The Role of a Deepening Trough Approaching an Area of Low Pressure
Analysis of a Rapidly Intensifying Cyclone on May 24th, 2011
I Introduction
Severe weather is anything but uncommon in May in the United States, and 2011 was no exception. In a particular stretch from May 22nd through May 27th, over 4,000 severe weather reports ranging from wind reports to tornado sightings came into the National Weather service as a massive storm system traversed the nation. (See Figure 1 for storm reports).
Figure 1: Storm reports from the Storm Prediction Center for May 22-27th from as the cyclone traversed the nation from the Southern Plains to the Northeast.
As tornadoes traversed the southern plains, straight-lined winds caused extensive damage across a large swath of the eastern half of the nation, and torrential rains caused significant flooding in the northeast, this outbreak went down as one of the deadliest and costliest in the nation's history.
What started as a stationary weak low-pressure center over the southern plains transformed into a powerful storm system as a deepening trough turning negatively tilted approached the plains. As a strong jet streak rounded the base of the trough over a very well defined dry line, it sparking off a very ferocious day of storms, and this would continue as the system moved east as the trough slowly began to weaken.
This case study will be looking at the stationary low pressure sitting over Oklahoma starting on May 22th and what factors led to its strengthening and what led it to propagate eastward. This will include the effects of the trough crossing over the low pressure, the moisture, temperature, and vorticity advection, and the approaching jet streak in the 500mb level.
II Impacts
A severe weather episode from May 22nd through the 26th resulted in widespread devastation across the southern plains all the way through to the Atlantic coast. Nearly 250 tornadoes were reported across half the continental United States. Seven billion dollars in damages was caused by the storms as they passed through many major cities. Over 180 people lost their lives during the outbreak of severe weather in five states, including 158 during the Joplin Missouri tornado alone.
The largest impacts were felt in two regions in the southern plains as two EF-5 tornadoes devastated large areas. In Joplin, Missouri, a multi-vortex tornado over a mile wide greatly affected the south side of the city leaving a 22.1-mile wide path of total destruction on May 22nd. The estimated cost of damage in the city of Joplin was $2.8 billion (See Figure 2 for damage path).
Figure 2: The damage survey conducted by the National Weather Service shows the area affected by the May 22nd 2011 tornado as it cut through Joplin.
In Oklahoma on May 24th, many long-tracked tornadoes raced across Oklahoma, including one that caused complete devastation in El Reno, Piedmont, and causing significant damage to the north side of Guthrie as an EF-5. Nine people were killed by this monster tornado, which was measured by the University of Oklahoma Doppler on Wheels to have winds gusting over 280 mils per hour.
As the system moved east, more damage was caused in its wake. The threat shifted from tornadoes to a linear, straight-lined wind threat and a flooding concern as the low pressure slowed down. Flash flooding as well as strong winds from severe thunderstorms was the greatest concern during this time. Thunderstorms dumped huge amounts of rain in a very short period of time in the northeast. Many areas in Vermont received over 3” of rain, with Plainfield, Vermont receiving the highest at 5.22” in only 24 hours as shown in Figure 3. Flooding along the Winooski River caused the river to go from normal to a record crest in a matter of 12 hours as seen in Figure 4 and forced the evacuation of 200 residents and forced a number of road closures. In the Atlanta metropolitan area, a strong downburst from a severe thunderstorm toppled trees across the county and killed three people they fell on their cars. Another nine people were injured in Pennsylvania as winds knocked down many trees onto houses and vehicles.
Figure 3: Rain totals from May 27th over the Burlington, Vermont National Weather Service county warning area.
Figure 4: River guage readings on the Winooski River showing the rapid rise in water to major flood stage, and a new record flood stage, on May 27th.
The event began early in the afternoon on May 22nd as tornadoes touched down in the city of Minneapolis, Minnesota just after noon. One person was killed, and over 30 were injured, two seriously as the tornadoes went through the city. By 5p.m., tornado watches issued by the Storm Prediction Center extended from Minnesota near the triple point of the cold front, warm front, and the low pressure, all the way down to Missouri along the cold front, and along the dry line south through Texas. Just after 5p.m., storms began exploding ahead of the front in Missouri, and a cluster of cells merged just southwest of Joplin, Missouri, and made a turn right for the city as a mile wide tornado touched down and traversed the city. As the night wore on, the storms along the front congealed into a squall line as they headed east, causing wind damage from Illinois and points south where a man was injured by a tree falling on his car.
As the system headed into its second day, storms began firing in Missouri on a newly formed dry line, and propagated eastward which resulted in a few tornadoes including one in Stewart County, Indiana injuring two people as well as several hundred wind reports ranging from Oklahoma to New Jersey.
On May 24th, the main show with the low pressure over Oklahoma began. A trough digging into Texas with a strong jet streak rounding the base assisted the low pressure to strengthen. In the warm sector, the warm temperatures and abundant moisture return over Oklahoma set up a very instable environment by early afternoon. A weak boundary crossed Oklahoma from the storms to the north clearing clouds and allowed more solar heating to occur during the peak heating period. A very tight moisture gradient along the dry line started to move eastward and was the cause for the storm initiation by breaking the capping inversion that was in place. Several supercells initiated and began dropping large hail and tornadoes across Oklahoma. One cell intensified rapidly with a very large tornado and affected the cities of El Reno and Guthrie. While the tornadoes were affecting Oklahoma, severe thunderstorms with strong winds paralleling the stationary boundary in Virginia caused multiple injuries and fatalities as track and field equipment was picked up and thrown injuring five at Landstown High School in Virginia, Virginia. Cranes also collapsed killing shipyard employees in Norfolk and Hampton.
By May 25th, the storm system was in a weakening mode, but a second straight high risk day for severe weather was present over the middle Mississippi River Valley. A very large outbreak of severe weather occurred as storms fired ahead of the cold front that was progressing eastward. Tornado watches were up for a line of storms from Ohio to Texas. The very pronounced warm sector had fuel from the Gulf of Mexico with abundant moisture and warm temperatures in the 80s while behind the front, temperatures were unseasonably cool in the low 60s. The wind was the primary threat for the line of storms as they tore roofs from houses and knocked over trees and power lines across nearly the entire eastern half of the country.
On May 26th, the low pressure began slowing down and the trailing cold front became nearly stationary from eastern Ohio, south to Georgia. The warm front from the low pressure in northeast Ohio to southern Maine stalled out and created a very heavy rain event starting the night of the 27th and into the 28th.
III Upper Air Pattern
The severe weather event occurring from May 21st through the 26th was primarily driven by the upper air patterns that had begun on the 21st. At 00z on the 22nd, a deep trough in the jet stream slightly negatively tilted with a cutoff low sitting over South Dakota is in the southern plains with a very strong ridge building into Michigan with a jet core of 90kts sitting over the tri-state region of Iowa, Missouri, and Illinois shown in Figure 5.
Figure 5: 300mb upper air chart at 00z on the 22nd.
By 12z on the 22nd, the trough weakened and began northward. The cutoff low over North Dakota began moving northeast as the jet streak subsided. A new jet streak is developing over the United States/Mexico border region oriented east-west on the base of the retreating trough as seen in Figure 6.
Figure 6: 300mb upper air chart at 12z on the 22nd.
On 00z of the 23rd, the ridge over Michigan moved much further east with the top of the ridge sitting over Maine, and the base of the trough is sitting over central Mississippi with the axis towards Missouri. The jet streak over the US/Mexico border now has a core of 100kts slowly progressing eastward. A new ridge is beginning to build over Colorado as a trough is beginning to dig down the west coast of the United States still off the coast of California with a jet streak near 100kts.
At 12z on the 23rd, the ridge is building over the Central US all the way to the Dakotas. The trough came ashore over California with a jet core rounding the base over 125kts. Another jet streak was developing over central Texas in excess of 90kts. Further out west over the Pacific, another much stronger trough approached the United States shown in Figure 7.
Figure 7: 300mb upper air chart at 12z on the 23rd.
At 00z on the 24th, the new trough coming up from the Pacific enhanced the little shortwave trough over Arizona adding a strong jet core of 125kts and causing the trough to dig further south. The jet streak had begun rounding the base of the trough butting southwestern Kansas in the left exit region of the streak. The ridge over the central United States had grown much wider across, and a cutoff high began to form over northern portions of Iowa seen in Figure 8.
Figure 8: 300mb upper air chart at 00z on the 24th.
At 12z on the 24th, the trough has dug further south to the New Mexico/Mexico border with the jet core rounding the base of 110kts still putting southwest Kansas in the left exit region. The ridge stopped growing with the crest of the ridge sitting over southeast North Dakota shown in Figure 9.
Figure 9: 300mb upper air chart at 12z on the 24th.
At the peak of the cyclone at 00z on the 25th, a cutoff low formed over southwest Kansas/Oklahoma panhandle region turning the trough strongly negatively tilted. The jet streak of 95kts set up over southern Oklahoma at the base of the trough with strong diffluence and divergence over central Oklahoma. The ridge crest moved into western portions of Wisconsin seen in Figure 10.
Figure 10: 300mb upper air chart at 00z on the 25th.
By 12z on the 25th, the cutoff low centered over central Kansas as the trough reached its fullest southern progression over the Texas/Louisiana border still negatively tilted. The jet core of 100kts sat over central Oklahoma as the jet streak almost fully rounded the base of the trough with the left exit region over southwest Missouri. Still, there is upper level divergence over the surface low. The ridge is sat over Michigan at this time shown in Figure 11.
Figure 11: 300mb upper air chart at 12z on the 25th.
At 00z on the 26th, the jet streak diminished almost fully with a small streak still over Oklahoma/Texas border region only at 70kts. The trough began a slight northward movement with a trough axis from northeast Missouri to northern Louisiana. The ridge moved further east now cresting over New York, and a new jet streak has formed just north of the ridge over Quebec, Canada over 130kts as shown in Figure 12.
Figure 12: 300mb upper air chart at 00z on the 26th.
12Z on the 26th the trough has lost its negative tilt with a trough axis from west central Illinois south-southwest to the Louisiana/Mississippi border. The jet streak over the Oklahoma/Texas border dissipated, but a new small jet streak has developed over southern Indiana. The ridge moved off the United States coast at this time shown in Figure 13.
Figure 13: 300mb upper air chart at 12z on the 26th.
As the final day of the cyclone began at 00z on the 27th, the trough didn't really moved much with the axis sitting in central Indiana south-southwest to the Gulf of Mexico south of Alabama. The cutoff low had also dissipated at this time, showing that the system had weakened significantly. A new small ridge began to form over central United States and a strong jet streak came ashore over Oregon over 130kts seen in Figure 14.
Figure 14: 300mb upper air chart at 00z on the 27th.
At 12z on the 27th, very strong diffluence occurred over the eastern half of the United States from the jet streak. The trough that has started this whole system is now paralleling the east coast of the United States as shown in Figure 15.
Figure 15: 300mb upper air chart at 12z on the 27th.
In Figure 16 on 00z on the 28th, what is left of the trough moved off shore into the Atlantic finally ending the story of this severe weather event.
Figure 16: 300mb upper air chart at 00z on the 28th.
IV Synoptic Level Pattern
The synoptic event began on May 22nd with a strengthening low pressure down to 994mb centered over Minnesota, and a trailing cold front that extended south all the way to Texas with an accompanying warm front that moved east through Pennsylvania. The low pressure began to weaken as the night went on and the cold front retreated westward. The warm front became stationary and was positioned southwest to northeast from New Mexico through Wisconsin. As the event went onward through the 28th, a stationary center of low pressure over Oklahoma began to strengthen due to an approaching trough in the jet stream, and eventually carried to the east. The progression of the low-pressure center can be seen in Figure 17.
An area of low pressure had been stationary over the Southern Plains for several days and latched onto the stationary front and started to deepen from 1004mb to 994mb in the matter of 24 hours on the 24th. A very strong dry line, seen in Figure 18, developed due to strengthening frontogenesis extending south to Texas from the low pressure which was now centered over southwest Kansas enhanced by moisture return from the Gulf of Mexico pushing at the dry line as very dry air from the west built up on the west side of the dry line which was exacerbated by the strengthening of the low pressure. The strengthening low and dry line can be seen in Figure 19, with the pressure lowering from 1001mb to 995mb within four hours. The dry line began to slowly move east with the low in the late afternoon and intensified. In Figure 20, a look at an analyzed surface map on the morning of the 25th, the low pressure began to follow a stationary front that extended from central Kansas all the way to the Northeast U.S.
Figure 18: A very strong moisture gradient along the dryline over western Oklahoma was the trigger for supercells to form in part due to the strong capping in place keeping storms discreet, and allowing for explosive growth. The gradient caused a strong thermal wind imbalance causing strong wind shear helping the storms to quickly rotate and go tornadic.
Figure 19: A look at the surface conditions over Oklahoma at from 16z to 20z on the 24th. Here we can see the strengthening dryline and low pressure. The METAR in Canadian, TX (NE corner of TX panhandle) went from a dewpoint of 28 to 9 with a westerly wind. Just about a hundred miles east, a METAR in West-Central Oklahoma has a dewpoint of 72 and a SSW wind changing to SSE. Also notice the METAR in Medicine Lodge, KS (south-central Kansas), the pressure is down to 995mb from 1001mb just four hours earlier.
Figure 20: By morning on May 25th, the center of low pressure at 996mb began to slowly track NE along the stationary boundary extending from Kansas eastward to New England.
The low pressure still at 996mb over northern Missouri tracked eastward along the stationary front with a cold front that extended south into Louisiana on the 25th. The cold front progressed eastward with the low pressure igniting more storms along the strong moisture and temperature gradient.
As the event moved into the 26th, the low pressure began to weaken down to 1002mb while still tracking along the stationary front to the northeast. The cold front progressed eastward following the low pressure. By the evening of the 26th, the low pressure was over central Ohio with a cold front that extended south through Alabama and still followed the stationary front set up through Maine.
On the 27th, the low pressure weakened to 1008mb and slowed down in speed almost becoming stationary still tracking northeast over the Northeast U.S. It sat over portions of the Northeast for several hours with a cold front nearly stationary strong along the east coast, and a warm front draped eastward from the low to the Atlantic.
On the 28th, the low pressure finally headed out to sea over the northern Atlantic where it would weaken down to 1012mb.
V Discussion
The strengthening of the low pressure on May 24th, 2011 is mainly attributed to the trough approaching the low-pressure center. Before dissecting each of the primary influences in great detail, they will first be summarized below.
The approaching, digging trough caused a disturbance in the thermal wind balance in return created warm air advection to occur in the higher levels. The warm air advection caused the air to rise following the Quasi-geostrophic Omega equation relation between warm air advection and rising motions. The rising air in return caused the low pressure to strengthen. On top of the warm air advection, a jet streak rounding the base of the trough put the area of low pressure in the left exit region causing ageostrophic circulations enhancing the rising motion.
Switching to the QG Height Tendency Equation, the differential temperature advection was the major player in regards to enhancing the low-pressure center. The warm air advection increased the thickness of the column of air over the low-pressure center causing the surface pressure to go down following the hypsometric equation. The fall in heights near the ground caused the fall in surface pressure.
Finally moving to the QG Vorticity Equation, before talking about each in much greater detail, the approaching trough increased the vorticity advection into the upper levels above the low pressure center, and this along with the rising motion caused the vorticity at the surface to stretch, in return, causing it to spin faster increasing the low pressure center.
Moving on to the upper level support, there was plentiful upper air divergence in the 500mb and 300mb levels with the diffluence amongst the height contours. The intense jet streak over the area of low pressure allowed ample upper level air divergence much greater than the surface convergence allowing the low pressure to intensify. The hydrostatic weight of the column became less allowing vertical motion to intensify.
As the jet streak rounded the base of the trough and was fully on the east side of the trough, the system began to weaken as the upper level divergence subsided and the rising motion became less vigorous. The system would eventually head out to sea where it would continue to weaken.
Following the QG Omega Equation, upward motion is proportional to the warm air advection above the surface as well as the positive vorticity advection increasing with height. In the case of the strengthening low-pressure center on May 24th, both were in place over Oklahoma allowing the low-pressure center to deepen quickly.
To show warm air advection, the laplacian of warm air advection will be used at the 700mb level. Looking at Figure 22, the laplacian temperature advection is displayed through the day on the 24th. Notice the placement of the greatest advection is right over the low-pressure center as seen in Figure 21. This intense warm air advection allowed the rising motion to intensify greatly as seen in Figure 21 as the low pressure drops from 1004mb to 996mb in just 12 hours.
This thermal advection raised the heights aloft creating an imbalance in the thermal wind between the coriolis and pressure gradient. In order for the system to get back into balance, the air diverges aloft, allowing continuity to take over forcing rising motion. As mentioned by David Billingsley in his Review of QG Theory-Part II The Omega Equation, thickness is proportional to the average temperature allowing the third term in the QG Omega equation to be the laplacian temperature advection. By his reasoning, “warm air advection is associated with upward motion and cold air advection is associated with downward motion” (Billingsey, 1997).
Figure 21: Mean Sea Level Pressure through the day on May 24th into the 25th. The stationary low pressure over Oklahoma greatly intensified through the day due to many quasi-geostrophic reasons.
Figure 22: The laplacian temerature advection of the 700mb level going through the day on May 24th. The warm air advection over Oklahoma allowed plentiful vertical motion enhancing the low pressure center at the surface.
The second half of the QG Omega Equation is that the positive vorticity advection increasing with height is also proportional to the rising motion. Looking at the 500mb level in Figure 23, the jet streak by 18z began to round the base of the trough and was ejecting into Oklahoma. The left exit region of the jet streak set up right over the area of low pressure at the surface (see Figure 21 for low pressure location).
Figure 23: A look at the 500mb pressure level with heights contoured along with wind speed from May 24th at 00z through May 25th at 00z. As the trough began to come ashore through California, a jet streak began to round the base of the trough. Moving into 18z on the 24th, the left exit region of the jet streak is set up right over the area of low pressure at the surface.
As the trough came onshore of the United States, the very strong cyclonic vorticity advection at the left exit region of the jet streak allowed the heights to lower causing a digging trough to take shape. The speed max began to approach the base of the trough tightening it. The vortmax continued to grow until the jet streak rounded the base of the trough causing the system to begin to weaken.
The left exit region of the jet streak provides cyclonic vorticity advection, which is positive vorticity advection. These vertical circulations have been known to relate to cyclogenesis as the surface (Reiter, 1969;Sechrist and Whittaker, 1979).
Along with what was mentioned above, the exit region of the jet streak forms an ageostrophic circulation in order to get back into balance. Rising cold air occurs in the left exit region further helping the development of the low-pressure center. The ageostrophic circulation would also strengthen the temperature gradient causing the warm air advection to occur.
The QG Height Tendency Equation had a great impact on the low-pressure center. The height tendency was affected as the positive vorticity advection began rounding the base of the trough via the jet streak. As the jet streak rounded the base of the trough, the heights began to fall, and the trough continued to deepen resulting in a digging trough. The vorticity maximum was located at the base of the trough between the area of height falls east of the base and the height rises west of the base.
The biggest affect on the QG Height Tendency Equation is the differential temperature advection. Looking at the differential temperature advection at 700mb in Figure 24, there is a large increase in the 700mb differential temperature advection in the hours prior and during the peak of the low-pressure center.
Figure 24: 700mb Differential temperature advection on May 24th showing the rapid growth of the temperature advection between the 850mb and 500mb levels. The temperature advection over the low pressure center provided the thermal expansion in the mid levels causing heights to rise aloft and heights to fall near the surface helping build the low pressure center.
As the trough approached the area of low pressure, this would cause the heights to decrease at the 500mb level, as seen in Figure 25, as the heights fall 6 meters in 6 hours. However, if you look at the 500mb level heights as the trough passed through in Figure 26, the height falls in Figure 25 should be the same across Oklahoma.
Figure 25: 6 hour height falls from 00z and 06z on May 25th.
Figure 26: 500mb Heights at 18z on the 24th and 00z on the 25th.
The height falls are not uniform due to the differential temperature advection aloft. The differential temperature advection here would raise the 500mb surface up actually increasing the height. Looking at Figure 27, the same six-hour height falls as in Figure 25 with the 700mb differential temperature advection overlaid. The 700mb differential temperature advection fits perfectly where the height falls should be greater over central Oklahoma where the low pressure was located. This is due to the fact the 700mb differential temperature advection had an impact on decreasing the amount of height falls that occurred by lifting up the air.
Figure 27: 6 hour height falls with 700mb temperature advection
While in the upper levels the differential temperature advection would raise the pressure level, at the surface, the height level decreases. As this happens, the low pressure at the surface, located directly below the peak of the differential temperature advection, would deepen. When talking about the terms that make up the QG Height Tendency Equation, “The terms qualitatively work in a similar manner with respect to geopotential height tendencies in that the low level warm air advection decreasing with height acts to increase heights above the local maximum while differential diabatic heating will increase the heights locally above the maximum. Below the maxima, heights will fall” (Ahsenmacher, 3).
As the column of air warms, the hypsometric equation requires that the thickness of the air increase. The heights will rise above the level of max heating, and the heights will fall below the level of max heating. So in summary, a fall in height of a pressure surface near the ground is accompanied by a fall in pressure at the ground.
Finally moving onto the QG Vorticity Equation, a lot depends on the vorticity advection and the vorticity at the surface. Looking at Figures 28 through 30, they show the vorticity in the 500mb, 700mb, and 1000mb levels. Notice the increasing vorticity with height, and the advection of vorticity over the area of low pressure at the surface, which is over central Oklahoma.
Figure 28: 500mb Vorticity on 5/24 at 18z (top left), 21z (top right) and on 5/25 at 00z (bottom).
Figure 29: 700mb Vorticity on 5/24 at 18z (top left), 21z (top right) and on 5/25 at 00z (bottom).
Figure 30: 1000mb Vorticity on 5/24 at 18z (left) and on 5/25 at 00z (bottom).
The vorticity advecting over the area leads to rising motion following the QG Vorticity Equation. The vortmax at 700mb is critical to the rising motion over the area of low pressure.
Now looking at the 1000mb surface, we will call this the surface vorticity. There is surface vorticity in the vicinity of the low pressure. From 18z on the 24th to 00z on the 25th, the vorticity maximum advects to the low-pressure center. This surface vorticity can now be stretched vertically by the rising motion, and spun faster. This stretching of the vorticity spins faster deepening the surface low, strengthening the low-pressure center.
After 00z on the 25th, the system begins to weaken as it heads east. The differential temperature advection begins to lessen causing the heights to fall aloft, and begin to rise at the surface. The laplacian temperature advection in the mid levels also begins to fade creating less of an imbalance in the geostrophic flow. As the imbalance decreases, the rising motion begins to subside as well.
Looking at Figure 31, the 500mb trough and jet streak began to lessen as well, and was in a weakening state as the jet streak rounded the base now primarily on the east side of the trough. The jet streak also lost its strength lowering the ageostrophic motion, therefore decreasing the rising motion in the left front quadrant of the jet streak.
Figure 31: 500mb heights and winds from 12z on the 25th to 00z on the 27th.
As all of the parameters began to lessen, the low pressure began to lose its strength as shown in Figure 32. The low pressure, which was down to 994mb, was up to 1016mb by morning of the 27th.
Figure 32: Mean Sea Level Pressure of the low pressure beginning at 00z on the 25th and going through to its eventual dissipation at 12z on the 27th.
So why did the low pressure strengthen so quickly then dissipate? Looking at Figure 33, the low-pressure center had ample and unobstructed supply of warm and moist air from the Gulf of Mexico as well as a supply of dry, cold air from the northwest. This contrasting airflow helped frontogenesis along the dry line triggering the storms that occurred and allowed an increasing gradient for the ageostrophic flow to increase. The low pressure was also not effected by any other upper air features allowing it to slowly spin over SW Kansas.
Figure 33: Conceptual model of the low pressure on May 24th, 2011. The low pressure had an unobstructed resource of warm and moist air coming from the Gulf of Mexico. This warm air was easily able to advect into the mid levels.
Moving into the day on the 24th, a very strong and now negatively tilted trough moved into the vicinity of the low-pressure center shown in Figure 34. There was plentiful divergence in the upper levels allowing continuity to occur allowing surface convergence to increase. The approaching jet streak put central Oklahoma in the left exit region of the jet streak where the low pressure was located. The added cyclonic vorticity advection and ageostrophic circulations through the Sawyer-Eliason Circulation allowed plentiful rising motion to occur by causing an imbalance in the thermal wind as shown in Figure 35. The added divergence enhanced the continuity (continuity diagram in Figure 36), and a tightening of the temperature gradient allowing the warm air advection forced the air to move upwards. This all combined to make the low-pressure center stronger.
Figure 34: Conceptual model as the approaching trough had plentiful upper level divergence, positive vorticity advection, and a potent jet streak putting the low pressure right in the left exit region.
Figure 35: Conceptual model of the jet streak causing cyclonic vorticity advection in the left exit region, and the jet streak creating ageostrophic flow around the exit region of the jet streak known as the Sawyer-Eliason Circulation. This enhanced the rising morion, and combined with the upper level divergence in the 500mb level, greatly enhance the low pressure center.
Figure 36: A model showing continuity. The surface convergence and increasing upper level divergence forced rising motion.
VI Conclusion
The sudden strengthening of a center of low pressure centered over Oklahoma on May 24th, 2011 was due to a number of ingredients that came together creating an environment very favorable to cyclogenesis.
Beginning in the morning of May 24th, a very strong trough digging east-southeast had with it a very strong jet streak rounding the base of the trough. The trough is what brought together all of the ingredients needed to form a very powerful low-pressure center.
Using the Quasi-geostrophic equations, it is very easy to decipher what made the low pressure deepen. Starting with the QG Omega equation, the upward motion was attributed to the positive vorticity advection over the area that was increasing with height and the warm temperature advection that took place in the mid levels. The warm air advection raised the heights aloft by increasing the thickness, which created a thermal wind imbalance between the coriolis and the pressure gradient. In order for the atmosphere to get back into a balance, there had to be divergence in the upper levels, which increased the continuity. In general with the QG Omega equation, warm air advection must move the air upwards creating a stronger low pressure.
The approaching jet streak put the low pressure in the left exit region where the ageostrophic circulation, called the Sawyer-Eliason Circulation, occurred right over the low-pressure center. This circulation caused rising motion over the low pressure. Finally, in regards to the QG Omega Equation, the jet streak caused cyclonic vorticity advection over the low pressure to form. This positive vorticity advection, increasing with height, caused vertical motion.
Moving on to the QG Height Tendency Equation, this follows the hypsometric equation in which heights rise above an area of warm air advection, and the heights fall below the area of warm air advection. So as the differential temperature advection in the mid levels (700mb) occurred above the low pressure at the surface, the thickenness of the column of air increased through thermal expansion, which rose the 500mb surface and sunk the 1000mb surface helping make the low pressure stronger.
Finally moving onto the QG Vorticity Equation, the increased vorticity advection over the surface low pressure had a great impact on the surface low. The vorticity over the surface low was stretched upward by the omega (rising motion) causing it to spin faster resulting in a stronger low-pressure center.
Without the approaching trough and powerful jet streak, the intense strengthening of the low pressure would not have occurred. Without the strong jet streak, it is very likely that this low-pressure center would have barely strengthened. The jet streak created the imbalance in the thermal wind, which in return created the upper level divergence, which cause rising motions through continuity. The jet streak was the source of the vorticity advection, and without it, the low pressure would not have grown so quickly, and the rising motion and stretching would have been minimal. The jet streak was also the source of the warm air advection into the mid levels creating the lowering of heights near the surface causing the low pressure to strengthen. The warm air advection created the larger thickness through the hypsometric equation. The fall in the heights over the pressure surface near the ground is accompanied by the fall in pressure at the ground.
As the system weakened, it was due to the weakening trough and jet streak. Without the jet streak providing the rising motions and strong vorticity advection, the low pressure began to weaken. The lack of differential temperature advection reduced the height rises in the upper levels and the lowering of heights near the surface. Without the upper level support, the low pressure began to subside and dramatically weaken only days later.
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