A COMPARISON OF SURFACE OBSERVATIONS AND ECHAM4-GCM EXPERIMENTS AND ITS RELEVANCE TO THE INDIRECT AEROSOL EFFECT

The observations of solar irradiance at the surface, total cloud cover and precipitation rates have been used to evaluate aerosol-cloud-interactions in a GCM. Records from Germany and US were available for the time period from 1985 to 1990 and 1960 to 1990. The model used here is the ECHAM4 GCM run for a 5-year period with a fully coupled sulfur chemistry - cloud scheme. We studied two experiments - one with an annual mean sulfate load of 0.36Tg S for the pre-industrial simulation and one with 1.05Tg S for the present day simulation.

Our goal was to indirectly confirm the existence of the indirect aerosol effect by finding indices for a better agreement of observations with the present day experiment compared to the pre-industrial experiment. We were able to draw such a conclusion only for the German data but not for the United States. The model correctly predicts the annual mean total cloud cover in Germany and the US, whereas global solar radiation is underestimated by 13W/m2. This deficiency stems from cloudy conditions. Clouds are either optically too thick or the vertical distribution of clouds is erroneous. This is confirmed by the modeled overcast solar irradiance, which is 27W/m2 lower than observed whereas for the clear sky model the observations agree. Precipitation rates are underestimated by 42% in the United States. The seasonal cycle of the precipitation rate is incorrect in all US regions. The modeled cloud cover is too low over the Central United States in July and August and consequently the solar irradiance exceeds the observations during these months. The opposite occurs in winter when the model overestimates the cloud cover and thus underestimates solar irradiance. We suggest the non-seasonality of vegetation and soil parameters as possible causes for these deficiencies. The convective precipitation formation might also contribute to these discrepancies.

On the other hand, this drying out effect of the inner continent is not as pronounced in coastal regions and in particular, the comparisons for the German grid-box provide indications for the validity of the indirect aerosol effect. The modeled annual cloud cover and solar radiation cycles for the present day aerosol load are in better agreement with observations. Furthermore, the model shows an interesting shift from low cloud reduction to cirrus formation in spring as a consequence of the indirect aerosol effect, a result which is confirmed by observational data.