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 for different regions of 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, QG height tendency: 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.
  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 theory.
  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.
  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.
  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. Because MCVs are convective, the convective parameterization of a model helps determine whether a model will forecast MCV development. Studies of past years (1999,2000) suggest the RUC has difficulties. Other models have problems as well. Remember that the 3dvar takes as a first guess the previous forecast; if a model does not predict MCV development, then the first guess field can be significantly wrong. In such a case, there must be plenty of data to resolve the MCV, which can easily slip between stations in the radiosonde network. If you are alert, this is an area where you can 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 dying MCV (is this a weak one?) 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<0, even in morning soundings (which explains the pre-Noon convection over Iowa on page 10). Normally, a mesolow at midlevels, and a meso-high at the surface, which high is associated with cooling. You can also have meso-lows at the surface. The pressure distribution is strongly affected by cloud microphysics. Surface heating also plays a role -- mesolow at surface much more likely if a boundary layer that is warmed.
  20. Some statistics. MCVs can be detected in satellite imagery, or by vortex-finding algorithms in numerical models, the former is more accurate. Up to 40 per year in RUC vortex finding algorithms.
  21. Some references. Fritsch et al. give good background info -- Davis/Trier are more recent contributors (Field experiment last Summer: BAMEX: Bow echo And MCV Experiment. St. Louis has a web page on it under science).
  22. Given that a numerical model can have difficulties regarding MCV genesis, if one does appear and you must forecast against the MCV-less model, you have to consider what the MCV adds to the environment: extra vorticity in mid levels especially, a potential moving cold-dome at the surface. More cloudiness and stability changes. Alterations in the moisture distribution induced by the circulation around the MCV.
  23. Some MCVs are well-predicted. This is especially so if the systems that generate them are well predicted. Maybe more common if MCV does not originate from an MCS.
  24. RADAR structure that looks MCV-like -- nice compact cyclonic swirl that occurs at the right time of day (shortly after sunrise). Twin radar views, one from Sullivan (KMKX), one from LaCrosse (KARX)
  25. However, this is a frontal wave. Definitely associated with an airmass change.
  26. Strong mid-level forcing (700-mb vorticity advection). There is thus strong mid-level shear, which further argues against MCV formation.
  27. Forecast problem: MCV formation on July 6th? (ironically, the day after the last day of BAMEX!) This was a case of conflicting model guidance. Up through the models starting at 1200 UTC on 5 July, AVN and ETA models tracked convection eastward along a boundary south of Wisconsin. Starting with the 1800 UTC AVN, however, the AVH consistently developed a strong convective system and tracked it over Wisconsin, while the ETA continued to insist the convection would primarily remain south of Wisconsin.
  28. Some forecast aids: water vapor imagery from MODIS shows a very strong shear zone over Idaho, and the resultant Kelvin-Helmholtzish features that grew on the zone. Image from 2025 UTC on 4 July 2003.
  29. The features are apparent in GOES-EAST and GOES-WEST as well. And the shear zone is associated with strong convection over the Northern Plains late on the 4th. Note that this shear zone is in close proximity to convection that occurs over South Dakota late on the 3rd and late on the 4th -- there is a vertical reach to this shear zone.
  30. 250-mb winds show the shear zone tracking across the Plains to the western Great Lakes by 1800 UTC 5 July. At the same time, a jet entrance zone is present.
  31. Low level moisture in abundance. Axis from northern Nebraska to central Illinois (the 15 C isodrosotherm @ 850 mb).
  32. RUC-40 500mb heights. Are there any pronounced shortwaves? Difficult to tell.
  33. Total precipitation generated from the ETA initialized at 1200 UTC 5 July. All of the precipitation is convectively generated. Note the convective generation of a vorticity maximum in north-central Iowa by 12z on 6 July.
  34. Total precipitation generated from the AVN initialized at 1200 UTC 5 July. The initial vorticity fields in this run show different amplitude from the ETA (especially the region of inertial neutrality over the Missouri River between Iowa and Nebraska). Convection forms over Iowa, but does not track into Wisconsin until very late in the day on the 6th.
  35. Total precipitation generated from the AVN initialized at 1800 UTC 5 July. The big changed between this loop and frame 33 is the generation of an MCV-like feature that moves out of Iowa at 1200 UTC 6 July into Wisconsin, accompanied by copious rains.
  36. Total precipitation generated from the ETA initialized at 0000 UTC 6 July. The ETA stubbornly continues to predict that convection remains south of Wisconsin.
  37. Total precipitation generated from the AVN initialized at 0000 UTC 6 July. A markedly different forecast, especially over Iowa. The AVN tracks an MCS across Iowa during the night on the 5th/6th and takes the MCV into Wisconsin the following day.
  38. ETA model starts to catch on. Forecast from 1200 UTC 6 July, and ETA now has precipitation falling over southern Wisconsin.
  39. How can you tell which model to believe? This slide compares AVN/RUC/ETA models using initial and early model divergence at 250 mb over the central Plains. Note first how the convection arcs from South Dakota to Indiana. For a model to reasonably predict the fields, the initial model divergence fields that support that convection should at least somewhat mimic the arc. Initial divergence fields may change quickly in a model that is poorly initialized (so maybe they shouldn't be used alone to determine whether a model starts out well); however, it is more likely for a model to mimic reality if the initial fields reflect observations than if the initial fields are different from reality -- you cannot expect a model that does not reflect reality to somehow move back towards observations. A more robust tactic is to see how the early evolution of the model matches observations. This slide shows initial and 06h forecasts of divergence. At 0000 UTC, The RUC divergence is strongest over the coldest tops, but parts of the fields don't match up with the IR imagery. In contrast, the AVN divergence stretches along the convection in a very believable way. The ETA model has divergence in approximately the right region/orientation, but the largest values do not overlay convection. Given these initial fields, it's not hard to suppose that the model evolution in the AVN will more closely match reality than the RUC. Indeed, of the 3 6-h forecasts, the AVN is most closely matching the strong convection evident in the satellite image at 0615UTC. The ETA model moves its initial divergence steadily east-southeastward, away from the developing convection. The RUC model had strong divergence upstream of the large MCS over western Iowa. Only the AVN has very strong divergence overlapping the MCS. The early evolution of the model runs should help you select the "right" model to believe when making a forecast for 6 July over WI. As you look at these fields, remember also that the forecasts will be used for initial fields in the 3d-var that initializes the subsequent model runs. Unless there are sufficient observations (which is difficult, as the MCV can fall between the cracks of the RAOB observational network), it's possible that subsequent initial model fields will underestimate the strength/position of the MCV, especially in the RUC model/eta model. The RUC cycle benefits from more rapid updates, but it still may take several cycles before the MCV is entirely and correctly integrated into the initial fields.
  40. Enhanced water vapor loop from 1215 UTC 5 July - 2015 UTC 6 July. A couple things to note. The shear zone that was evident in the MODIS water vapor imagery persists on 5 July. A shortwave is apparent in the WV imagery, and as that shortwave approaches the Black Hills of South Dakota, convection initiates. The convection moves southeastward towards western Iowa, then it turns to the east and east-northeast.
  41. AWIPS Screen grabs of 11 micron IR data with METARs overlain. Note how the convection that forms the MCV moves towards the highest dewpoints.
  42. This is the tail end of the MCV -- this part of the loop is included to highlight the development of convection over Michigan/Indiana associated with the MCV circulation.
  43. Note how the MCV and shear patterns evolve so that the MCV ends up in the region of lowest shear. This forecast data is from the eta model -- which did not do a good job of predicting MCV onset. However, the shear values are very similar to the shear values in the AVN, which did. Again, this underscores the importance of the convective parameterization in developing MCVs in a model.
  44. The deleterious effect on forecast high temperature from the MCV at Madison and at Green Bay.
  45. Precipitation totals for 6 July 2003. Precipitation was widespread over much of the southern 2/3rds of the state. Several totals exceeding 1" over the southern counties.
  46. Forecast 500-mb heights/precipitation valid at 1800 UTC 6 July from the 0600 UTC, 0900 UTC and 1200 UTC RUC runs. Note the development of precipitation and the troughiness that develops, consistent with MCV mid-level spin. In fact, the RUC model tends to have the precipitation associated with the MCV linger too long in Iowa. A better forecast tool here would be simple translation of the MCV.
  47. RUC forecast summary. Note that the RUC has been shown to have difficulties predicting MCV onset (papers by Davis and Trier). Remember: it can take several forecast cycles for a model to finally accept that an MCV is well and truly present. There must be sufficient numbers of observations to resolve the MCV and convince the 3d-var that the initial model fields have gone astray and that they should be significantly altered by the (correct) observations.
  48. Summary of 6 July Case. System that may have forced MCS visible in WV imagery 2 days earlier; classic presentation on satellite imagery, and classic effect on forecast temperatures; upper-level divergence could be used to gauge model initializations; MCV forced downstream convective development (although that development did not then evolve into another MCV).
  49. Next two cases include unusual weather associated with MCVs. MCVs are associated with "extra" moisture and "different" shear. Also, the circulation associated with an MCV can move moisture.
  50. MCV associated with a Gulf Surge in the Gulf of California (the Gulf Surge is associated with a Pacific Hurricane). How will this MCV affect the weather? Can it be used to help forecast?
  51. Consider using the approaching MCV as a marker for a more moist environment (and one with less shear -- otherwise the MCV would decay). As it approaches during days to be shown in the loop, note how the diurnal convection over the southwestern US increases in coverage. As the MCV moves up the coast, the atmosphere is moistening. Both AVN and ETA (and sounder!) raise RHs and precipitable water.
  52. The region is under a ridge, with a weakness in the ridge off the Pacific coast of Baja.
  53. AVN forecasts for 1800 UTC 19 August (42-h, 24-h, and 00-h forecasts) show progressively stronger mid-level circulation approaching Arizona from old Mexico. Both 500-mb and 700-mb troughs increase in magnitude as the verification time approaches. (d(prog)/dt not a bad indicator in this example).
  54. Two cyclonic circulations are moving up the west coast of Mexico in this loop, and both are associated with MCVs. At 1415 UTC 18 August, notice the MCS at the southern edge of the map. At 0815 UTC 19 August, an MCV that seems to have originated from this MCS is over southern Baja, and is moving up Baja towards the USA. Simultaneously, strong convection over Mexico at 0415 UTC 19 August has spawned/is associated with an MCV moving into Arizona. This feature spins cyclonically as it moves northwestward across Arizona on the 19th and 20th. It falls apart over the higher terrain of Nevada late on the 20th and on the 21st. Nevertheless, there is more convection over the desert Southwest when this MCV is nearby. Again, if you think of an MCV as a tracer of moisture, this makes sense.
  55. Again, shear is low where the MCV is. Necessary for MCV maintenance, especially when there is not a lot of convection to help maintain the circulation.
  56. Left-hand side: Band 11-12. This difference acquires positive values where low-level moisture is pooling (in the absence of high-level cirrus, which degrades the utility of the difference). These yellow and orange into red value occur over Nevada as the MCV approaches, signaling an increase in the moisture as the atmosphere that supports the MCV moves over the area. Right hand side is the 'normal' 11-micron window image. The cyclonic motion is especially noticeable early in the image, before diurnal convection blossoms. What might you expect to occur with copious moisture in a region of low shear?
  57. Rains associated with diurnal convection in this especially moist airmass that supported MCV formation caused flash floods in Las Vegas.
  58. This next example shows an MCV-like feature also; however, it's not a true MCV because the shear is high, and the MCV feature was associated with several tornadoes. Cyclogenesis occurred over New York during July during the afternoon -- will it be accompanied by severe weather?
  59. There were several short waves of interest in this case study. This is the AVN 500-mb initialized heights/vorticity. There are two features to focus on. One shortwave is rotating around the Polar Vortex over northeast Canada. The shortwave drops southeastward from Manitoba to Michigan, losing strength as it drops. A second shortwave moves through the longwave ridge over the Canadian Rockies, sliding from Alberta (12z 20 July) to Illinois (06z 21 July) before spinning up to a very tight, strong vortex at 12z on the 21st over Ohio. Then it moves northeastward over western New York in conjunction with the cyclogenesis.
  60. The large-scale aspects of this case were well-forecast. Convection over Indiana/Ohio induced surface cyclogenesis that then continued into New York state. This case is interesting because of the small scale of the induced cyclone/large scale of the MCV. It becomes unclear which vorticity maximum is dominating: the convectively reinforced one over Ohio or the one dropping down around the Polar Vortex over Michigan.
  61. Loop of window channel on the 21st. Some things to note. The convection over northern Ohio seemingly weakens as the convection over southern Ohio strengthens -- this may reflect the southern system interrupting the flow of moisture to the northern system. Note how southeastern Pennsylvania in this loop is sunny for most of the day. Great for destabilizing the atmosphere there. Once the northern system can tap moist destabilized air over Pennsylvania, it reinvigorates. This does not have a classic MCV presentation on satellite -- it looks far more frontal wave in nature, and it is evolving in a region of relatively high shear -- so this is not a classic MCV. Definitely a hybrid system, yet knowledge of MCV dynamics could be helpful this day in nowcasting/forecasting. Note further the presence of the 2nd vorticity center over lower Michigan, especially visible at the end of the loop.
  62. Forecasts show cyclogenesis just where you'd expect it. Just downstream of mid-level trough. Note the implied surface winds over Pennsylvania in this case would be southwesterly, or at most southerly along the ridges.
  63. Despite the forecast for southwesterlies, MSAS and surface winds show southeasterlies. It is tempting to speculate that the MCV-like feature over Northwestern Pennsylvania at this time (near Bradford/KBFD) is affecting the low level wind field. A good (unanswered) question: to what extent does the induced low-level circulation attending the MCV draw northward the moist, unstable air that develops over SE PA?
  64. Regional Radar from KBGM, hourly from 08z on the 21st through 00z on the 22nd. Initially some kind of boundary at the NY/PA border along which the convection is tracking. The very cyclonic wrapped-up feature moving along the New York/Pennsylvania border is associated with the mid-level vorticity spawned by convection over the midwest early on the 21st. As it interacts with the unstable air over south-central Pennsylvania, severe weather develops.
  65. This annotated KBGM radar shows when/where tornadoes and funnel clouds/TVSs occurred. The severe weather is plainly associated with the vorticity center that evolved out of the midwestern convection earlier in the day. How could knowledge of MCV dynamics help here? Heavy precipitation associated with the convective system will help maintain its circulation; if precipitation had been lighter this day, the wrapped up circulation would have more quickly decayed. However, as a forecaster you would have to consider the effects of the MCV because it will persist.
  66. Kinzua Viaduct blew down. At the time of its construction, it was the tallest iron bridge in the world.
  67. Comments on this system. This was not a classic MCV -- it persisted in a region of high shear because lots of latent heat release maintained it. Understanding how the circulation of the MCV affected the redistribution of low-level moisture is critical to understanding the tornadogenesis that happened that day.
  68. Wrap-up. MCVs form in regions of relatively small mid-level (850 to 500mb) shear. Typical values are 10-15 knots between 850 and 400 or 500 mb. There can be strong shear below 850, but mid-level shear must be weak. MCVs will persist if there is little mid-level shear, or if there is a lot of precipitation (or both!). Numerical models can have a difficult time predicting MCV onset, thus the appearance of MCV may mean it's time for a forecast revision. MCVs can be used as tracers of airmasses that are especially moist and that have small values of mid-level shear.