Changes in the Productivity of the Warm Rain Process in Deep Convective Clouds Resulting from Regional Climate Change over the Continental U.S

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 in the thermodynamic and dynamic aspects of the storm environment, and in the various microphysical processes that yield the precipitation in convective storms, including warm rain and ice processes. As a first step in assessing this problem, 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, and a 1D warm rain microphysical model was run with these profiles to investigate differences in the productivity of the warm rain process alone.  The model is initialized at the cloud base using estimates of cloud condensation nuclei (CCN) appropriate for the region, and is run up to the -12 °C level, above which the neglect of ice processes can no longer be justified.  Over 100 runs of the warm rain model were conducted for over 50 sites across the U.S. On average, the air at the cloud base is warmer and moister, with an increased temperature of 2°C in the future environmental profiles. The cloud depth below the freezing level in the future scenario can be much greater, but is partly compensated by greater cloud updraft speeds.  Overall, there is an enhancement of the warm rain process, but less than expected from considering cloud depth alone. CCN variability is less sensitive at some locations in the future runs and will be explained.