Abstract
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.