Thematic description of CGMS level 1
Data Acquisition and Checks
The meteorological station data consists of:
- Station information.
- Raw daily meteorological data.
- Processed daily meteorological data.
The stations are limited to those for which data not only are regularly collected but which can also be received and processed in semi-real time (Burrill and Vossen, 1992). Relevant information of stations includes station number, station name, latitude, longitude and altitude. This data are available in the table WEATHER_STATION.
The following text describes how (historic) data are collected for the European application. This gives some insight in the problems that can be expected when working with station data. Some of the historic meteorological data are purchased directly from various national meteorological services, others are acquired via the Global Telecommunication System (GTS). As the data are obtained from a variety of different sources, considerable pre-processing is necessary to convert them to a standard format. Two different procedures are applied for distinct subsets of the data set. The historic data were ordered directly from national meteorological services. Around 1992 they represented approximately 380 stations in the EU, Switzerland, Poland and Slovenia with data from 1949 to 1991 (Burrill and Vossen, 1992). Later the historic set have been extended with stations in eastern Europe, western Russia, Maghreb and Turkey. The historic data were converted into consistent units and were 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 near real time from the GTS network for different hours within one day. The data are pre-processed and quality checked using the AMDAC software package (MeteoConsult, 1991) which extracts, decodes and processes the GTS data. After decoding, the following data are checked for consistency and errors: air temperature, dew-point temperature (humidity), pressure at sea level, wind speed, amounts of precipitation, clouds, and sunshine duration. This error checking compares each observation with the corresponding values of the surrounding stations and compares that particular observation with observations at other times in the same day at the same station. Obvious errors in the observations are corrected automatically and a message is written to a log file; other errors are flagged for possible correction by an operator (Burrill and Vossen, 1992). Finally, the data are converted into daily values. This comprises the selection of minimum and maximum temperature, the aggregation of the rainfall, cloud cover and sunshine duration, the calculation of mean vapor pressure etc.
The processed daily meteorological data consists of 30 meteorological parameters including various cloud cover indicators, air temperature, vapour pressure, wind speed and rainfall. Because European stations follow different measurement schemes many records contain blank fields for parameters which are never registered. Stations often also include blank fields for parameters which where not available for limited periods. However, the stations selected for inclusion in the database are those which normally report at least the minimum and maximum daily air temperature, rainfall, wind speed, vapour pressure (or humidity) as well as either global radiation, sunshine hours or cloud cover (Burrill and Vossen, 1992). Each day the processed daily meteorological data are inserted into the CGMS database in table METDATA. However CGMS only needs a sub set of the 30 variables described above and thus only 9 variables are described in underlying user manual and table METDATA.
