PNNL

Abstract

The initiation of deep moist convection initiation is governed in part by the horizontal width of updrafts near cloud base, which limits the deleterious effects of entrainment-driven dilution on buoyant thermals ascending through the free troposphere. However, the factors controlling cloud-base updraft widths, which in turn dictates cloud depth are not well understood. We track the evolving three-dimensional structure of the mesoscale sub-cloud vertical forcing, cloudy updrafts developing along it, and near-cloud thermodynamic ingredients within a simulated high-resolution ensemble of 7 realistic daytime orographic convection initiation events to determine their roles in controlling cloud width and depth. Statistical analysis of ~5000 cloudy updraft samples indicates that the most important contributor to the width of cloudy updrafts across the ensemble is the depth of the sub-cloud mesoscale ascent. However, the depth achieved by clouds is more consistently predicted by the near-cloud ambient convective available potential energy and the relative humidity within the lower to middle free troposphere. Therefore, while the width of cloudy updrafts may be partly set at low levels by the mesoscale vertical mass flux, the probability of deep moist convection is governed by the generation of positive buoyancy within cumulus thermals and entrainment-driven dilution that reduces it. Persistence of the low-level mesoscale forcing locally consolidates and vertically transports boundary layer moisture, helping to reduce updraft dilution. However, these factors vary in relative impacts on cloudy updrafts across individual cases, indicating multiple pathways for deep convection initiation.