Microphysical Differences Resulting from Regional Climate Change in Simulated Deep Convective Storms

It is currently unknown how the precipitation resulting from convective storms may change as regional climates change.  The complexity of this issue results from changes not only in the thermodynamic and dynamic aspects of the storm environment, but also in the microphysical aspects of the storm itself, including both warm rain and ice processes. NCAR CCSM3 model output was used to generate average atmospheric vertical profiles over various U.S. sites representative of past (1970-1999) and future (2070-2099) regional climates. On average, cloud bases are warmer (by 2°C) and moister in the future.  Overall, there is an enhancement of the warm rain process in the future due to a greater cloud depth beneath the freezing level, but faster updrafts can buffer this response. The warm rain process is less sensitive to the number of cloud condensation nuclei at some locations in the future runs, especially for heavily precipitating storms. Preliminary estimates suggest that in the future clouds, freezing of the larger raindrops formed by the enhanced warm rain process may increase the amount of riming growth of graupel, potentially altering the total rainfall of the storm. To explore these tendencies further, and in greater depth, new 3D simulations with the Weather Research and Forecasting (WRF) model will be presented for the past and future climate scenarios.  These simulations include interactions between warm rain and ice processes, and illustrate various buffers in the microphysical chain of events leading to precipitation in convective storms.