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Cloud-Resolving Model Simulations and a Simple Model of an Idealized Walker Cell

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  • 1 Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts
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Abstract

An idealized Walker cell with prescribed sea surface temperature (SST) and prescribed radiative cooling is studied using both a two-dimensional cloud-resolving model (CRM) and a simple conceptual model. In the CRM, for the same SST distribution, the width of the warm pool (area of strong precipitation) varies systematically with the magnitude of the radiative cooling, narrowing as radiative cooling is increased. The simple model is constructed to interpret these behaviors. Key aspects of the simple model include a surface wind determined from the boundary layer momentum budget, which in turn sets evaporation assuming a spatially uniform surface relative humidity, prescribed gross moist and dry stratification as a function of column water vapor and precipitation, and a gustiness enhancement on evaporation in areas of precipitation. It is found that the gustiness enhancement, likely due to mesoscale systems, creates a feedback that narrows the warm pool. This process has not been included in previous formulations of the simple model and its role is emphasized here.

Corresponding author address: Jonathan Wofsy, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138. E-mail: jwofsy@post.harvard.edu

Abstract

An idealized Walker cell with prescribed sea surface temperature (SST) and prescribed radiative cooling is studied using both a two-dimensional cloud-resolving model (CRM) and a simple conceptual model. In the CRM, for the same SST distribution, the width of the warm pool (area of strong precipitation) varies systematically with the magnitude of the radiative cooling, narrowing as radiative cooling is increased. The simple model is constructed to interpret these behaviors. Key aspects of the simple model include a surface wind determined from the boundary layer momentum budget, which in turn sets evaporation assuming a spatially uniform surface relative humidity, prescribed gross moist and dry stratification as a function of column water vapor and precipitation, and a gustiness enhancement on evaporation in areas of precipitation. It is found that the gustiness enhancement, likely due to mesoscale systems, creates a feedback that narrows the warm pool. This process has not been included in previous formulations of the simple model and its role is emphasized here.

Corresponding author address: Jonathan Wofsy, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138. E-mail: jwofsy@post.harvard.edu
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