The Effect of Updraft Entrainment on Convective Cell Deepening in Realistic Large-Eddy Simulations

Abstract Entrainment of surrounding cooler and drier air into convective updrafts is one of the key processes that influence deep convection initiation and growth. Numerous studies have investigated the effect of entrainment on isolated convective cloud growth in idealized simulations, but the importance of this effect in realistic conditions with many interacting convective clouds remains uncertain. We examine the impact of entrainment on the depth reached by convective clouds in realistic large-eddy simulations (LES) over central Argentina during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign. Cloudy updrafts and their associated properties are assigned to convective cells tracked with radar reflectivity signatures. Several thousand convective cells are tracked over two high convective available potential energy (CAPE) and two low CAPE cases that support cells of varying depths and intensities. Entrainment is calculated explicitly as the fluxes of air into the outer surface of each cloudy updraft. Single-predictor logistic regression models are used to determine the relative importance of updraft, near-updraft, and preconvective initiation atmospheric conditions in predicting whether convective cells become deep. We then build a multiple-predictor regression model pairing important updraft and meteorological metrics with fractional entrainment rate. The probability of cells transitioning to deep convection is most sensitive to ambient 600-hPa relative humidity (42% of total metric contribution to cloud depth predictability), followed by low-level CAPE (28%), cloud-base updraft width (19%), and fractional entrainment (11%). Thus, the initial width of the updraft along with potential buoyancy and its dilution through the midtroposphere collectively determine whether deep convection will result from shallower clouds. Significance Statement Storms are an important component of weather and climate. They are influenced by many processes that are difficult to quantify, limiting their predictability. One such factor is the mixing of cooler and drier air into the cloud. This effect has been mostly studied in highly idealized simulations with isolated clouds and controlled surrounding meteorological conditions. We examine this mixing process in more realistic situations where clouds interact with one another in highly variable atmospheric conditions. We find that mixing of drier air into storm cores reduces their ability to grow deeper. However, the dryness of the air is more impactful to storm vertical growth than the amount of air mixed into the storm.