Suitability of medium-range predictions of classical meteorological parameters and of the duration of leaf wetness for biometeorological forecasts

A classification of daily weather types is used to obtain typical deviations of daily maximum and minimum air temperatures, global radiation, amount of precipitation, mean daily relative humidity at 1400 hours local time, meteorological water balance, grass minimum temperature at a height above the turf of 5 cm and duration of daily leaf wetness for SW Germany (Stuttgart area) from the corresponding monthly means. The period of reference was the growing season from May to September for the 5 years 1980-1984. Medium-range weather forecast maps for the ground surface and the 850 hPa level are issued daily for 5 days in advance by the medium-range weather forecasting centre at Reading (ECMWF). The forecasts are valid for Western Europe and have been used to transform the forecasted flow pattern and air pressure distribution, both on the ground and at the 850 hPa level into Central European weather types, following the classification mentioned above. The seasonal variability (from May to September) has been found to be small enough to be able to present the results as a single table for the whole vegetation period. The deviation of the forecasted values from the observed values are compared, using as test period the 1988 and 1989 seasons. The forecasts were: (i) persistency forecasts (the same deviations are forecasted for the next 5-days period); (ii) supposed weather types during the next 5 days, derived from ECMWF with expected deviations of the meteorological elements from the seasonal mean; and (iii) "real" weather types, classified officially from the German Weather Service using mean deviations of the meterological elements from the seasonal means. The means obtained on the 5-day deviations are discussed. The values were, for example, 2 K for minimum temperature. about 3 K for maximum temperature, 20% of the values for global radiation, 1.5 mm for the amount of daily precipitation and 16% for the daily duration of leaf wetness. The mean relative deviations differed between the various meterological elements, being smaller for elements mainly related on the macro-/mesoscale than for elements such as minimum temperatures that are mainly relate to the microclimate.