Control of convective precipitation over warm tropical oceans

Recent work suggests that variations in convective inhibition

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.