HR: 0800h
AN: A51A-0010    [Abstracts]
TI: A Simple Model of Subtropical Stratocumulus Response to Changes in Subtropical and Tropical Sea Surface Temperature
AU: * Caldwell, P M
EM: caldwep@atmos.washington.edu
AF: University of Washington Department of Atmospheric Sciences, Box 351640, Seattle, WA 98195-1640
AU: Bretherton, C S
EM: breth@atmos.washington.edu
AF: University of Washington Department of Atmospheric Sciences, Box 351640, Seattle, WA 98195-1640
AU: Wood, R
EM: robwood@atmos.washington.edu
AF: University of Washington Department of Atmospheric Sciences, Box 351640, Seattle, WA 98195-1640
AB: Despite widespread recognition of the importance of subtropical stratocumulus clouds to the planetary energy budget, the strength and even the sign of stratocumulus response to increased levels of greenhouse gases is still unknown. This is due both to uncertainties in the relevant boundary layer processes that maintain the cloud and uncertainties in how the large-scale temperature, humidity and subsidence profiles and SST will change in these regions. Subtropical stratocumulus form over cold SSTs in the subsiding branch of the Hadley circulation. The above-boundary layer temperature profile in these regions is strongly controlled by tropical deep convection in the Intertropical Convergence zone (ITCZ), so accurate prediction of changes in these clouds is predicated on accurate portrayal of changes to the ITCZ and to the Hadley circulation. In this study, we apply a weak temperature-gradient assumption and an above boundary-layer radiation-advection energy balance (with radiative fluxes computed by a 2-stream model) to derive above-boundary-layer thermodynamic and subsidence profiles for the Southeast Pacific stratocumulus region based on sea surface temperature (SST) in the ITCZ. By coupling these forcings to a cloud model it is possible to create a framework for predicting stratocumulus properties based on the SST in the ITCZ and in the stratocumulus region. One strength of this framework is its flexibility; in our study we compare results obtained with a mixed layer model, a single column model, and a large eddy simulation. Mixed layer model results suggest that the choice of entrainment parameterization has a large impact on the resulting mean state, but little influence on how the mean state responds to increasing SST. Because the validity of the mixed layer model depends on the mean state, this result suggests that the specifics of the entrainment parameterization determine the range of applicability of the model, but not conclusions regarding stratocumulus climate response within the domain of validity. Examining the surface energy budget under the assumptions that oceanic heat transport remain constant and heat storage is negligible suggests that the stratus region SSTs should warm roughly half as much as the ITCZ SSTs and that shortwave cloud forcing should remain constant as the planet heats up. This second finding is important because it suggests that changes in stratocumulus cloud properties may not be a significant feedback to global warming.
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3307 Boundary layer processes
DE: 3310 Clouds and cloud feedbacks
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3374 Tropical meteorology
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005