92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012: 9:15 AM
A System-Wide Response of Tropical Convection to Aerosol Indirect Forcing
Room 244 (New Orleans Convention Center )
Susan C. van den Heever, Colorado State University, Fort Collins, CO

It has been demonstrated by a number of different studies that the impacts of aerosol indirect forcing on cloud and microphysical properties may vary both in magnitude and in sign depending on the cloud regime and the environmental characteristics. Thus, while aerosol indirect effects may be large on local spatial and temporal scales, the total domain-wide forcing may be significantly reduced when considering a cloud population consisting of numerous cloud types developing under a variety of environmental conditions. Many of the previous modeling studies conducted in order to examine aerosol indirect forcing on cloud processes have focused specifically on one cloud type or on the response under a specific set of conditions. While this allows for the assessment of the local effects of aerosol forcing, it makes evaluating the system-wide response difficult to evaluate. The goal of the research presented here is to further examine the response of tropical convective storms to aerosol indirect forcing on both a local and a system-wide scale.

As the tropical atmosphere is never far from a state of radiative convective equilibrium (RCE), previous cloud resolving modeling (CRM) studies using such a framework have proven highly successful in experiments focusing on the feedbacks between radiation, clouds, water vapor and convection in the tropics. For the research presented here, large-domain, high-resolution, long-duration three-dimensional CRM simulations have been conducted using the Regional Atmospheric Modeling System (RAMS) under such a RCE framework. The three-dimensional setup allows for a more accurate representation of mesoscale convective systems than is provided by a two-dimensional framework. Such systems play a key role in tropical convective processes. RAMS is a sophisticated CRM that includes the prognosis of aerosol concentrations. The model grid used for these experiments utilizes a horizontal grid spacing of 1km, variable grid spacing in the vertical, and periodic lateral boundary conditions. The lower boundary is an oceanic boundary with a fixed sea surface temperature.

The CRM is first run until RCE is reached, a process that takes approximately 50 simulation days. The model is then restarted at day 50 and aerosols that can potentially serve as cloud condensation nuclei (CCN) are introduced in one half of the domain. No aerosols are introduced into the other half of the domain. The amount of aerosol available for activation is progressively increased from one sensitivity experiment to the next (still within only half of the domain) thus representing a range of aerosol conditions from pristine to polluted atmospheres. The sensitivity tests are run for a further 50 days. The design of these experiments allows us to examine aerosol indirect forcing on the large-scale circulations that develop between the clean and polluted portions of the domain, as well as for side-by side comparisons of the cloud and microphysical characteristics of the tropical convective systems developing in the clean and polluted environments.

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