Aerosol‐induced mechanisms for cumulus congestus growth

Abstract

Tropical convection has been observed to contain three cloud modes, the middle of which is cumulus congestus clouds. Congestus clouds act to moisten the tropical atmosphere, may be mixed‐phase, and on occasion surpass the freezing level inversion from where they may develop into deeper convection. This study investigates the impacts of enhanced aerosol concentrations on the growth of congestus clouds produced in idealized cloud‐resolving model simulations run under a state of radiative convective equilibrium (RCE). High‐resolution, long‐duration simulations were completed using the Regional Atmospheric Modeling System (RAMS). Aerosol concentrations between 2 and 4 km above ground level were varied from clean to polluted conditions in order to represent the advection of Saharan dust over the Atlantic Ocean. The congestus populations within each aerosol simulation are statistically analyzed using 10 days of model output after the simulation reaches RCE. Results indicate that congestus in more polluted conditions produce greater amounts of cloud water and ice mass, enhanced updraft strengths, and an increase in the number of congestus cloud tops that extend above the freezing level. Enhanced vapor depositional growth on the populations of more numerous, smaller cloud droplets in the polluted conditions, and the subsequent increase in latent heat release in the warm phase regions of the cloud, is found to be important factors in convective invigoration of these cloud systems. Aerosol feedbacks associated with cold pools and condensate loading also influence the updraft strength and act in opposition to the warm phase invigoration processes.

Aerosol–cloud interactions in mixed-phase convective clouds – Part 1: Aerosol perturbations

“…In contrast, our simulations suggest that the precipitation response is mainly driven by changes in latent heating below the 0 • C level. The idealised studies of Sheffield et al (2015) and Lebo (2014) have also found changes in the warm-phase section of the clouds to dominate over latent heating changes in the mixed-phase part. The increase in warm-phase latent heating in Sheffield et al (2015) and Lebo (2014) is due to a more efficient vapour deposition on the more numerous cloud droplets in polluted conditions.…”

Anthropogenic Aerosol Influences on Mixed-Phase Clouds

“…Freezing releases latent heat aloft and invigorates these cloud systems [47], but also in the lower levels of the cloud more latent heat is released by enhanced condensational growth [88,93]. This is partly counteracted by the reduced buoyancy due to increased water loading.…”

A global lightning parameterization based on statistical relationships among environmental factors, aerosols, and convective clouds in the TRMM climatology

“…[] and Sheffield et al . [] contend that aerosols are responsible for the transition from shallow convective to congestus or deep convective modes, especially in pristine environments such as over the West Pacific Warm Pool. Storer et al .…”