|HABYTIME MINI-LECTURE 79:|
CONVECTIVE EROSION OF CAP
METEOROLOGIST JEFF HABY
This writing addresses one process that can erode a cap and that is the process of solar heating of the boundary layer. In the diagram below
the boundary layer is the “unstable layer” labeled at the surface. This layer is made unstable by solar energy warming the ground which
is turn warms the air above it creating a significant temperature decrease with height within the boundary layer. Within this unstable
layer, air will convectively rise. With enough moisture present, fair weather cumulus will develop. In this diagram, fair weather cumulus
are forming instead of storm producing cumulus since there is a capping layer above the boundary layer.
The capping layer is a layer of warmer and thus stable air aloft that prevents the penetration of convective elements through it. When the
cumulus enters the capping layer, its vertical uplift slows and ceases within the capping layer. A capping layer can make the differences
between fair weather and stormy weather. As convective elements try to penetrate the capping layer one after another it will cause the
capping layer to weaken. If the capping layer can be weakened enough then it will eventually break.
Convective elements that try to penetrate the capping layer result in the capping layer weakening. One reason this occurs is because moisture
from the clouds is introduced into the capping layer. The evaporation of cloud droplets within the capping layer results in a cooling
of the capping layer. Another reason is because some of the air from the boundary layer is mixed with the capping layer air. Thus the
capping layer is losing mass due to getting mixed out with the unstable boundary layer below it. This process is typically most
evident in the afternoon when solar heating is at a maximum and convective elements are bombarding the capping layer. When a cap
breaks it will often occur in the afternoon when cumulative solar heating and convective motions are maximized.
Once a convective element is able to penetrate the capping layer then it can rise through the unstable air that is above the capping layer. This
results in deep convective storms. If the cap is very strong, then solar heating and convective motions in the boundary layer may not be
able to break it. This situation can result in a “bust” in convective storms if it was previously expected that the cap would break in the afternoon.