Research: Mesoscale convective vortices (MCVs) and their relationship to precipitation

The latent heating in a large complex of deep moist convection will often produce a cyclonic vortex at midlevels---generally known as a mesoscale convective vortex (MCV). If the conditions are right, these vortices can then go on to initiate additional convection the next day. As a result, they represent a complex forecast challenge: for a numerical model forecast to make an accurate "day 2" forecast, it must make a correct prediction of the location and timing of the convection on day 1 (which is itself difficult); it must represent the vertical structure of latent heating that leads to the development of the MCV; it must correctly capture the evolution of the vortex; and it must determine whether the lifting associated with that vortex will initiate convection again.

In some cases, the "day 2" convection that develops moves slowly and produces excessive amounts of rain. Numerous factors contribute to the favorable environment for heavy rain near MCVs: the atmosphere is usually very moist, which itself leads to efficient precipitation; the vortex circulation provides lifting and destabilization on the downshear side; the relative humidity is high, so there is little evaporation of rain; the lack of evaporation inhibits the development of strong cold pools (which tend to make convection move faster); and the winds are usually weak in the midtroposphere, which means that everything moves relatively slowly. In particular, when a strong low-level jet intersects the vortex, there is enhanced lifting, a source of moisture, and increased convergence, which can lead to quasi-stationary, heavy-rain-producing MCSs.

These processes have been summarized, through the analysis of observations and numerical simulations, in the figures below. The first is a 3-D visualization of an extreme-rain-producing MCS that occurred on 6--7 May 2000 in Missouri. The second is a schematic diagram based on composite analysis of several MCV-related heavy rain events.

Three-dimensional picture highlighting the primary processes at work in the 6--7 May 2000 extreme-rain-producing MCS. In this figure, the viewer is looking at the system from the south-southeast. The MCV and its associated potential vorticity anomaly is shown by "+PV;" the curved arrows indicate the associated midlevel circulation. The orange isosurface at the bottom represents the 297 K isentropic surface; the low-level gravity wave activity can be seen under the convective system. The colored "ribbons" represent parcel trajectories originating at different levels: yellow at 0.4 km AMSL; blue at 0.8 km; green at 1.5 km; red at 2.1 km. The trajectories in yellow and blue show near-surface air that generally rises over the gravity waves but passes underneath the deep convection, whereas the parcels shown in blue and red are approaching from the southwest and rise in deep updrafts. `LOW signifies the surface mesolow.

Schematic diagrams showing important processes in the development and maintenance of extreme-rain-producing convective systems associated with midlevel circulations. (a) Plan view. A schematic representation of the radar reflectivity structure of an MCS is shown in color, in relation to the location of a midlevel vorticity maximum (dark gray shading and curved arrows). The thick dashed curve indicates the flow in the upper troposphere (e.g., 250 hPa). Thick black arrows show the location of an LLJ, and the light gray shading shows the location of high-θe air at low levels (e.g., 925--800 hPa). (b) Southwest-to-northeast cross section. Representative isentropes (every 5 K) are shown by the thin black lines; the wind profile (including LLJ) is shown by the vectors on the left. A reference vector and length scale are shown at bottom. Green shading indicates areas with relative humidity >90%; gray shading indicates high values of absolute cyclonic vorticity. The thick dashed arrow shows air approaching the circulation from the southwest, which is undergoing isentropic upglide and destabilization.

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