Numerical Simulations of Stratus Clouds and Their Sensitivity to Radiation—A RACE Case Study

Abstract

The three-dimensional Canadian Mesoscale Compressible Community model has been run at high resolution (?x = 2 km, ?z = 50 m) to simulate stratus clouds observed on 1 September 1995 during the Radiation, Aerosol and Cloud Experiment (RACE) conducted near the Bay of Fundy, Canada. A new explicit cloud scheme and the Canadian operational radiation scheme were validated at this resolution for the first time. The simulations show a reasonable agreement between the observed and modeled stratus cloud system. The cloud structure, position, cloud water content, temperature, and the qualitative properties of longwave and shortwave radiative fluxes were verified against the satellite imagery, lidar, and aircraft measurements taken during RACE. The simulated cloud thickness (?150 m) was thinner than the observed one (200?250 m). The differences in the simulated and observed radiative fluxes were mainly due to errors in the simulation of cloud thickness. Sensitivity experiments demonstrate that the simulated cloud is extremely sensitive to longwave and shortwave radiation. Longwave (shortwave) radiation substantially increased (decreased) the total water path.