The Relationship between Intense Convection and Off-Equatorial, Upper Tropospheric Temperature Maxima

Well-known theories for the convectively coupled, large-scale dynamics of the tropical atmosphere predict the relationship between precipitation, tropospheric temperature, and near-surface thermodynamics. Forms of convective quasi-equilibrium assert that tropospheric temperature is tied to boundary layer properties (temperature and humidity, as combined into equivalent potential temperature, θe) by convection. When coupled to a set of momentum equations, this leads to a closed system that can be used to describe the locations of precipitation and free-tropospheric temperature, given a distribution of surface sensible and latent heat fluxes. One well-known solution has a local maximum in free-tropospheric temperature and near-surface θe positioned poleward and westward of a prescribed, off-equatorial maximum of surface heat fluxes.

Yet classic convective quasi-equilibrium theories seem incomplete given that free-tropospheric moisture has been found to greatly influence moist convection, that convective motions are known to be deeper and more intense over land than ocean, and that moist convection is observed to organize in time and space to form meso- and large-scale structures. Detailed observations of the climatology of tropical precipitation underscore this complexity: the regions of highest rainfall accumulation are sites of very frequent rainfall that occurs in organized convective systems having large stratiform components. In contrast, the most intense convection typically happens in land regions with relative little accumulation. For example, in South East Asia, most monsoon precipitation falls on the Bay of Bengal and on the coastal mountains of Myanmar, but the strongest convection is about 1,000 km to the northwest near the western edge of the Himalayas.

This work explores the climatological relationship between rainfall frequency, convective intensity, surface θe and upper-tropospheric temperature in observations and in a hierarchy of model simulations. It focuses on monsoon regions, where satellite observations are used to show that maxima of upper-tropospheric temperature lie off the equator in continental locations where moist convection tends to occur in intense, sporadic bursts of isolated cumulus cells. Comparisons are made with simple, convective quasi-equilibrium models of the response to off-equatorial heat sources, and with numerical models having detailed treatments of moist convection.