A Modeling Study of Factors that Control the Vigor of Deep Cumulus Convection

Yale University, New Haven CT

Investigators

Abstract

Observations show substantial variations in the intensity of both tropical and midlatitude-summertime deep convection (e.g., thunderstorm activity) that are not well explained by established measures of the atmosphere's instability based on simple principles. These classic treatments address properties of undiluted parcels of air ascending from a uniformly heated flat surface relative to their surrounding environment to determine the maximum updraft intensity and height achieved by convective clouds. This research will employ model calculations capable of resolving multiple factors influencing such clouds to better understand and quantify the role of variable mid-level atmospheric moisture, heterogeneity in surface heating, and surface orography (i.e. sloped and elevated terrain) in determining their vigor. Resulting improvements in simplified representations of these intricate effects (termed "parameterizations") will ultimately improve global climate models. The primary cloud-resolving numerical system used in this exploration will be the Weather Research and Forecasting (WRF) model, which will be supplemented by more idealized calculations to help understand departures of convective cloud vigor from that predicted by simple parcel theory and to more fully disentangle cause and effect. This strategy will involve carefully designed groups (ensembles) of WRF model runs to systematically explore changes in cloud behavior as a function of key environmental factors. The intellectual merit of this work rests on advancing our basic understanding of moist buoyantly-driven atmospheric convection through a hierarchy of conceptual and numerical models, which will allow researchers to develop and test basic hypotheses concerning factors that control the vigor and heat transport properties of convection, and ultimately the role of such clouds as a form of "turbulence" within the global atmospheric circulation. Broader impacts of this work will include contributions toward more accurate simulations of local/regional weather and global climate, as well as means to more reliably anticipate expected changes in convective activity in the presence of climate change. Training of an undergraduate student in basic elements of design and operation of cloud-resolving models (suitable as an introduction to a chosen career in meteorology for that student) and enhanced exposure of public school students to science through the "DEMOS" program (a Yale University undergraduate organization that does outreach in the surrounding area) will be additional impacts of this effort.