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 mesobeta 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<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 mesolows 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. Vortex-finding
algorithms in the RUC find ~40 MCVs (some dry) each year.
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. Again,
steady translation speed will help you predict when the MCV will clear
out.
23. Some MCVs are well-predicted. This is especially so if the systems that
generate them are well predicted.
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, 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 3 rd 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 6 th.
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. Vorticity moves east-southeastward from IA to IL.
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 eastsoutheastward, 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 effect on forecast high temperature from the MCV at MSN & GRB.
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 and to understand more fully
what is going on in the atmosphere?
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 19 th 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.
This is a hybrid system at sub-synoptic scales, so it is difficult to apply
conceptual models to it.
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 also the presence of the vorticity center
over lower Michigan 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.