Impact of horizontal resolution on climate model forecasts of tropical precipitation and diabatic heating for the TWP-ICE period

In order to study the impact of horizontal resolution on climate model simulations of tropical moist processes, short-term forecasts using the Community Atmospheric Model (version 4) at several resolutions are performed for a time period encompassing the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). TWP-ICE occurred in the environment of Darwin, Australia in January and February 2006. The experimental period encompasses a number of atmospheric phenomena, such as an MJO passage, mesoscale convective systems, monsoon trough, and active and dry conditions. The CAM is run with four horizontal resolutions: 2 degrees, 1 degrees, 0.5 degrees, and 0.25 degrees latitude-longitude. Simulated profiles of diabatic heating and moistening at the TWP-ICE site show that the model parameterizations respond reasonably well for all resolutions to the sequence of varying conditions imposed by the analyses used to initialize the model. The spatial patterns of global model biases in time mean precipitation are largely unchanged over resolutions, and in some regions the 0.25 degrees model significantly overestimates the observed precipitation. However, there are substantive positive aspects of finer resolution. The diurnally forced circulations over the Maritime continent are more realistically captured by the 0.25 degrees simulation, which is able to better resolve the land-sea breeze. The intensity distribution of rainfall events is also improved at higher resolution through an increased frequency of very intense events and an increased frequency of little or no precipitation. Finally, the ratio of stratiform to convective precipitation systematically increases toward better agreement with observational estimates with increases in resolution.