Aerosol indirect forcing on a range of tropical cloud systems developing within a radiative convective equilibrium framework

Tropical convective clouds are distributed in a tri-modal manner and play an important role in the tropical climate. Much is still not understood about how these three modes of convection interact, the roles that they serve in the global water and energy balance, and how these modes may change when perturbed by external effects such as aerosol indirect forcing. The goal of the research to be presented here is to further investigate the impacts of aerosol indirect forcing on the properties and organization of these three modes of tropical convection. The tropical atmosphere is never far from a state of radiative convective equilibrium (RCE), and RCE cloud-resolving model (CRM) studies have been successfully used in a number of experiments focusing on the feedbacks between radiation, clouds water vapor and convection in the tropics. Numerous numerical simulations have thus been conducted using the Regional Atmospheric Modeling System (RAMS) within a RCE framework. RAMS is a sophisticated CRM model that allows for the prognosis of aerosol concentrations. Two-dimensional simulations have been performed using a grid that spans approximately 10,000 km in the zonal direction, with a horizontal grid spacing of 1km, variable grid spacing in the vertical, and periodic lateral boundary conditions. The model is initialized using the 00 GMT 5 December 1992 TOGA COARE sounding and convection is initiated by randomized perturbations to the potential temperature. The model is first run until radiative convective equilibrium is reached which takes approximately 50 days. Sensitivity tests are then conducted in which a layer of aerosol that can potentially serve as cloud condensation nuclei (CCN) and/or ice nuclei (IN), is introduced between 2 and 4 km AGL. The amount of aerosol available for activation is then progressively increased from one sensitivity experiment to the next. The experiments are then run for another 50 days. The use of such large-domain, high-resolution and temporally long simulations allows for the investigation of aerosol indirect forcing on the wide range of different cloud types and systems that develop under the variety of environments that evolve.

Initial results from these experiments demonstrate that enhanced CCN concentrations result in stronger updrafts that cover greater fractions of the domain; that the precipitation response to greater CCN concentrations is dependent on the storm type; that heavy rainfall producing systems become more intense but less in frequency; that the ratio of liquid water to ice mass increases with increased CCN concentrations; and that greater concentrations of CCN produce a decrease in the frequencies of the shallow mode, an increase in the frequency of the congestus mode and a weak increase in the deep convective mode. The impacts of aerosol on the relative contributions of the three modes to the liquid water path, ice water path and surface precipitation have also been examined, as well as the impact of enhanced aerosol concentrations on all the terms contributing to the water vapor budget. These results will also be presented.