Special Symposium on Aerosol–Cloud–Climate Interactions


The impacts of aerosol indirect forcing on tropical convection

Susan C. van den Heever, Colorado State University, Fort Collins, CO

Tropical convective clouds appear to play an important role in the tropical climate. Recent observations indicate that they are distributed in a tri-modal manner with the cloud tops of the shallow convective mode reaching 2 km, those of the congestus mode reaching 6 km, and the deep convective mode extending to the tropopause. Shallow convection has been observed to serve as a source of moisture in the boundary layer and plays a role in maintaining the trades and its circulation, while the cumulus congestive mode appears to influence surface radiation and precipitation, and is important in moistening the middle troposphere. However, 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 forcing. Aerosols represent one such external forcing that may have a significant influence on the microphysical, dynamical and precipitation characteristics of tropical convection. 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 tropical convection.

One way to consider the important connection between convection and the energy budget of the Earth is through the concept of radiative convective equilibrium (RCE). The tropical atmosphere is never far from a state of 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. In order to achieve the goal of this research, numerous numerical simulations have 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. Both two- and three-dimensional simulations have been performed using a grid that spans approximately 10,000 km in the zonal direction, and with horizontal grid spacings ranging from 1 to 2.4 km. When the model is run in three dimensions, a channel configuration is utilized due to memory constraints. 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 lower boundary is a fixed ocean surface with a temperature of 300K, and the lateral boundaries are periodic. The model is first run until radiative convective equilibrium is reached. This 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) depending on environmental conditions, 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 impacts of the enhanced aerosol concentrations on numerous aspects of tropical convection will be presented including the tri-modal distribution of convection, the water budget of the tropics, the partitioning between the liquid water and ice species, and the impacts of aerosols on the updraft strength and development.

wrf recording  Recorded presentation

Session 2, Impact of aerosols on convection and weather
Tuesday, 13 January 2009, 11:00 AM-12:00 PM, Room 131B

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