PNNL

Abstract

The environments of tracked mesoscale convective systems (MCSs) are assessed over a 20-year period. We aim to provide novel information on observed MCS environments for global atmospheric models, to assess and improve their ability to simulate MCSs. The analysis is performed in two ways, providing different insights into the environmental conditions that occur before and during observed MCS occurrence. In the first, environmental conditions are investigated at different spatial scales up to 24 h before MCS formation and following MCS track locations after their formation. Several environmental variables are found to show marked changes before MCS initiation, particularly over land. One variable, the vertically integrated moisture flux convergence, shows a robust signal across different regions and throughout the diurnal cycle. When the analysis is performed at different spatial scales, signs of scale dependence between 200 and 500 km are found, providing new evidence of a natural length scale for use with MCS parametrization. In the second analysis, the likelihood of MCS occurrence for given environmental conditions is evaluated, by considering the environments under which the MCS core and shield occur over the course of all tracked MCS lifetimes. These are compared to analogous non-MCS environments which are not part of the tracked MCSs, allowing discrimination between MCS and non-MCS occurrence for the first time. Three environmental variables are found to be useful predictors of MCS occurrence: total column water vapour, relative humidity and moisture flux convergence. Such relations could be used as trigger conditions for parametrization of MCSs, thereby strengthening the dependence of the MCS scheme on the environment.