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 not only thermodynamic and dynamic aspects of the storm environment possibly changing, but also changes 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 30 year averages 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 in these environments to investigate differences in the productivity of the warm rain process alone. The model is initialized at the lifting condensation level of each sounding with estimates of cloud condensation nuclei appropriate for the region, and run up to the -12°C level, above which the neglect of ice processes can no longer be justified. The production of drizzle/raindrops at the 0°C and -12°C levels is evaluated and compared for the past and future soundings, not as a prediction per se, but an evaluation given a particular climate scenario. In nature, raindrops may be lofted into the colder regions of the storm where they encourage graupel formation and may eventually result in more precipitation reaching the ground, which will be the goal of future work with a 3D model with representations of both warm rain and ice processes.

In the present study, over 76 runs of the microphysical model were conducted for over 38 sites over the U.S., and regional dependencies of the productivity of the warm rain process are assessed. On average, the cloud base temperature increases 2°C in the future environmental profiles and the depth from the cloud base to the freezing level increases by 500-750 m, allowing the warm rain process to act in deeper portions of the cloud. As a result, the liquid water content at the freezing level increases by approximately 0.7 g/m³ for almost all the sites from the past to the future, implying more condensed water for the accretional growth of ice particles may be available in the future storms.

However, a regional dependency in the results is evident. The highest temperatures are observed in the Midwest and Eastern part of the U.S. in this future climate scenario, and as a result, the modeled liquid water content and precipitation water content at the freezing level, in addition to the depth from cloud base to the freezing level, increases from the Western to the Eastern U.S., consistent with the increase in cloud base temperature. In the Midwest and Eastern U.S., updraft speeds are also greater, however, and at some sites offset the increase in the cloud depth below the freezing level, illustrating the complexity of the problem. The results show an increase in drizzle/raindrop mass of 0.7, 0.8 and 0.9 g/m³ for the Eastern, Midwestern, and Southwestern U.S., respectively, with only a very minor increase (0.1 g/m³) in the West. The drizzle/raindrop number concentration at the freezing level also exhibits a regional dependency. These results illustrate that not only the cloud base conditions, but also the distribution of the buoyancy over the cloud depth, affecting the updraft speed in the storms, can affect the productivity of different microphysical processes, and thus must be considered in evaluating precipitation changes resulting from regional climate change.