Category Archives: lee cyclogenesis

The Role of Deep, Moist Convection and Diabatic Heating In Association With A Rapidly Intensifying Cyclone: A QG/PV Analysis

After a long hiatus, I have returned with a new article (finally). I continue my discussion on PV dynamics (I am always learning something new each day), but this time focus more on PV non-conservation. My motivation for this case study/paper came from a challenging winter storm we had to deal with at the North Platte WFO. Numerical models poorly simulated the rapid intensification of a cyclone ejecting the Rockies, and the event ended up being a local high impact storm for the CWA. It was also a null event across portions of North Dakota as the cyclone rapidly intensified and stalled.  The culprit for rapid intensification was the initation of deep, moist convection near the center of the cyclone/warm front and the generation of low level PV through differential diabatic heat release which influenced the low level mass fields and advection patterns. It was a unique case study, and I hope it is of use to other forecasters/weather enthusiasts out there. Please take the time to closely view the images (the small details made all the difference in this event!), especially the “dprog/dt” analyses of PV generation/destruction. The link is the local office case study (still in review by my SOO) in .pdf format (6 MB’s). As always, any questions/comments/criticism/feedback are always welcome.

My next article, I hope, will stray away from rapid cyclogenesis and delve into “negative” feedbacks to cyclone development–a forecast which can be equally as challenging as rapid positive feedback cyclogenesis.

Also, take a look at the references. The Brennan paper on PV non-conservation is superb.

Feb28_29thCaseReview

 


High Plains Convection, the Ineffectiveness of Surface-6 km Bulk Shear, and the Effectiveness of IPV in Synoptic Diagnosis.

Summer time atmospheric flow is characterized by the retreat of the Polar Front deep into the high latitude reaches of the Northern Hemisphere.  The westerly flow often times becomes dominated by shallow PV anomalies passing through the flow.  The lack of snow cover and long, intense insolation result in strong surface heating and the development of terrain induced diurnal flows.  This effect is most commonly seen across the High Plains of New Mexico, Colorado (Front Range), and portions of eastern WY where mountain valley circulations and smaller scale mountain slope flows can result in favored regions of convective initiation/enhancement.

The unique and very complex terrain of the Colorado mountains and High Plains can result in enhanced convective initiation and local wind fields spanning the meso-gamma to meso-alpha which go on to strongly influence eventual storm mode.  Areas of enhanced convective initiation include the Front Range, Sangre de Cristos (and the various sub-ranges including the Culebra Range, Crestone Range, Spanish Peaks, etc.),  and the Palmer Divide to name a few.

On the afternoon of Saturday, June 11, surface observations shows the combined unique effects of moderate cross-barrier westerly flow impinging upon the high terrain of the Rockies (leeside troughing) and the diurnal mountain-valley circulations resulting in the southeast upslope flow:

Often times the “parameter” of first choice regarding severe convective potential and storm mode is the surface-6 km bulk shear parameter.  In short, surface-6 km bulk shear is simply the length of the hodograph (the addition of shear vectors) from the surface to 6 km.  The amount of shear has a strong influence on storm mode and behaviour (in combination with CAPE/instability/synoptic flow pattern).  Sufficiently strong shear (associated with increasing winds with height which is then correlated to synoptic scale disturbances) through the effective updraft can result in rotating mesocyclones and associated supercells.

A quick look at the NAM surface-500 hpa bulk shear (typically the same as surface-6 km) at 21Z shows shear values less than 30 knots across southeast Colorado.

SPC mesoanalysis also shows less than 30 knots of surface to 6 km bulk shear values:

This would be indicative of non-supercell type multicell thunderstorms and/or single cell updrafts.  However, a quick look at the sounding is far more telling:

Note the strength of the wind fields above 400 hpa and the additional shear available to high based parcels.  This environment would be conducive to high based supercells with rotating mesocyclones owing to the vertical pressure gradient forces (VPGF) associated with a sheared and unstable environment.

As we will see later, convective initation was favored across high and thin mountain ranges across southeast Colorado–a common location for summertime DMC across Colorado.  This favored location is due to the terrain induced slope flows associated with differential heating of high terrain (with respect to the lower plains) which results in upslope anabatic flow during the day (katabatic drainage flow during the night).  Given many of the ranges across southern Colorado range from 10,0000-13,000 feet with isolated higher peaks, it is likely updraft bases are higher than 600 hpa (over 15000 feet MSL or 10,000 feet AGL with respect to the height of the High Plains).

Indeed, observations from across the area showed thunderstorms in the area with clear skies (ASOS ceilometers only hits on clouds below 12,000 feet AGL), suggestive of the very high bases.

KLHX 111953Z AUTO 15015G26KT 10SM CLR 31/12 A2988 RMK AO2 PK WND 15026/1949 LTG DSNT SE AND S SLP055 T03060117

It should also be noted that the mountain terrain would likely locally enhance dewpoint/temp profiles (owing to the localized slope flow induced convergence/moistening) which would likely enhance the thermodynamic properties (enhance CAPE) of bouyant parcels initiating across the mountains.

The secondary question quickly becomes what does IPV/PV have to do with this?

Typical summer flow across the intermountain west is dominated by shallow upper tropospheric PV anomalies passing through the mean flow.  Often times there is little to no reflection in the 500 hpa height field evident in the flow as the “depth” of the anomaly is only relegated to the highest portions of the troposphere.  Unfortunately, most meteorologists are trained to make use of the 500 hpa height/vorticity field only, and indeed most atmospheric synoptic plots only include this level.

Note at 18Z, June 11–there is a relatively “flat” height field across Colorado:

The mean trough is well to the west over the West Coast.  This would be indicative of possibly low/weak convective potential as the synoptic flow (using 500 hpa charts) is not conducive to forced ascent and enhanced synoptic convergence and sufficient upper level shear.  However, this is misleading, and a quick look at the 1.5/2.0 PVU surface shows the presence of a number of shallow anomalies aloft.

Note the anomalies are rather shallow in nature and do not extend much beyond 300-375 hpa.

Worth noting is the well defined 500 hpa “kink”/wave in the height field which develops as the anomaly ejects out of the Rockies and translates eastward through the High Plains:

This is likely due to the vertical stretching of the upper level anomaly as it both ejects out of the Rockies and interacts with the low level thermal anomaly across the plains east of the Rockies.  This results in the development of cyclonic vorticity owing to the conservation of potential vorticity along theta surfaces.  This can also be explained via QG theory.  In other words–imagine the path a low level theta surface follows as it ejects out of the Rockies (downslope).

Image Courtesy of CIRA

Image Courtesy of University of Wyoming

As expected, the presence of these shallow anomalies enhances the upper level wind fields:

which has a strong influence on storm mode and development owing to the sheared environment.

So what eventually occurred?

A cluster of supercells initiated off the high mountains of southern Colorado across the zone of enhanced high level vertical shear where the upper wind fields were maximized associated with the ejecting anomalies (see image above).

A close-up view of initiation associated with enhancement via terrain:

Note the high level shear–clearly evident in the long anvils.

It is clear that care must be taken when evaluating the atmospheric environment.  The atmosphere does not work solely at 500 hpa or with simplistic parameters such as surface-6 km bulk shear.  An understanding of local terrain, climatology, storm environment, relevant synoptic features must all be considered or significant forecast “surprises” and/or errors will result.

As always–meteorology is such a beautiful thing when it makes sense.

Weather is always cool.


The Froude Number and Stable Flow: Mountain Blocking

A powerful low amplitude shortwave ejected into Montana this morning in association with a 160 kt Pacific Jet.

The 0Z NAM from yesterday clearly depicts this feature:

Large scale and mesoscale ascent developed rapidly as the jet core amplifed over the region.  Note the large increase of high level moisture associated with a region of strong vertical ascent:

0545Z:

Three hours later at 0845Z:

Low amplitude intense shortwaves such as these have a tendency to develop significant upward vertical velocity/downward vertical velocity couplets which support rapid cyclogenesis and regions of strong pressure gradients over small areas (i.e. rapid intensification, or the second partial of p with respect to x, gradient of the gradient).

Note the rapid pressure rises, on the order of 8+ mb’s / 3 hours over northern MT as extreme cold air advection set in behind the front.

The surface analysis depicts the strong surface ridging associated with the extreme subsidence mainly owing to strong cold air advection behind the cold front.  Also note how surface ridging amplifies as the high pressure region interacts with the Rockies.  The Rockies “block” the subsident air from progressing westward, therefore air builds at a faster rate east of the Continental Divide resulting in stronger surface ridges:

The Great Falls sounding at 0Z shows the flow was mainly out of the N in the low levels and NW in the mid levels.

Great Falls is around 3700 feet, so in this sounding, stable N flow extended to nearly 10,000 feet, or over 6000 feet AGL.

The Belt Range south of Great Falls extends to around 6000-8000 feet and reaching top elevations greater than 9000 feet.  Also note they form a “bowl” type shape around the region.  This makes it very difficult for air to flow around the mountains.

The Froude number,

relates the inertial forces to the gravitational force.  Think of it as a relation of kinetic energy to potential energy where V is velocity, N is the brunt vaisala frequency, and L is the height of the mountain.  Therefore, think of it as relating KE= 1/2mv^2 to PE = mgh.  The brunt vaisala frequency is: 

Note the gravity term (remember mgh) and the static stability d-theta/d-z (the more stable the air mass is, the greater the kinetic energy will need to be for air to ascend the range).

A series of radar images shows how stable N-NW flow “bunches up” into the valley as stable flow is blocked by the mountains south of the valley.  Low level stable air builds into the valley and it acts to “uplift” air above it, much like Cold Air Damming:

Note in the surface obs the heaviest snow develops coincident with rapidly rising pressure as stable air builds into the valley while V simultaneously weakens (weak V, which means lower kinetic energy, therefore the flow can not ascend the mountain).  Note also that downslope flow into the valley was not able to kill of the qpf.  Also note the powerful cold front (green) with G into the 60s.

High res models were trying to show a large weather hole over Great Falls associated with downsloping into the valley.  A good example showing high res models can struggle mightily in compex terrain:


Satellite Loop of the Powerful Northern Plains Storm

First storm animation (more are coming) of the record breaking low.  This animation is more than 3.5 days long from the 24th-27th of October (make sure to watch in 720):

I suggest clicking the link to see it in a slightly larger view (use the expand) from the youtube website.


Incredible Jet Stream Divergence

No amount of superlatives can describe the storm taking shape over the central and northern plains.  My projections of surface intensity in the previous post were completely wrong (I believed 966 was too low).  With any sort of extreme weather system, any particular dynamic and/or kinematic field is expected to be impressive.  This storm, however, is displaying an incredible amount of jet stream divergence which shows up in spectacular fashion on satellite imagery.    Let’s take a look.

IR satellite image at 15Z with the center of the jet stream noted at 250 hpa with the red line.  The green circle denotes the upstream jet streak in excess of 160-180 knots.

12Z 300 hpa analysis with winds and divergence plotted (thanks to http://www.patricktmarsh.com/, I never knew jet stream level winds with divergence plotted existed!):

As analyzed by the 12Z GFS @ 18Z (250 hpa):

Note the increasing jet level winds on the eastern side of the upper level trough from 12Z to 18Z.  Let’s investigate further.

The 12Z HPC surface analysis has the cold front analyzed in northern CO:

Note the well defined lee cyclone in the Front Range of CO extending into New Mexico, an atmospheric response due to the cross-barrier flow blocking effect of the Rockies.  Large and long mountain ranges block the otherwise orderly flow of cold air advection, resulting in a geostrophic adjustment process.  Lee cyclogenesis acts to enhance the low-level south flow and, in the case of the US, the flow of warm and moist Gulf air northward.  The “blocking” of cold air into the plains acts to “displace” the cold air aloft from the low level warm air in the plains in the vertical.  Mentioned in the previous post as well, the thermal wind equation comes into play here.  http://amsglossary.allenpress.com/glossary/search?id=thermal-wind-equation1

The change in the geostrophic wind with height (vertical shear) is related to the thermal gradient.  The jet stream, therefore, is a manifestation of intense baroclinic zones and upper level fronts, not the other way around.

Let us put it together a little more.  Take a look at the GFS 12Z analyzed 1000-500 mb thickness fields:

The location of the surface trough is noted with the green line with strong surface ridging behind the front (as expected).

Note that, at 12Z, the upper level cold air isotherm packing is lagging behind the low level cold air:

The effects of large scale flow blocking become much more apparent here as we put things together.  The effect of the broad and high Colorado Rockies is to block or retard the low level progression of otherwise orderly cold air advection.

BY 18Z, the region of cold air aloft has now become superimposed over the region of lower level cold air associated with the low level front, currently being blocked by the high terrain of the Rockies.

Oh, but wait.  What did the thermal wind equation state?  The picture is becoming slightly more clear now.  The juxtaposition of cold air aloft and at low levels along with the continued effect of lee cyclogenesis due to cross barrier flow results in southerly warm air advection in the low levels of the high plains.  These processes work to enhance the baroclinic zone along the mountain barrier.

18Z GFS forecast shows how much tighter the 1000-500 mb thickness field has become due to the aforementioned processes.  Also note the high-low pressure couplet that has developed across CO with the decrease in the surface pressure of the lee cyclone, now to 984 mb.  Cold bora winds downslope into the plains as the cold air “pours” over the Front Range.

As expected from the thermal wind equation, our jet stream has now become stronger on the eastward side of the curved jet stream over our now enhanced baroclinic zone across the high plains (circled).:

Also worth noting here are some of the terrain flows that can develop under such circumstances.  In the case of the Front Range, mesoscale terrain flows can develop around or over regions of decreased height in the Rockies.  Extreme pressure gradient forces are relaxed through relatively narrow regions of the terrain, resulting in terrain enhanced gradient forces.

Both the Ferris Mountains and the Laramies reach elevations above 10,000 feet with the Snowy Range (named the Medicine Bows in Colorado) extending to over 12,000 feet.  Gaps in the terrain extend down to 7500 feet in Laramie, WY before reaching approximately 4600 feet in Akron, CO.  With I-80 along southern WY being the only large scale “outlet” for subsident air over the Great Basin, winds can become rather extreme.

The obs from Akron, CO clearly show frontal passage (boxed red) with the typical pressure falls preceding the front followed by rapid pressure rises.  Of course, peak winds occur during the time period of rapid pressure rises (boxed green) and strong descent due to efficient mixing in the convective boundary layer acting in conjunction with descent on the backside of the frontal circulation (circled red).

Let’s move on.

Curved jet dynamics result in regions of strong ascent/descent (ascent on the exit region, descent on the entrance region) on the poleward (cold) side of the jet stream.

Also note the increasing amplitude of the trough and the “digging” nature of the jet.  Is this a result of QG Chi interepreted height falls associated with abnormal thermal advection patterns noted earlier?  Think about that.  Do jets “dig” or do heights fall?  I will let the readers decide.

Goes satellite derived WV winds at 18Z suggest both the NAM and GFS are under observing the jet streak winds on the downstream portion of the trough which would result in even greater values of jet divergence.  Circled isotach at 120 kts (18Z GFS peaked at 90 kts from 300-200 hpa).

This jet stream divergence was manifested in spectacular fashion on satellite imagery:

And on multi-spectral satellite imagery:

Here is an animation of the cloud patterns associated with this divergence over Colorado.  This is the best way to see the divergence pattern and associated cloud field:

Also note the “folds” oriented perpendicular to the flow (easily seen in the visible sat images).  Personally, I have no explanation for these features.  It seems plausible the N-S oriented CO Rockies have an influence, but I personally have no reasoning.  Anyone with ideas or explanations please let me know.

Update:  The expected smooth nature of the jet cloud pattern over WY is typical earlier in the day.  As the system interacts with the Front Range of Colorado, the folds seem to originate in the region where enhanced vertically propagating mountain waves often develop.  This seems like a plausible explanation, but I will have to do more of an analysis before coming to such a conclusion.

This analysis ends here, but note this is just one explanation (also the more simplistic and less mathematical approach and reasoning) of lee cyclogenesis and further baroclinic development associated with an intense jet stream (lee troughing is possible with little to no jet stream/weak baroclinity).  Other authors have proposed a QG explanation (Bluestein uses this approach in his Synoptics in Midlatitudes) as well as potential vorticity reasoning.  In general, differing “theories” and interpretations seem to come to relatively similar conclusions (in the last 20 years at least).

Meteorology is a beautiful thing when it makes sense.

Additional reading for those interested.

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(1989)117%3C0154:NAOTIO%3E2.0.CO;2

http://www.atm.helsinki.fi/~dschultz/pubs/19-SchultzDoswell00.pdf


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