Summer Drying Problem
A cooperative effort with the Hungarian Weather Service
Holger Göttel and Vera Schlanger (Hungarian Weather Service)
Within the EU funded project MERCURE (Modelling European Regional Climate: Understanding and Reducing Errors) a strong bias towards an extensive drying of the soil was detected during summer months in large areas of the Danube river basin. This feature is common to all participating limited area models. The largest differences occur in the Hungarian Lowlands and in the Wallachia.
Start of the cooperation
The Hungarian Meteorological Service (HWS) and Max Planck Institute for Meteorology (MPI-M) established a cooperation to analyse the summer drying problem. The HMS sent two scientists to MPI-M for a period of two months. Their tasks were to compare REMO results against observation data as well as to learn how to use REMO.
24 year high resolution RCM run with REMO - horizontal resolution 1/2 deg., 20 vertical levels - for Central Europe under present day climate conditions. Initial and boundary values are interpolated from ECMWF reanalysis/analysis data between 1979-2002.
First results of this validation are shown below. The validation of REMO shows that:
- Precipitation fits well to the observations except for late summer months (Fig. 1)
- Only for the summer months the model overestimates the 2m temperature (Fig. 2)
The results don't differ from the findings of the MERCURE project.
Figure 1: Precipitation [mm/month] over 16 stations in Hungary. The solid line is the multi station mean of precipitation for all years from 1979-2002. The dashed lines are the min and max of monthly means in precipitation.
Figure 2: 2m-air temperature [ºC] for 16 Stations in Hungary. The solid line is the multi station mean of temperature for all years from 1979-2002. The dashed lines are the min and max of monthly means in temperature.
Other meteorological fields (e.g. pressure, 10m wind and surface solar radiation) have been compared to observed data. The model data shows a slight overestimation of the 10m-wind velocity and an definitive overestimation of surface solar radiation. The high solar radiation is correlated with the underestimation of cloud cover.
- Problems in advection
- Deficiencies in the convective mass flux scheme
The first hypothesis is that probably drier air is transported into this region. For this the advection processes were compared for one month between simulated and driving data.
Figure 3 shows the differences in the advection processes (for 3 days) through Hungary. Comparing the air mass advection for REMO (left panel) and ERA15 (right panel) reveals that in the REMO simulation Hungary is influenced by continental and in ERA15 by mediterranean air. A detailed investigation is in preparation.
The reasons for the disagreement between simulated and observed precipitation and temperature in the Danube river basin is not clear yet. The solution is complex and requires a more detailed investigation.
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