Comparison of monthly mean precipitation rates from GPCP observations and ECHAM5 simulations

Clouds and precipitation play an important role in the hydrological cycle of the earth. Changing precipitation patterns due to climate change result in shifted vegetation zones, have influence on water quality, soil structure/erosion and runoff into the rivers and oceans. Through feedback processes, this changed precipitation rates have an impact on cloud formation and microphysical processes which, on their part, influence the precipitation rates. The prediction of precipitation is, therefore, an important issue for the climate modeling community. In order to obtain reliable results and to evaluate the model behavior it is essential to compare precipitation rates from model simulation with observation. This study uses observational data from the Global Precipitation Climatology Project (GPCP) to evaluate model simulations done with the ECHAM5 GCM.

The GPCP combines satellite data with surface rain gauge measurements to retrieve precipitation datasets from 1979 up to now. The satellite measurements include microwave (SSM/I) and infrared precipitation estimates as well as low earth orbit estimates from TOVS (TIROS Operational Vertical Sounder) and OPI (Outgoing Longwave Radiation (OLR) Precipitation Index).

The ECHAM5 is a general circulation model developed at the Max-Planck-Institute in Hamburg, Germany. In this study a version with a 2-moment bulk microphysics scheme for cloud water is used. The precipitation (i.e., rain and snow) is treated diagnostically such that within on model time step rain and snow which do not evaporate or sublimate below cloud base are completely removed from the model.

For the statistical analysis monthly mean values of the precipitation rates are used. The resulting frequency distributions are grouped according to the seasons and are, furtheron, separated into total, land, and ocean precipitation. Initial results yield that the model has most problems with capturing heavy precipitation events in the northern and southern midlatitudes. Therefore, sensitivity studies with different cloud microphysical parameters will be carried out and presented at the conference.