Category Archives: terrain

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:


Subtle Terrain Effects Across Eastern South Dakota

The morning satellite imagery shows the subtle effects of terrain on sensible weather.  The Coteau des Prairies in Eastern SD (sometimes referred to as the Sisseton Hills) rises above the local terrain by 600-800 feet.  A good colored relief map clearly shows this feature:

Strong nearly unidirectional NW flow was present over the entire region, and the synoptic flow pattern was not conducive to any significant regions of forced ascent.  Upper level heights were rising with strong subsidence owing to the effects of differential anticyclonic vorticity advection aloft.  However, a weak mid-level trough axis was present owing to weak thermal advection as the powerful low continued to “wrap” warm and moist air around the large upper trough.

700 hpa trough axis noted on both the 12 GFS and NAM.  Also note the mass convergence at this level–a response to the forced ascent, not the other way around.

Note the region of weak warm thermal advection.

And weak resulting synoptic ascent:

The 0Z sounding from Aberdeen depicts the moist inversion sandwiched in between two dry layers, one aloft and one near the surface.

The subtle effects of terrain across the region had very visible and profound weather effects.  Stable low level NW flow and subsequent downsloping resulted in drying of the low level air mass over Aberdeen with subtle upslope effects clearly visible over the higher terrain.  Let’s look a little closer.

Radar depicts the showers over the area at 21Z.  Note the radar “hole” around Aberdeen (red) and the regions of enhanced returns over the higher terrain (green).

The observations at Watertown (KATY) and Aberdeen (KABR) also show the effects of terrain enhancement.  Note the dewpoint depression at Aberdeen hovers around 4-5 degrees C while Watertown hovers around saturation to 1-2 degrees C.  Ignore the -SN reports from Aberdeen–seems to be a sensor error (also note it reports a constant 9SM visibility, a good indicator something is not right).

Aberdeen:

Watertown:

The visible satellite image the day after clearly shows the terrain enhanced snowfall over the higher terrain in Eastern SD (red).  Also note the west slope favored snowfall across the Bighorns, Black Hills, and Laramies (green).  Also worth noting here is the lack of snow cover over the whole of the Coteau des Prairies region.  This suggests the air mass precipitated out before before reaching farther south and east.

This shows up in even more spectacular fashion on the MODIS multi-spectral image.

This is a clear example  the significant effects even “subtle” terrain can have on weather.  In this case, strong and stable low level flow won out over weak mid level ascent.  If precipitation were heavier, it is unlikely terrain would have played nearly as significant of a role.


%d bloggers like this: