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On the controls of vertical motion top-heaviness
  • Miguel Bernardez,
  • Larissa Back
Miguel Bernardez
University of Wisconsin-Madison

Corresponding Author:bernardez@wisc.edu

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Larissa Back
University of Wisconsin–Madison
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Understanding what controls vertical motion profile shape is fundamental to understanding tropical precipitation patterns. There are two controls that have been studied previously: the thermodynamic profiles of the environment and the dynamics imposed by sea surface temperature (SST) patterns. To fit these two perspectives together, we focus on two regions with distinctly top and bottom-heavy vertical motion: The Western Pacific and the Central Eastern Pacific. These regions have roughly the same column-integrated water, precipitation, and column-integrated horizontal moisture advection, however the shape in the West is top-heavy while the East is bottom-heavy. The top-heaviness angle is introduced to describe this difference. To study thermodynamic controls on vertical motion profile shape, we use weak temperature gradient (WTG) simulations. We are able to simulate the shape differences between our two regions from the thermodynamics. We then show that the dry static stability and the underlying SST are the most important for the vertical motion shape differences between our two regions. We then show that the qualitative shape differences can be explained using a simple entraining plume model. The entraining plume model accepts the temperature and moisture profiles as inputs and outputs the plumeâ\euro™s buoyancy, which is directly related to the vertical motion profile shape. We find that increasing the dry static stability leads to bottom-heaviness. We hypothesize that the SST gradients lead to an equilibrium temperature that is cooler than an identical atmosphere with no gradient. The cooler boundary layer leads to a thermodynamic environment that is more conducive to bottom-heaviness.