Evaluation of an Explicit One-Dimensional Time Dependent Tilting Cloud Model: Sensitivity to Relative Humidity

Abstract

The primary goal of this study is to evaluate the performance of the Explicit One-dimensional (1D) Time-dependent Tilting Cloud Model (ETTM), which will be potentially used in a cumulus parameterization scheme. The Weather Research and Forecasting (WRF) model was used in a cloud-resolving mode to study 3D cloud characteristics under two sheared environments, one from the Rain in Cumulus over the Ocean (RICO) field experiment and the other from the International H 2 O Project (IHOP). Then, WRF 3D simulation results were used to evaluate ETTM performance. WRF simulations were performed with different radii from 1 km to 10 km of thermal bubbles for initiation. The three-dimensional cloud features were quite different between RICO and IHOP due to their environments, which were sub-tropical maritime sounding and mid-latitude continental sounding, respectively. ETTM 1D cloud simulations, corresponding to each of the WRF simulations, were conducted. The simulated 1D clouds were too weak when the original thermal bubbles were similar to those used in the 3D cloud simulations (i.e., no additional moisture within the thermal bubbles). The sensitivity of model results to relative humidity was tested by imposing a lower bound of 88% (ER88) and 95% (ER95) humidity to the thermal bubble in ETTM simulations. When compared with the original simulations, 1D results from ER88 and ER95 showed clear improvements, but they were still underestimated relative to 3D clouds, and the results from IHOP were slightly worse than those from RICO. Sensitivity tests with a zero-degree cloud tilting angle and with a different radius of the downdraft were also examined. Results show that the downdraft due to the tilting of the cloud slightly improved ETTM's performance in terms of the heat and moisture fluxes, while the influence of using different downdraft sizes on 1D simulation results is not clear.