100 Years of Research on Mesoscale Convective Systems

Recognition that convective clouds can have “mesoscale” dimensions, that is contiguous horizontal dimensions of 100s of kilometers, has changed the way convective clouds are understood both in weather forecasting and in the context of the role of convective clouds in the general circulation of the atmosphere. Prior to the mid 20^th Century, there were some hints of the existence of convection occurring in storms of mesoscale dimensions. During WWII military forecasters in sub-Saharan West Africa recognized and described mesoscale squall lines in the tropics. After the invention of radar during WWII and the launch of weather satellites in the 1960s, mesoscale convective systems (MCSs) became much better understood. The Global Atmospheric Research Programme’s Atlantic Tropical Experiment (GATE) in 1974 and the development of the WSR-88D radar network in the U.S. in the 1980s led to further understanding of MCSs and shown the variety of MCS behaviors, with some propagating as squall lines and others not. Satellites with radars on board (TRMM, CloudSat, and GPM) have shown the global reach and regional distribution of convection organized on the mesoscale. A series of field experiments in the tropics and midlatitudes over the subsequent decades of the 1990s, 2000s, and 2010s have continued to add to the knowledge base of both of the internal dynamics and microphysics of MCSs and their interactions with larger scales of motion. The field experiments have shown that the precipitation in MCSs, whether of the squall line or non-squall type, is subdivided into convective and stratiform components and that the air motions and microphysics in the convective and stratiform regions are distinct. The stratiform regions are marked by evaporation and cooling below the mid-troposphere and heating in the upper troposphere. When combined with the convective-region heating profile, the heating profile of an MCS is top heavy, and the greater the proportion of stratiform rain, the more top heavy the heating profile. Because potential vorticity generation is proportional to the vertical gradient of heating, the reaction of the large-scale circulation to convective heating convection occurring over a region of Earth is influenced by the extent to which the convection is in the form of MCSs. The convectively disturbed regions of tropical synoptic and planetary scale circulations, such as the Madden-Julian Oscillation, contain MCSs that affect their structures. In midlatitudes, larger and more long-lived MCSs can spin up midlevel quasi-balanced cyclonic circulations that lead to that can lengthen the lifetimes of MCSs and strengthen synoptic-scale troughs. The genesis stages of tropical cyclones are often marked by MCSs that subsequently amalgamate into balanced cyclonic storms. Current research problems include how aerosol may or may not influence MCSs and why lightning occurrence varies between some MCSs and others. This talk will chronicle some of these key developments in the story of the discovery of mesoscale organization of convective storms.