Mesoscale Convective Vortices Talking Points



Mesoscale Convective Vortices Talking Points

1. Title Screen. Note the typical presentation of the MCV on this visible image. This early morning shot is typical of the structure during this time of day. A cyclonic swirl in mid-level clouds, partially obscured by higher-level cirrus. Sometimes an anticyclonic swirl in the cirrus is obvious. The overlay describes typical soundings in relation to the MCV.

2. Nomenclature. MCVs have been identified for 20+ years, and it took a while to settle on a name. You may also see them referenced as MVCs, for Mesocale Vorticity Centers. MCV is preferred because of the Convective element. Because Ned Johnston was the first to ID the phenomenon from satellite imagery, Neddy or Neddy Eddy is sometimes used.

3. Why do we care about MCVs? Because they screw up a forecast. If the environment allows it, an MCS can spawn an MCV. Should that happen, instead of decaying cirrus debris, you have regenerating convection or stratiform rainfall over your forecast area. Models can have a very difficult time in accurately predicting MCV onset. That's not to say they don't predict them -- there is a False Alarm Rate with the prediction. Always remember that these are convective -- how well they are simulated depends on the accuracy of the model parameterization of convection. Recognizing when conditions are favorable for development and being vigilant can help you out-forecast the forecast models. This is when the human element becomes crucial -- You can become aware of when the models are going awry before the models do, and react. Model must contend with inertia.

4. Teletraining objectives: Show some examples to refamiliarize you with satellite presentation of MCVs. Discuss the lifecycle of MCVs, and what they need to form (small values of mid-level shear, lots of moisture/instability), persist (ongoing convection), or decay (high shear, little convection). Relate 'runaway convection' in a model to MCVs. Give you hints on how to decide when if MCV will or will not form when a model does or does not forecast one. Show some examples of how the presence of MCVs can effect weather.

5. Basic lifecycle points of an MCV. Stratiform rain region (i.e., latent heating) supports the development of a potential vorticity anomaly/ cyclonic spin-up. PV anomaly forms in a region of enhanced static stability. Or, from the QG height tendency perspective: local heating gives height falls below, height rises above. If mid-levels warmth is maintained, rather than allowed to radiate away in the form of gravity waves, the MCV can persist. Convection maintains the MCV, shear erodes it. There's a balance between the two. MCV vorticity advection -- or if you want to consider it this way, flow up over the surface cold dome (warm advection) -- can re-trigger convection to help maintain the MCV structure. Vorticity advection gives the extra lift that might help start the supporting convection.

6. How might a moving MCV induce rising motion, leading to sustaining convection? Cross-sectional view of propagating MCV. Mid-level potential vorticity anomaly is linked to surface cold dome. As dome moves, moist air is forced up. May or may not be convection. Clouds on up-shear side, clearing on down-shear side.

7. Spin (vorticity) in center of MCV might also originate from strong vorticity generation at the end of convective lines. That generated convection can then propagate along the lines towards the center of the MCV, where it accumulates / spins. This is the Line-End Vorticity Plume.

8. Basic needs for MCV persistence: Access to the warmest most moist air. If the MCV is heading towards cooler air, it probably will die. Too much shear? Vortex will be torn apart -- analogous to a hurricane, although the ultimate energy source is much different. Recall, however, that any individual sounding may be contaminated by convection and have considerable shear. The low-shear requirement is at meso-alpha or meso-beta scale, not at convective scales.

9. x-z schematic of MCV development. A region of mid-level diabatic heating is accompanied by upper level height rises/lower level height falls (Think Q-G height tendency equation). Similarly, the increase in stability at mid levels will increase the potential vorticity, inducing a cyclonic spin. Height rises/falls lead to divergent/convergent flow. If the mid-level diabatic heating relaxes, then the divergence/convergence will lead the atmosphere back to balance as it returns to its (approximate) pre-convective state. Energy from latent heating has propagated away as gravity waves, or radiated away.

10. Sometimes, the low-level convergence can lead to more convection, more diabatic heating, which invigorates the PV anomaly/upper level divergence/low-level convergence.

11. This is a GOES-EAST loop of band 4 (11 microns), unenhanced IR imagery. Convection over Nebraska evolves into an MCV that is visible in the imagery by ~0900 UTC. As the MCV tracks into Iowa, convection continues to develop during a time of day when it might not be expected (14-15 UTC). This particular MCV did not regenerate the following night. Such regeneration is rare. Steady translation can be noted in this loop -- simple translation is a good predictor of when it will clear out.

12. Here is a visible image loop of the same MCV. If you're making a forecast out of Davenport, the implications of the MCV development are obvious. Rather than decay and debris, you have continued rains/clouds/convection. Typical soundings in the MCV environment include moist stable in the cool rain region, slight inversions behind the MCV, and convective instability in the inflow.

13. Past studies suggest the RUC model has difficulty with MCV onset -- probably related to the convective parameterization. Indeed, all models find MCV formation problematic. Always remember that if a model does not predict an MCV, and one forms, there has to be plenty of data to convince the model that the model is actually wrong. MCVs are scaled such that they can easily slip between radiosondes and be poorly resolved. If there is an MCV, and a model has not predicted one, look very carefully at the initial fields to ascertain whether or not the MCV is well initialized. (This is true to some extent if the model does forecast the MCV, too, of course). If you are alert, this is an area where you can really out-do the model.

14. One of the clues to MCV formation: have they formed already, earlier, in the airmass? This doesn't help with the first one to form, but because they tend to form in clusters (i.e., if there's one today, chances are increased that one will form tomorrow), it can help at times. MCVs are easy to see in satellite loops. They can be harder to see in single images.

15. For example, look at this one image. There is an MCV present, but it's hard to see because it's a swirl of mid-level clouds over central Tennessee, northwest Alabama and northeast Mississippi. The monster MCS over eastern Oklahoma may include an MCV, but you can't see it in this image.

16. Enhancements don't always help you see MCVs. In this example, my eye is still drawn to the MCS over Oklahoma to the detriment of the MCS to the east. I played around with different enhancements, and the results were always similar.

17. This is a loop of band 4 IR imagery from GOES EAST. Note that the MCV originally over TN translates to the east and as it does, convection forms along its southern flank. It is interesting to speculate that the MCV would have persisted longer had it moved towards the higher theta-e air to the south, the presence of which is assumed because that's where the convection forms. There also appears to be a weaker MCV moving out of the massive MCS over Oklahoma -- there is a cyclonic swirl over northeast Arkansas around 2100 UTC on 30 June that apparently spawns convection over western Kentucky and Tennessee as it tracks over. The MCV over Alabama loses its identity over the Smoky Mountains -- terrain features may have disrupted its circulation, or perhaps there is a deformation zone there -- deformation knocks the stuffing out of MCVs.

18. MCVs form in regions of very low mid-level vertical wind shear. This is analagous to the low-shear environment that supports hurricane formation. In the range of low shear, relatively high values support secondary convective development.

19. One thing common to MCV environments: very unstable. LI ................
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