [-]
COUNTRY_CODE country code (only for identification, not used) [-]
DISTANCE_TO_COAST distance to the nearest coast [km]
PERCENT_COMPLETE availability threshold [-]

 

Additional weather calculation
Global radiation and potential evaporation as required by CGMS (see Chapter 4) are calculated at station level and stored in the table CALCULATED_WEATHER which contains the following fields:


 

WMO_NO station reference number (possibly WMO) [-]
DAY day [d]
E0 potential evaporation from a free water surface [mm d-1]
ES0 potential evaporation from a moist bare soil surface [mm d-1]
ET0 potential transpiration from a crop canopy [mm d-1]
CALCULATED_RADIATION daily global radiation [KJ m-2d-1]

 

To calculate E0, ES0, ET0 and the global radiation, a set of regression constants for each weather station is needed. The program uses one of three different regression formulae for the global radiation calculation, depending on availability of the observed meteorological parameters. These three formulae, Angstrom, Supit and Hargreaves, per weather station, have their own set of regression constants, which depend on the station location. The constants are stored in the table SUPIT_CONSTANTS, containig the following columns:


 

WMO_NO station reference number of the station (possibly WMO)
ANGSTROM_A interpolated Angstrom A
ANGSTROM_B interpolated Angstrom B
SUPIT_A interpolated Supit A
SUPIT_B interpolated Supit B
SUPIT_C interpolated Supit C
HARGREAVES_A interpolated Hargreaves A
HARGREAVES_B interpolated Hargreaves B

 

This table is filled by a utility program called SupitConstants. This program estimates the regression constants for an arbitrary location by interpolating constants from a known location (see Subsection 4.1.3 ). These constants have been established for a number of weather stations covering a large part of Europe, and are stored in table SUPIT_REFERENCE_STATIONS. This table contains the following information:
 

STATION_NAME station name for identification, not actually used
LATITUDE latitude in decimal degrees
LONGITUDE longitude in decimal degrees
ALTITUDE altitude (m)
ANGSTROM_A calculated Angstrom A
ANGSTROM_B calculated Angstrom B
SUPIT_A calculated Supit A
SUPIT_B calculated Supit B
SUPIT_C calculated Supit C
HARGREAVES_A calculated Hargreaves A
HARGREAVES_B calculated Hargreaves B

 

Information in table SUPIT_REFERENCE_STATIONS represents a significant body of data and should not normally be changed by the user. The regression constants have been calculated with up to twenty years of meteorological data for some stations (Supit & van Kappel, 1997).
 

Missing data
A station can be found suitable for use in the interpolation even if meteorological data are not available for the whole period for which the interpolation is to take place. In this case, missing data will be substituted by data from table REFERENCE_WEATHER. This table, filled by the utility program ReferenceWeather, contains an entry for each station for every possible day of the year (i.e. 366 days). The data in this table represents the daily long-term average data for this station. The DAY field in the table contains dates from 1-1-1956 until 31-12-1956, where the year 1956 was chosen solely for the fact that it is a leap year. The table contains the following fields:
 

 

WMO_NO the (possibly WMO) reference number of the station [-]
DAY day (year is 1956) [d]
MINIMUM_TEMPERATURE minimum temperature (°C) [°C]
MAXIMUM_TEMPERATURE maximum temperature [°C]
CALCULATED_RADIATION daily global radiation in KJ/m2/day [ KJ m-2d -1]
WINDSPEED mean daily windspeed at 10m [m s-1]
RAINFALL mean daily rainfall [mm]
VAPOUR_PRESSURE mean daily vapour pressure [hPa]
E0 pot. evap. from a free water surface [mm d-1]
ES0 pot. evap. from a moist bare soil surface [mm d-1]
ET0 pot. transpiration from a crop canopy [mm d-1]

 

Some of these fields can contain NULL data, e.g. the rainfall field will not contain data for a station that only supplies temperature data.
 

Weather data availability
The availability of a weather station and its role in the interpolation is determined by the weather data availability calculation. For all stations in the WEATHER_STATION able, the program will assess if the amount of data available for the station in the requested period equals or exceeds the threshold value PERCENT_COMPLETE. This assessment is done for each of the three ‘labels’ RAIN, TEMP and REST, and checks the relevant data subsets, e.g., the check for REST only involves the CALCULATED_WEATHER data. Whilst checking the availability for a station, the program will make sure that for those days that data is missing the REFERENCE_WEATHER table can provide a substitute value. If this is not the case, the station will not be available. results of the availability check are stored in the WEATHER_DATA_AVAILABILITY table, which contains the following fields:
 

YEAR year for which the station is labelled
WMO_NO station identifier
AVAILABILITY_TYPE "RAIN", "TEMP", or "REST"

It is clear that a station that can provide a full set of data will have three entries in this table for a given year. If the period for which the availability check is being performed is in the current year (as defined in the SYSCON table), then the check will only be performed for the period from Jan 1st until the last day of the period requested. Otherwise the check is being performed using the data for the whole calendar year.
 

Grid weather
Once the additional data for the individual station has been calculated and the station availability has been established, interpolation can start. The first part of this process involves the scoring’of the weather stations for each grid, in order to establish exactly which stations to use for the interpolation. The second step is the actual interpolation. Interpolation results are written into table GRID_WEATHER which contains the following fields:
 

GRID_NO grid number [-]
DAY day [d]
MAXIMUM_TEMPERATURE maximum temperature [°C]
MINIMUM_TEMPERATURE minimum temperature [°C]
VAPOUR_PRESSURE mean daily vapour pressure [hPa]
WINDSPEED mean daily windspeed at 10m [m s-1]
RAINFALL mean daily rainfall [mm]
E0 pot. evap. from a free water surface [mm day-1]
ES0 pot. evap. from a moist bare soil surface [mm day-1]
ET0 pot. transpiration from a crop canopy [mm day-1]
CALCULATED_RADIATION daily global radiation [KJ m-2 d-1]

 

The grid weather table cannot contain NULL data. Maximum distance between a weather station and the grid centroid is set in the SYSCON table. Stations beyond this distance are not considered for interpolation. For control purposes, information about the use of the stations for interpolation for a particular grid is written to the table STATIONS_PER_GRID containing the following fields:
 

GRID_NO grid number
YEAR year
RAIN_STATION the station selected for the rainfall data
TEMP_STATION_1 up to four stations used for temperature
TEMP_STATION_2  
TEMP_STATION_3  
TEMP_STATION_4  
REST_STATION_1 up to four stations used for the rest
REST_STATION_2  
REST_STATION_3  
REST_STATION_4  

 

Note that this information is only stored for the year in which the interpolation is taking place. If the process is run again for a different period in the same year and the weather data availability has changed, then these values will change.

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