Impacts of Regional Climate Change Upon the Warm Rain Process and Surface Precipitation from Deep Convective Storms: A Numerical Modeling Study

Multiple studies have stated that precipitation changes resulting from climate change are reflected mainly in the heavy and extreme daily precipitation events, at the expense of more moderate events. Thus convective precipitation may be more sensitive to increases in global temperatures than stratiform precipitation. Observations in the contiguous U.S. over the 20th century have shown that these precipitation changes have been larger in the summer months, potentially leading to higher occurrences of flooding. However, the details of how precipitation production within deep convective storms will be modified as regional climates change are still not well understood, owing to the complex interactions between microphysical processes including both warm rain and ice processes, thermodynamic aspects of the atmosphere, the cloud dynamics, and the large-scale environment. Previous studies have shown that heavy rain-producing storms may have a very active warm rain process. This study aims at determining if storms developing in a warmer, moister future environment will indeed have a more active warm rain process, and if so, its effects on ice processes aloft and precipitation at the surface as well.

The Weather Research and Forecasting (WRF) model with the double-moment Morrison microphysical scheme is used to simulate continental convective storms at different sites within “past” (1970-1999) and “future” (2070-2099) environments derived from NCAR CCSM3 model output. The storms are initiated with warm bubbles to force the convection in an idealized sense.

Initial results show dynamically stronger future storms with shorter lifetimes that produce greater amounts of rain within the storms and higher rainrates at the surface, and less graupel/hail. The warm rain process is found to play more of a role in the initial rain production in the future storms; the melting of graupel/hail contributes an equal amount at later times. Graupel/hail production starts slightly later in the future storms owing to a higher freezing level where the freezing of raindrops initiates rimed particles; the faster rain production in the future storms also decreases the amount of cloud water that is available for collection by graupel/hail. Thus the contribution of ice processes to surface rainfall is decreased in the future storms. Calculations of the time-varying precipitation efficiency of the storms show higher values in the future storms. The consistency of these results over multiple sites around the U.S. will be examined. The causes and implications of the greater rainrates, surface rainfall and precipitation efficiency in at least some, if not all, of the future storms will also be discussed.