PNNL

Abstract

A four-year climatology (1997-2000) of warm boundary layer cloud properties is developed for the U.S. Department of Energy Atmospheric Radiation (ARM) Program Southern Great Plains (SGP) site. Parameters in the climatology include cloud liquid water path, cloud base height and surface solar flux. These parameters are retrieved from measurements produced by a dual-channel microwave radiometer, a millimeter-wave cloud radar, a micropulse lidar, a Belfort ceilometer, shortwave radiometers and atmospheric temperature profiles amalgamated from multiple sources, including radiosondes. No significant interannual differences are observed, but nighttime liquid water paths are consistently higher than daytime values. The summer months of June, July and August have the lowest liquid water paths and the highest cloud base heights. Model outputs of cloud liquid water paths from the European Center for Medium Range Weather Forecasting (ECMWF) model and the Early Eta Model for 104 Model Output Location Time Series (MOLTS) stations in the environs of the SGP central facility are compared to observations. The ECMWF and MOLTS mean and median liquid water paths are 3 and 4 times greater, respectively, than the observed values. The MOLTS data show lower liquid water paths in summer, which is consistent with observations, while the ECMWF data exhibit the opposite tendency. A parameterization of normalized cloud forcing that requires only cloud liquid water path and solar zenith angle is developed from the observations. The parameterization, which has a correlation coefficient of 0.81 with the observations, provides estimates of surface solar flux that are comparable to values obtained from explicit radiative transfer calculations based on plane-parallel theory. This parameterization is used to estimate the impact on the surface solar flux of differences in the liquid water paths between models and observations. Overall, there is a low bias of 50% in modeled normalized cloud forcing resulting from the excess liquid water paths in the two models. Splitting the liquid water path into two components, cloud thickness and liquid water content, shows that the higher liquid water path in the model outputs is primarily a result of higher liquid water content. On the other hand the cloud thickness in both observations and models is comparable.