STORM-SCALE STRUCTURE OF THE PAMPA STORM
by Bruce Jensen, Mark Mabey, Tim Marshall, and Erik Rasmussen
(Reprinted from the 13th Conference on Storms, Tulsa, Oklahoma, October 1983, Severe Local p. 85-88.)
During the evening of 19 May 1982 a severe thunderstorm produced eight tornadoes in the vicinity of Pampa in the Texas Panhandle. The storm was observed by several storm "chasers" and two intercept teams from Texas Tech University. The tornadoes occurred in cyclic fashion with each tornado forming at about the same time as, or prior to the dissipation of the previous tornado. Tornado-genesis apparently occurred above the location of the surface gust front associated with the previous tornado and near the mesofront separating the cooler air near the precipitation core from the warmer inflow air.
This paper combines visual observations of cloud features and tornadoes below the updraft region of the storm with intercept team measurements of dry and wet bulb temperatures, pressure, wind direction and estimates of wind speed. These measurements document the locations and evolution of mesoscale frontal systems associated with the tornadogenesis and evolution.
Several features of this storm are particularly unusual. Intercept teams commonly observe cloud-free slots (which may or may not be filled with precipitation) to propagate cyclonically around wall clouds as tornadoes descend to the ground. These slots were not present with some of the Pampa tornadoes. Also, this storm produced at least three anticyclonic tornadoes. In all cases these were associated with nearby cyclonic circulations. The anticyclonic circulations ranged from a very thin eddy rotating about a major cyclonic wall cloud, to a major meso-anticyclone and updraft several kilometers south of a cyclonic maxi tornado.
2. MESO-BETA SCALE OBSERVATIONS
The initial thunderstorm activity formed in the area just northeast of Amarillo along a dryline. Intercept team visual observations of the line of scattered towering cumulus along the dryline suggest that a dryline bulge developed in the area south of Amarillo just after the first storms began. The most severe storms occurred ahead and just to the northeast of the bulge. Another possible triggering or organizing mechanism vas weak upslope components in the surface wind near the Caprock escarpment.
Notes provided by the Amarillo office of the National Weather Service indicate that the first intense storms formed northeast of Amarillo near Borger, and that the Pampa storm subsequently developed about 30 km further south. The initial echo movement found by plotting regular and special radar observations was from about 200 degrees, but the storm turned sharply to the right after the first hour and in the second hour began moving from 270 degrees. Radar and field observations indicate that part of this change in movement was due to propagation as the storm incorporated discrete new updrafts from the south of the main updraft.
The storm was in a stage of cyclic tornado production from 1800 until about 2000 (CDT). After that time the storm appeared to be a supercel1 with a large, steady and continuously propagating updraft. It should be noted that theta-e values computed for the surface inflow toward the main updraft on two occasions compare most favorably with values for regular observing stations in SW Oklahoma, which suggests that there may have been a cyclonic low-level circulation on the meso-beta scale transporting air from the eastern Texas Panhandle and western Oklahoma toward the storm.
3. MESO-GAMMA SCALE EVOLUTION
The analysis of events on the meso-gamma scale in the Pampa storm is based on a variety of data sources. First, the two intercept teams made observations of dry and wet bulb temperature, pressure, and wind. The temperature observations were made using an aspirated psychrometer, the pressure readings with a highly accurate aneroid barometer, and wind speed and direction were estimated by experienced intercept personnel. The pressure readings were corrected for elevation changes by using the known vehicle positions and 1:24000 scale topographic maps. These values are correct to the nearest .01 in (.3 mb). Additional detailed information was taken from the two vehicle logs and tape transcriptions. A few supplementary wind observations were made by observing the motion of grasses in movies made at various sites. In addition, the observations of two other experienced storm chasers have been utilized (they are Neal Rasmussen of Norman, OK and Jim Leonard of Miami FL). Including all of these sources provides about 40 wind, 25 pressure, and 15 temperature observations, and many more observations of cloud structure, movement, and weather.
The observations are too numerous to present in detail here. Instead, conceptual drawings of the low-level flow and frontal structure combine all of the pertinent observations. Figure 2 is a map showing some enroute observations and details of the damage paths of the eight tornadoes. Figure 4 is a series of maps on the same scale used in figure 2 which detail the meso-gamma scale evolution. Shown are highways and the outline of Pampa (heavy lines) and streamlines and frontal positions at various times. In figure 4 the cold front symbol denotes the gust front position along the leading edge of a spreading rear flank downdraft (Lemon and Doswell, 1979). The stationary (or warm) front symbol represents the mesoscale frontal system separating cool air near the precipitation core from warm inflow air.
Prior to 1823 CDT a vigorously rotating wall cloud had been in existence for about 50 min. The first tornado formed Just west of the wall cloud and was anticyclonic. Although the intercept teams were not close enough to observe cloud motions, it is possible that a band of strong northerly winds began wrapping around the wall cloud and that the tornado formed in the region of anticyclonic shear Just west of the wind band. The tornado was fairly narrow and shoved strong descending motion on one side of the funnel and rising on the other.
Figure 4a shows the apparent flow pattern at 1823. The second tornado had Just reached the ground, but the gust front must have already surged ahead of the tornado by that time because it arrived at the filming site at 1830. A north-south band of lower clouds advanced away from the wall cloud and toward the filming site and may have been above the gust front. The surface wind on the west side of Pampa was strong easterly and the temperature was 76 degrees F which implies that the meso-warm front was to the south.
At 1830 (fig. 4b), the gust front reached the west side of Pampa, while the tornado was moving slowly north and northwest, apparently being slightly off-axis in the parent mesocyclone. The gust front passage was preceded by a 3mb/ 1-min rise in pressure about 3 min before passage. Winds became light southwesterly and then calm, possibly indicating that this particular gust front was weakening. The flow pattern illustrated is supported by apparent motion in movies shot of tornado number 2. A band of dust moved steadily away (toward the ENE) from the tornado and eventually extended for over a mile. It appeared to be advancing along the edge of the rain core. At about 1830 a new wall cloud formed just NI4 of Pampa and must have been within 1.5 km of the gust front position.
Tornado t2 continued as the new mesocyclone developed rapidly at the 1~ edge of the city. The flow pattern illustrated is substantiated by several observations made by one intercept team. This vehicle traveled north to a position immediately SE of the developing wall cloud were sustained at about 50 kt from the ESE. After placing a sound recorder at that site the team retreated south and experienced winds from the WNW at an estimated 55 kts with gusts to around 70 kts several minutes later. This surge of air implies that the gust front surged eastward as the new tornado was developing. In the figure, two gust front positions are plotted; the eastern-most position is the dissipating gust front associated with tornado number 2, and the front Just west of it is the vigorously surging gust front associated with the development of tornado 13.
By 1845, one intercept team had moved to the east side of the city while another was moving around the south side. At this time, widespread damage to trees, power lines, windows, etc. was occurring as the gust front moved through Pampa. Moderate south winds were observed south of the gust front, while strong east winds continued north of the meso-warm front. Also note that tornado t2 was still in progress, tornado t3 was newly formed, and a new wall cloud was observed over the NE side of the city.
Ten minutes later, at 1855, a major cyclonic tornado had formed from this new wall cloud, with tornado number 3 still in progress. Figure 2 shows that tornado number 4 followed a broad looping path. It was a single condensation funnel for a few minutes, disappeared, and then reformed as a multiple vortex tornado. Figure 3 shows two photos of this tornado. After turning back to the southeast and becoming surrounded with rain it destroyed the Pampa Industrial Park causing over $3 million in losses. Several other very interesting phenomena were occurring at about this time. The intercept team south of Pampa observed the formation of anticyclonic rotation in an area about 2-~km across in the updraft base south of tornado number 4. Associated with this feature was an anticyclonically rotating funnel extending quasi-horizontally to the south. At about the same time the vehicle east of tornado 14 observed another anticyclonic funnel to form near overhead on the east side of the wall cloud. Movies show this funnel to be in a zone of strong anticyclonic shear vorticity outside a band of locally very strong cloud-base winds which were propagating around the wall cloud cyclonically. Although the anticyclonic funnel south of the wail cloud was embedded in a larger anticyclone, it also appeared to be near this shear zone. The weak low-level anticyclone illustrated is partially supported by an observation of light easterly winds near the same storm-relative location a few minutes earlier. A 2-mb pressure deficit was also measured in this area well south of tornado 14, which probably indicates the development of strong updrafts in the area of the anticyclone. Also note that another new wall cloud was forming at 1855 EKE of tornado 14.
Tornado number 5 was a small anticyclonic tornado that occurred ENE of tornado number 4 as shown in figure 2. By 1909 CDT, the new wall cloud had become well developed and three minutes later a maxi cyclonic tornado formed. Initially the condensation cloud was almost a mile wide at the ground. One intercept team encountered baseball-size hail as this tornado followed them on highway 60. As tornado number 6 moved ENE, the meso-anticyclone was becoming more organized to its south. In the early stages a cloud-free vault was observed to extend veil up into the CB, with scud fragments moving rapidly toward and up into the vault. This area showed obvious anticyclonic rotation, and evolved into a wall cloud, with a clear slot (the first observed) propagating anticyclonically around the north side. To the immediate north of the clear slot was heavy precipitation associated with the hook echo around the south side of tornado number 6. By 1950 a significant anticyclonic tornado formed and coexisted with the cyclonic tornado for at least another 20-min. Tornado number 7 did not become encircled with precipitation as did number 6. Tornado number 8 was a small tornado observed under a northwest storm flank and was not associated with the cyclic series just described.
Other storms have been documented to produce anticyclonic tornadoes in conjunction with cyclonic ones (e.g. Marshall and McDonald, 1982). Several authors have discussed the mechanisms generating rotation in thunderstorms (e.g. Davies-Jones, 1983, and Rotunno and Klemp, 1982). It can be shown that the drawing-up of initially horizontal vortex tubes should produce a cyclonically rotating updraft and anticyclonic vortex downstream of the updraft. However, the smaller anticyclonic vortices in the Pampa storm appeared to be associated with eddies forming on the outside (with respect to the cyclonic vortex axis) of a band of strong flow which extended from the ground up into the cloud base and located behind a surging gust front. This implies that the outer portion of the cyclonic vortices were not irrotational (as in a Rankine-combined vortex), but had regions of substantial negative vorticity. Further, the development of the mess-anticyclone is not easily explained by the drawing up of vortex tubes. A possible mechanism for this development is the converging of vertically oriented negative vorticity by persistent updrafts Just south of the surging gust fronts.
The cyclic nature of this storm is very similar to that of the Tulia storm (Rasmussen, 1982). A surging gust front was shown in that case to be associated with cyclic tornado production, with new tornadoes forming east of ongoing tornadoes as the gust front surged ahead. The overall intensity of the Tulia storm on radar appeared to pulse as new updrafts increased storm strength, and then the downdraft and occlusion process weakened the storm. Unfortunately adequate radar information is not available for comparisons with the Pampa storm.
These are some of the features documented in the Pampa storm through storm intercept observations:
1) Anticyclonic vortices appeared to occur on two scales. Small eddies formed in anticyclonic shear zones near cyclonic wall clouds, and a mess-anti-cyclone formed south of the region of surging outflow.
2) Cyclic tornadogenesis appears to be related to surging behavior of gust fronts. The Pampa storm gust fronts weakened and slowed after surging several kilometers east of a tornado. Then a new surge would occur as a new mesocyclone and tornado developed in the vicinity of the old gust front.
3) Rear flank downdrafts were not evidenced by clear slots in all cases.
We are grateful to Mr. Roy Britt of Richmond, VA for driving one of the intercept vehicles and alloying the use (abuse) of his car. Also, thinks goes to the personnel of the National Weather Service in Lubbock and Amarillo Texas for their continuing support of storm intercept work in vest Texas. Meal Rasmussen and Jim Leonard provided valuable storm observations.
Davies-Jones, R.P,, 1983: The onset of rotation in thunderstorms. Preprints, 13th Conf. Severe Local Storms, Tulsa, Oklahoma (to he published in this volume).
Lemon, L.R, and C.A.-Doswell III, 1979: Severe thunderstorm evolution end Mesocyclone structure as related to tornado-genesis. Mon. Wea., Rev. 107, 1184-1197.
Marshall, T.P. and J.R. McDonald, 1982: An engineering analysis of the Grand Island tornadoes. Proceedings, 12th Conf. Severe Local Storms, San Antonio, Texas, 293-296.
Rasmusen, E.N., 1982: The Tulsa Outbreak Storm: Mesoscale Evolution and Photogrammetric Analysis. M.S. thesis. Dept. Geosciences, Texas Tech University, 180 pp.
Rotunno, R. and J.B. Klemp, 1982: The influence of the sheer-induced pressure gradient on thunderstorm motion. Mon. Wea. Rev., 110, 136-151.