Apr 25, 2004
thanks Jeff, I just got the latest on SPC MD#0771 that was just issued.

side question if this is relevant or not

whats an MCV versus and MCS/MCC. I know what an MCS is but im seeing MCV lately, esp last night around 2am. Im guessing its a cell out ahead of main line of convection that moves in different direction relative to the overall flow. Not really sure.

MOD: Split off from the 5/5 NOW thread...
An MCV is a mesocale convective vortex. I don't know a lot about the dynamics of MCVs (anyone wanna chime in?) but I think you can imagine them as a convectively-induced mesoscale low pressure system. Sometimes, the intense latent heating (along with other processes) w/in an MCS can lead to the development of a mesoscale low. Such features tend to develop several hours into an organized convective event, since, again IIRC, they derive some of their rotation energy from Coriolis effects (which we know, through scale analysis, doesn't usually directly affect convection given the limited spatial and temporal scales involved with individual thunderstorms).

An MCC is a subset of MCSs... I believe the formal requirements for what qualifies an MCC can be found if you search the AMS journals (one requirement involves the ellipticity of the cold cloud tops as seen in IR imagery, another involves the extent of very cold cloud-top temperatures, etc). Some MCSs are fast-moving (like a typical bow echo), but MCCs tend to move quite a bit slower (certainly not always, just a tendency).
As far as the MCV is concerned, it forms as a result of the rear-inflow jet, which induces a cyclonic vortex on the north side of the squall line/MCS and an anticyclonic vortex on the south side. Since the Coriolis parameter is greater on the south side, it tends to counteract the anticyclonic rotation and you are left with an inertially stable vortex on the north side which can persist for hours and trigger new convection downstream through vorticity advection processes.
From Davis and Weisman (1994):
The reason for the formation of MCVs has been speculated upon by Zhang and Fritsch (1987), Hertenstein and Schubert (1991), Raymond and Jiang (1990), and Raymond (1992, hereafter R92), among others. Zhang and Fritsch performed simulations using The Pennsylvania State University / National Center for Atmospheric Research mesoscale model (MM4) with parameterized convection and found a cyclonic vortex formed within the stratiform precipitation region between 1 and 5 km AGL. Their explanation centered on enhanced vortex stretching because of the diabatic heating, either by a preexisting disturbance of by circulations within their simulated MCS.
The MCV formation is favored by system-scale convergence in the presence of earth's rotation, as shown by S94. Without rotation, end vortices form at first through the interaction of diabatic cooling in downdrafts with the ambient shear. As the MCS matures, however, the strong vertical circulations and associated diabatic heating become more important. The primary contribution to the low-level end vortices is the generation of PV anomalies through the correlation of heating with easterly shear beneath the maximum front-to-rear flow. Evaporative cooling in the westerly shear beneath the descending rear-inflow jet complements this process but has a secondary importance.[/b]

Davis, C. A., and M. L. Weisman, 1994: Balanced dynamics of mesoscale vortices produced in simulated convective systems. J. Atmos. Sci., 51, 2005–2030.

The important thing it take away from this is that an MCV can last well beyond the MCS that produced it, and can significantly affect convective threats during the following days.