control the frequency of moist convection over warm tropical
oceans. Convective inhibition is modulated by two distinct
mechanisms, variation in boundary layer moist entropy and
fluctuations in the intensity of the capping just above the
boundary layer. These two mechanisms lead to very different types
of convective systems. In convectively active regions, boundary
layer moist entropy is controlled primarily by a balance between
surface moist entropy fluxes and convective downdrafts, leading
to a dependence of this type of convection on strong surface
winds and high sea surface temperatures, both of which augment
boundary layer moist entropy. Rapidly moving waves such as
convectively coupled equatorial Kelvin waves most effectively
produce capping layer fluctuations and tend not to depend much on
surface entropy fluxes.
Additional evidence shows that the saturation fraction or column
relative humidity of the troposphere produces a strong modulating
effect on the precipitation produced by moist convection. Both
observations and cloud-resolving numerical models show a very
steep dependence of precipitation rate on saturation fraction.
Cloud-resolving models also show that the typical virtual
temperature profile in convectively active regions of the
tropics, with cooling at low levels and warming at upper levels,
results in greater precipitation than in undisturbed regions.
Tropospheric saturation fraction is controlled by a mixture of
large-scale and convective processes. The convection itself can
either promote or discourage future convection in the same
location by its effect on the moist entropy budget; if lateral
flows in and out of the convective region tend to increase the
moist entropy in the convective region, then the saturation
fraction increases and future precipitation is augmented.
Conversely if these flows tend to decrease the moist entropy,
then precipitation is reduced and precipitation is self-limiting.
The column-integrated entropy tendency is intimately related to
the concept of gross moist stability. The sign and magnitude of
the gross moist stability in different situations is currently an
interesting area of research.