Aerosol and Low-Level Thermodynamic Sensitivities of the Cumulus-to-Congestus Transition Within the CAMP2Ex Environment

Abstract:

The tropical trimodal distribution of convective clouds has been recognized as a ubiquitous feature for over 20 years. The three modes of cumulus, congestus, and cumulonimbus all play significant roles in the transport of heat and moisture in the tropics. The cloud top height of each mode corresponds to a prominent stable layer present in the tropical climatological mean, and cloud properties are further characterized by other aspects of the local thermodynamics and by aerosol concentrations. While the impacts of aerosols on warm phase cloud droplet concentrations and collision coalescence processes are relatively well understood, aerosol effects on the thermodynamics, dynamics, and precipitation of cloud systems are less so. A number of studies have examined the independent influences of the thermodynamic environment and aerosol concentrations on tropical convective cloud species and their properties, but few have covaried aerosol and thermodynamics. The goal of this research is therefore to examine the relative importance and covariance of the thermodynamic and aerosol environments on the trimodal distribution, with particular focus on the cumulus-congestus transition.

To address our goal, we use a suite of idealized cloud-resolving numerical simulations, as well as observations of each of the trimodal cloud species made under a variety of thermodynamic and aerosol conditions around the Philippines during the recent NASA Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP²Ex). We use these comparisons to examine how varying the initial aerosol concentration and low-level static stability affect the morphology of tropical maritime convective clouds and impact the cumulus-congestus transition. These simulations were run using the Regional Atmospheric Modeling System (RAMS) under thermodynamic and aerosol environments based on CAMP²Ex observations. Using the model data, we have examined the impacts of the aerosol and environmental perturbations on cloud type frequency, cloud top height, maximum updraft strength and altitude of maximum strength, condensate microphysical properties and profiles, and cloud and clear-sky thermodynamic profiles. We will also present and discuss those processes which led to differences in these quantities and to the cumulus-congestus transition.