A Review - Mesoscale Convective Systems

Mesoscale Convective Systems, or convection of mesoscale proportions accompanied by stratiform precipitation account for a large fraction of the Earth’s precipitation. MCSs, especially in the middle latitudes, often bring high impact weather events including heavy rain, damaging winds, frequent lightning and occasional tornadoes. Therefore they present serious challenges for short term weather forecasts. MCS problems in the tropics include heavy rain production, impacts on the large scale due through heating profiles and impacts on the upper ocean through wind and rainfall.

One of the dominant MCS patterns is the leading squall line with trailing stratiform precipitation. The dynamical patterns of this archetype have been described from theory and conformed by observations, containing a jump updraft, overturning downdraft and the overturning updraft. Extensive rear inflow of mesocale proportions invades the convective and reinforces the overturning downdraft. The trailing stratiform is marked by a sharp radar “bright band” induced my melting ice particles. The stratiform rain regions is driven by ice particles detrained from the convective line, growing by mainly deposition in a mesoscale updraft. Melting and evaporative cooling drive a mesoscale downdraft. Many interesting surface features have been documented including a meso-high and wake low. Lightning patterns in MCS are also of interest, including the production of sprites and other types of high altitude lightning caused by enormous charge reservoirs contained within the stratiform region. Important deviations from the so called linear MCS include the asymmetric MCS, which often contains a pronounced mid-level mesovortex, and the mesoscale “bow echo” or derecho, accompanied by strong, damaging winds.

MCSs in both the tropics and middle latitudes are still under intense study based on satellite and ground based radar observations. The talk will conclude with some recent observations of MCSs and a brief discussion of convective and stratiform rain fractions. Classical papers on middle latitude and tropical MCSs from Prof. Robert A. Houze Jr. will be highlighted throughout the presentation.