Convective Updraft and Downdraft Kinematics of Mesoscale Convective Systems in Convection-Permitting Model Across Different Model Grid-Spacings

Abstract:

Mesoscale Convective Systems (MCSs) help regulate the global energy cycle through their extensive cloud coverage and the exchange of latent heat. However, it is challenging to represent MCSs in global climate models, in part because this necessitates parametrizing processes that interact across a wide range of scales. In convection-permitting models (CPMs), significant improvements have been made in addressing bulk MCS properties. However, large uncertainties remain in simulating detailed processes, since these models operate in the gray-zone of turbulent motion where convection is not fully resolved.

In this study, we explore the ability of CPMs to capture finer-details of MCS kinematic properties, and to investigate discrepancies in those properties as a dependent on model grid-spacing and MCS lifecycle phase. We perform ensemble MCS simulations across a wide range of model grid-spacings (from 4 km to 250 m) using the WRF model for continental U.S. conditions. The fidelity of simulated MCS kinematic properties are anchored by comparisons against those revealed by unique multi-year radar wind profiler (RWP) dataset from DOE ARM SGP site. Consistent with the RWP observations, the probability of simulated updraft/downdraft increases as a function of low-level radar reflectivity. Convective updrafts are less frequently observed in higher resolution runs, indicating a difference in probability up to 20% between 4km and 250m. A bimodal distribution in convective updraft probability is consistently found at all model grid-spacings. Convective updraft intensity is relatively overestimated at the coarser-resolution models, especially during the mature stages of the MCS. The convective downdraft intensity and probability are underestimated by all simulations when compared to the observations, with the coarse resolution behaviors least reflective of RWP statistics. The more significant discrepancies are found within compensating downdrafts at the mid-levels during the developing stages of the MCS. The improved downdraft performance in the higher resolution models is attributed to more frequent generation of graupel at these heights. A heightened rain percentage at mid-levels also suggests a potential higher downdraft source level leading to improved downdraft intensity in higher resolution runs.