A Moist Conceptual Model for the Boundary Layer Structure and Radiatively Driven Shallow Circulations in the Trades

Ann Kristin Naumann Max Planck Institute for Meteorology, Hamburg, Germany

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Bjorn Stevens Max Planck Institute for Meteorology, Hamburg, Germany

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Cathy Hohenegger Max Planck Institute for Meteorology, Hamburg, Germany

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Abstract

A conceptual model is developed to analyze how radiative cooling and the effect of moisture and shallow convection modify the boundary layer (BL) structure and the strength of mesoscale shallow circulations. The moist BL allows for a convective mass flux to modify the BL mass balance, which enhances inversion entrainment compared to a dry case and acts as a moisture valve to the BL. The convective mass flux is found to be insensitive to the applied radiative cooling and in the absence of heterogeneities cloud-free conditions exist only for unusual large-scale forcings. The model is able to explain the moderate range of BL heights and humidities observed in the trades. In a two-column setup, differential radiative BL cooling causes a pressure difference, which drives a BL flow from the cold and moist column to the warm and dry column and couples them dynamically. The small inversion buoyancy jump of the moist BL yields a stronger BL flow of 4 m s−1 instead of 1 m s−1 in the dry case. For typical conditions of the subsidence-dominated tropical oceans, a radiatively driven shallow circulation is stronger than one driven by sea surface temperature (SST) gradients. While the strength of the SST-driven circulation decreases with decreasing SST difference, the radiatively driven circulation is insensitive to the radiative BL cooling difference. In both cases, convection is suppressed in the descending branch of the shallow circulation and enhanced in the ascending branch, resembling patterns of organized shallow convection.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Ann Kristin Naumann, ann-kristin.naumann@mpimet.mpg.de

Abstract

A conceptual model is developed to analyze how radiative cooling and the effect of moisture and shallow convection modify the boundary layer (BL) structure and the strength of mesoscale shallow circulations. The moist BL allows for a convective mass flux to modify the BL mass balance, which enhances inversion entrainment compared to a dry case and acts as a moisture valve to the BL. The convective mass flux is found to be insensitive to the applied radiative cooling and in the absence of heterogeneities cloud-free conditions exist only for unusual large-scale forcings. The model is able to explain the moderate range of BL heights and humidities observed in the trades. In a two-column setup, differential radiative BL cooling causes a pressure difference, which drives a BL flow from the cold and moist column to the warm and dry column and couples them dynamically. The small inversion buoyancy jump of the moist BL yields a stronger BL flow of 4 m s−1 instead of 1 m s−1 in the dry case. For typical conditions of the subsidence-dominated tropical oceans, a radiatively driven shallow circulation is stronger than one driven by sea surface temperature (SST) gradients. While the strength of the SST-driven circulation decreases with decreasing SST difference, the radiatively driven circulation is insensitive to the radiative BL cooling difference. In both cases, convection is suppressed in the descending branch of the shallow circulation and enhanced in the ascending branch, resembling patterns of organized shallow convection.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Ann Kristin Naumann, ann-kristin.naumann@mpimet.mpg.de
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