The Influence of Cloud Microphysics and Radiation on the Response of Water Vapor and Clouds to Climate Change

Abstract

Uncertainties in representing the atmospheric water cycle are major obstacles to the accurate prediction of future climate. This project focused on addressing some of these uncertainties by implementing new physics for convection and radiation into the NCAR Community Atmosphere Model (CAM). To better understand and eventually better represent these processes in this major national climate model, we modified CAM3.5 to use the convection and cloud schemes developed by the Massachusetts Institute of Technology (MIT) and the RRTMG rapid radiation code for global climate models developed by Atmospheric and Environmental Research, Inc. (AER). The impact of the new physics on the CAM3.5 simulation of convection on diurnal and intra-seasonal scales, on intra-seasonal oscillations and on the distribution of water vapor has been investigated. In addition, the MIT and AER physics packages have been incorporated and tested in combination within the Weather Research and Forecasting (WRF) regional forecast model for the purpose of evaluating and improving convective and radiative processes on time scales appropriate to weather simulations. It has been found that the application of the AER radiation and MIT convection produces significant improvements in the modeled diurnal cycle of convection, especially over land, in the NCAR climate model. However, both the standard CAM3.5 and the modified CAM3.5 with the new physics are unable to capture the proper spectrum and propagating characteristics of dynamical intra-seasonal oscillations such as the Madden-Julian Oscillation. In addition, it has been shown that the new physics methods modify, but do not substantially improve, the distribution of upper tropospheric water vapor in CAM as established through the comparison of modeled and observed satellite radiances. This suggests that continuing regional discrepancies in water vapor amounts in the climate model may not be solely related to convective or radiative processes. The major results of this project have been described in more detail in a journal article titled The Impacts of AER Radiation and MIT Convection on the Water Cycle Simulated by CAM3.5 that will be submitted for publication during Fall 2010.