Evaluation of MM5 Optically Thin Clouds over Europe in Fall Using ICESat Lidar Spaceborne Observations

Abstract

The description of clouds in mesoscale models has progressed significantly during recent years by improving microphysical schemes with more physical parameterizations deduced from observations. Recently, the first lidar in space, the Ice, Cloud, and Land Elevation Satellite (ICESat)/Geosciences Laser Altimeter System, has collected a valuable dataset that improves the knowledge of occurrence and macrophysical properties of clouds, and particularly high-altitude clouds, which are usually optically thin. This study evaluates the capability of the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) to reproduce optically thin clouds using the ICESat October?November 2003 dataset. Initial and boundary conditions are prescribed from NCEP products and MM5 run over the European continent with a 40-km spatial resolution. Spaceborne lidar profiles are diagnosed from model outputs and compared with the observed ones at the same location and time. One month of simulations?observations comparisons shows that the model correctly reproduces cloud structures on average, but underestimates the thinnest clouds (by 0%?20%) and overestimates less thin clouds in the upper troposphere (altitude >6 km). The total low-level water cloud amount (altitude <6 km) appears fairly well reproduced, although the masking effect of higher clouds does not allow for a firm conclusion. The clouds are rarely simulated and observed simultaneously, 50% for high clouds and 20% for low clouds. The lack of high-altitude very thin clouds is possibly due to dry biases in the upper-troposphere humidity fields used to force the model. The overestimation of optically less thin cloud may be due to an overestimation of the cloud lifetime or water vapor supersaturation around ice clouds that is not taken into account in the model. When the upper troposphere and low warm clouds appear in the model at the same time and location as in the observations, they are optically too thick, likely because their water/ice content and particle concentration are overestimated simultaneously.