Comparison of model predicted cloud parameters and surface radiative fluxes with observations on the 100 km scale

Cloud parameters and surface radiative fluxes predicted by regional atmosph eric models are directly compared with observations for a 10-day period in late summer 1995 characterized by predominantly large-scale synoptic condit ions. Observations of total cloud cover and Vertical cloud structure are in ferred from measurements with a groundbased network of Lidar ceilometers an d IR-radiometers and from satellite observations on a 100 kilometer scale. Groundbased observations show that at altitudes below 3 km, implying liquid water clouds, there is a considerable portion of optically non-opaque clou ds. Vertical distributions of cloud temperatures simultaneously inferred fr om the groundbased infrared radiometer network and from satellite can only be reconciled if the occurrence of optically thin cloud structures at mid- and high tropospheric levels is assumed to be frequent. Results of three re gional atmospheric models, i.e. the GKSS-REMO, SMHI-HIRLAM. and KNMI-RACMO, are quantitatively compared with the observations. The main finding is tha t all models predict too much cloud amount at low altitude below 900 hPa, w hich is then compensated by an underestimation of cloud amount around 800 h Pa. This is likely to be related with the finding that all models tend to u nderestimate the planetary boundary layer height. All models overpredict th e high-level cloud amount albeit it is difficult to quantify to what extent due to the frequent presence of optically thin clouds. Whereas reasonably alike in cloud parameters, the models differ considerably in radiative flux es. One model links a well matching incoming solar radiation to a radiative ly transparent atmosphere over a too cool surface, another model underpredi cts incoming solar radiation at the surface due to a too strong cloud feedb ack to radiation, the last model represents all surface radiative fluxes qu ite well on average: but underestimates the sensitivity of atmospheric tran smissivity to cloud amount.