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Mesocyclone Passes over NSSL Surface Station

Edwin Kessler National Severe Laboratory, NOAA 1313 Halley Circle, Norman, Okla. 73069

Abstract author's home near Criner, Okla., in support of a study of energy. An apparent passed directly over a surface station During daylight hours of 13 September, in Oklahoma, sur- that was equipped with both digital and strip chart recorders near face were light and variable, widely scattered mild Criner, Okla. Peak gusts were 18 m s-1 (35 kt); within 3 min the sur- face wind changed from strong southeasterly to strong westerly. The thundershowers drifted from west to east, and high tempera- pressure dropped 5 mb in 52 min and recovered 4 mb in 6 min. There tures were in the mid-30s(°C). Before retiring, in anticipation was only a trace of precipitation. The wind system was probably an of nighttime storms, I turned on the recorder connected to an inertial remnant of circulation formed with a whose anemometer 16 m above ground, on the same tower that peak intensity occurred about three hours before the was holds a wind-driven electrical power generator. I also left observed. open the east-facing bedroom window, in which a fan pro- vided cooling at night. 1. Mesocyclone observations After I had been asleep about an hour, I was awakened by an easterly or southeasterly wind nearly strong enough to Late in the evening of Sunday, 13 September 1981, a mesocy- blow the fan out of the window, and I went outside as a pre- clone passed over an NSSL station with automatic recording caution to observe other more serious weather threats, if any. of weather parameters. Several tens of such stations are Occasional lightning flashes revealed low moving rap- available for operation as a mesonetwork in Oklahoma dur- idly from the southeast, and there were a few small - ing spring periods of intensive observations. The station dis- drops. I went to my garage to observe the recorder, then indi- cussed here has been maintained throughout the year at the cating speeds above 15 m s"1 (30 kt), but during the one or two minutes it took to return to the house, the wind rapidly declined. I stopped and listened. As the local wind became 0003-0007/82/121380-07$05.75 nearly calm, I heard a distant rustle, which became pro- © 1982 American Meteorological Society nounced over a minute or two and then was suddenly mani-

FIG. 1. Wind speed in knots recorded from an anemometer 16 m above ground at the author's home near Criner, Okla. 1380 Vol. 63, No. 12, December 1982

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with wind speed under 5 m s_1 (10 kt), around the time of the wind shift, lasted less than 2 min.1 Figure 2 is a photograph of the record of a microbaro- graph in a standard shelter 20 m southeast of the wind gener- ator tower. The duration of the pressure perturbation ap- pears to be about 90 min, including about 20 min of rapidly rising pressure before an even faster decline. The first pres- sure maximum is higher than the second maximum, which occurred shortly after the minimum pressure. Also, the max- imum rate of pressure fall is less than the rate of rise after minimum pressure. Features of fall and rise of pressure are opposite to those shown by the trace of the very intense meso- cyclone at Newton, Kans., first presented by Ward (1972) and discussed further by Davies-Jones (1982). Figure 3 shows a portion of the high-resolution record made by the equipment that records various parameters auto- matically every minute, about 15 m east of the taller tower. FIG. 2. Microbarograph record made at the author's home. Hori- Wind and corona current are sensed near the top of a tower 7 m zontal lines of intermediate weight represent pressure gradations of tall, and other parameters are sensed in a standard screen. 0.1 in Hg, or about 3.4 mb. The sharp dip in pressure occurred During the approach of the mesocyclone center, the dry bulb around 2200 CST, 13 September 1981. temperature declined about 2°C, while the wet bulb tempera- ture declined about 1°C. These temperatures each rose about 0.5°C after passage of the center. The maximum one-second fested as a strong wind from the west. I realized that there wind speeds at 6.5 m height during each minute reach a peak had just occurred a more pronounced wind shift than I had about 5 min before the time of lowest pressure. The average ever before witnessed. speed during each minute shows a clear minimum at 2208 Figure 1 shows the record of the anemometer at 16 m CST, contemporaneous with the deep minimum in Fig. 2. height. The peak gust is 18 m s_I (35 kt), the period of easterly and southeasterly winds identifiable with the weather entity is about 40 min, and the period of associated winds with a 1 The minimum of 2.5 m s 1 (5 kt) must have been greater than the westerly component is about 15 min. The nearly calm period actual minimum, owing to pen drag.

FIG. 3. Transcription of digital data recorded by an automatic station at the author's home. Data are sampled each second, and minute averages are recorded. "Max gust" is the peak one second vajue during each minute, selected from 60 samples of u and v made at 1 s intervals. "Av Speed" isyju2 + v2, and thus typically underestimates the true average by about 1/2 m s"1.

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FIG. 5. Upper air sounding made by National Weather Service personnel at Oklahoma City, 1700 CST, 13 September 1981.

variations. The minimum wind occurred about a minute after the minimum pressure. (In reporting these asymme- tries, the author does not imply surprise. Few atmospheric whirls are symmetrical, and off-sets of wind and pressure centers are an expected consequence of air accelerations in a moving pressure system.) The wind variations suggest a Rankin structure very roughly; in other words, the wind had a prominent maximum some time (distance) from the time (location) of lowest pres- sure. Figure 1 indicates that the period between the wind maxima, when (by definition) the "core" was over the sta- tion, was about 15 min in duration. The automatic station recorded corona current through a steel needle installed at a height of 7 m by Christopher Church, of Miami University, Oxford, Ohio. Church's inter- pretation of the record (Fig. 3) notes that a positive corona current is indicative of net negative charge in the lower region of the overhead, or, at least, a negative potential gra- dient in the region between the cloud and the ground; this is the usual situation beneath an active thunderstorm. A typical value for corona current during with winds of a few meters per second is about 1 /JLA, indicating a potential gradient of 3 to 10 kV m"1. In stronger winds, such a current FIG. 4. Weather maps,0600 CST, 13 September 1981. a) Surface; is sustained in weaker potential gradients. Prior to minimum b) Winds at 500 mb with a contour of 500 mb height. Temperature over the whole region shown at 500 mb was between —5°C and pressure at the station, there was a 15-min period with —10°C. (After U.S. Dept. of Commerce, Daily Weather Maps.) marked negative currents. Since the wind was much stronger during the second period, it seems likely that the potential gradient then was not so large as before. While negative cur- rents are often associated with downdrafts, according to The record of average u and v components during each min- Church, there is little other evidence for strong downdrafts in ute shows that the wind was mostly SE or SSE before the time the situation discussed here. During the 5 min period of min- of lowest pressure, and mostly westerly and WNW after- imum recorded pressure, the corona current was negligible; ward. There was a 2 or 3 min period of NE winds about 9 min this was followed by a 15 min period of moderate positive after lowest pressure. The wind observations, including a current. While there seems little that can be definitely de- moment of near calm, suggest that the center of the mesocy- duced about the specific dynamics of the situation from these clone passed over or very near to the station. data, they support the thought that the minimum pressure The record of average pressure during each minute shows region was devoid of deep convection, and was perhaps the that a pressure fall of nearly 5 mb took place over a 52 min site of gentle subsidence, and the data indicate that major period, and a rise of nearly 4 mb over a 6 min period. The charge variations occurred on a 30 min time scale, corres- pressure fall began about 10 min before the wind began to ponding perhaps to a 19 km (10 n.mi.) space scale. increase, but appreciable northwesterly wind continued 10 to Radar data, discussed further below, indicate that showers 15 min after the pressure rise had given way to irregular small were moving approximately from 290° at 10 m s-1 (20 kt)—

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FIG. 6. Images from the GOES-East Satellite. The arrow in the first frame indicates a very early stage of the disturbance which de- veloped into a thunderstorm reported at Hobart, Okla., as shown in the visual data recorded at 1630 and 1800 CST. Between 1900 and 2000 CST, the infrared satellite data show the disturbahce to become displaced southward and merged with a widespread cloud system which developed along the Red River. we therefore deduce that the overall cyclone diameter was be- south, the wind over Norman was nearly calm. The wind di- tween 41 and 56 km (22 and 30 n.mi.), the former figure cor- rection drifted from east through south and thence to NW, responding to the inferred surface wind system, the latter to from where it blew up to 3.5 m s_1 (7 kt) from 2145 to 2330 the area of pressure influence. The 15 min interval previously CST. Thereafter, the wind became practically calm again. related to the cyclone core is similarly identified with a di- Had the Norman station been significantly influenced by the ameter of about 9 km (5 n.mi.), larger than the 5.7 km aver- mesocyclone directly, the wind in shifting from east to age signature diameter reported by Burgess (1976) but well northwest would have passed through north rather than within the range of his reported extremes. south. The inappreciable influence at the Norman station of There was little or no indication of this mesocyclone sys- the wind system observed near Criner lends credence to its tem in the record made at another automatic station, at deduced small size, based on its probable motion and on its NSSL in Norman, 30 km to the NNE; this is the weather sta- duration at Criner. It also lends credence to the notion that tion closest to that at my home. At NSSL, the winds had been this was a whirlwind and not a squall line. light easterly during the afternoon, declining to nearly calm Evidence from surface locations farther south is lacking, at sunset. For 45 min centered on 2115 CST, there was an en- since there is no weather station for a distance of 240 km, the hanced easterly breeze with some gusts to 4 m s_1 (8 kt). From former FAA reports from Ardmore having been discon- 2150 to 2230 CST, as the mesocyclone center passed to the tinued effective 21 August 1981.

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2. General weather situation Features of the surface weather map for 0600 CST 13 Sep- tember 1981 are weak (Fig. 4a), but the surface wind shows cyclonic curvature over Oklahoma and north-central Texas, and several stations in this region reported towering cumulus clouds. At 500 mb (Fig. 4b) there was a well marked cyclonic circulation over central Oklahoma. Eleven hours after map time and 4 h before the detailed records reported here, the atmosphere over central Oklahoma showed moderate con- vective instability, as illustrated in Fig. 5. The scattered thundershowers previously referred to produced a trace or more of rainfall at 40% of 178 stations distributed across the state on September 13; five stations reported more than 25 mm (1 in) of rain and the maximum amount was43.7 mm(1.72 in) at Gage, in the north-western part of the state. The only damaging reported this date in Storm Data occurred near Antlers, in the southeastern quadrant of Oklahoma.

3. Other evidence from satellite, radar, and local hourly observations Data acquired with the WSR-57 radar at Oklahoma City and from the GOES-East satellite have been consulted to re- late the entity that passed over my house to some larger weather system. Figure 6 shows a series of satellite photo- graphs abstracted from a slightly larger set. The photo at 1530 CST, 13 September, indicates beginnings in far western Oklahoma of a shower system, which is shown to be well developed by 1800 CST. The image of clouds over western Oklahoma at 1530 corresponds to weak radar echoes first observed at about the same time with the Oklahoma City radar at 270°-280°, between 165 and 185 km (90-100 n.mi.) (Fig. 7, 1527 CST). By 1557, radar echoes had intensified at 277°, 88 n.mi. The echo top to the west of Oklahoma City was reported to be at a height of 11 km at 1530 CST. At 1630, the top had increased to 15 km, and the echo on the Oklahoma City radar was then, as shown in Fig. 7, a single prominent cell about 9 km (5 n.mi.) in diameter, with a core intensity corresponding to a rainfall rate of about 25 mm h_1 (1 in h"1). Note the considerable enhancement shown also in the satel- lite photos between 1530 and 1630 CST, Fig. 6. After 1700 CST, some additional small echoes of moderate intensity ap- peared near the first and the cluster drifted ESE while weak- ening slowly. The infrared satellite photographs in Fig. 6, and the radar data, show that the original rather discrete small entity was gradually replaced by an enlarging area of cloud and rain, which intensified over the Red River border

FIG. 7. Portions of photographs of the plan-position indicator of the WSR-57 radar at Oklahoma City, 13 September 1981, made dur- ing development of thundershowers thought to harbour a mesocy- clone, which subsequently drifted eastward. The triangle in photo made at 1732 CST denotes the location of Hobart, Okla. Range cir- cles are at intervals of 46.3 km (25 n.mi.)

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FIG. 8. Photographs of the plan-position indicator of the WSR-57 radar at Oklahoma City, 13 Sep- tember 1981. Range circles are at intervals of 46.3 km (25 n.mi.). The records of surface data discussed here were made at azran 171°, 45.4 km (24.5 n.mi.) shown by the triangle marker on the photo at the left. Pictures at left and right were made with raised antenna; the center picture presents a scan at 0° elevation angle.

area of Texas and Oklahoma during late evening.2 diameter about 9 km (5 n.mi.) ESE of the author's station, At 1630 CST, the significant echo discussed above was likewise embedded in widespread weak echo. Other radar produced by a shower located at azimuth 292°, 181 km photographs at intervals of 5 or 6 min also have been con- (98 n.mi.) with respect to the weather station at my home. sulted. Clearly, the mesocyclone is not associated with any The weather station nearest this shower was at Hobart, well established echo category, nor is any new association about 67 km (36 n.mi.) to the SSE. At 1555, Hobart reported suggested by the radar pictures. Rather, it appears that when towering cumulus NW with a local wind 210°, 2 m s"1 (4 kt). this prominent anomaly in the field of horizontal wind was At 1654, there were towering cumulus in all quadrants about observed at Criner, it was not associated with a correspond- Hobart. At 1745, a thunderstorm began at Hobart (cf. radar ingly intense structure in the field of precipitation, nor of ver- photos in Fig. 7 at 1732 and 1802 CST); there was a peak wind tical velocity through a depth necessarily associated with of 15 m s"1 (30 kt) from the north at 1815 and total precipita- precipitation production. tion was 19.8 mm (0.78 in). If a mesocyclone formed between 1630 and 1815 CST a little northwest of Hobart, its drift to- ward the east or east-southeast at a rate between 8.7 and 10.3 m s"1 (17-20 kt) would have brought it near to or over 4. Concluding remarks my home at 2200 CST. The Oklahoma City radar did not show any echo patterns An asymmetric mesocyclone passed over an automatic re- characteristic of around 2200 CST, the time of cording station near Criner, Okla. The circulation's circle of mesocyclone passage at my home. At 2100 CST, an irregular influence was about 37 km (20 n.mi.) in diameter, the core area of moderate showers was located 55-92 km (30-50 n.mi.) diameter was about 9 km (5 n.mi.), maximum wind was 18 ms_1 south of Oklahoma City, drifting eastward. Maximum echo (35 kt), and the pressure perturbation was 5 mb. intensity in these showers corresponds to a rainfall rate in the It seems probable that the mesocyclone originated three or range 13-25 mm h"1 (0.5-1.0 in h"1). There is no indication of four hours earlier, in a thunderstorm system which at 1800 any significant shower system west of the author's station, al- CST was located near Hobart, 148 km (80 n.mi.) west of though that area is blanketed with weak echo. At 2121 CST, Criner. As this storm weakened after dark, the mesocyclone an azimuthal scan made in part with the antenna elevated which it spawned persisted, as an inertial remnant of the (Fig. 8) shows a moderate echo aloft of nondescript shape, earlier, presumably more vigorous circulation. Imbedded in about 9 km (5 n.mi.) WSW of the author's station, and em- an area of very light precipitation, and overall less than 0.5% bedded in rather widespread weak echo. At 2127 CST, a of the area of Oklahoma, with a central core less than a tenth normal radar scan shows widespread weak echoes but no as large, its identification was fortuitous. Many more such significant radar echoes from my location or within a west- entities must exist than are correctly identified. Together they ward semicircle 65 km (40 mi) in radius. At 2214, the radar represent a substantial noise content in the flow of weather shows a moderate echo aloft only about 5.5 km (3 n.mi.) in observations. They create forecasting difficulties in at least two ways—first, most are unreported until and unless they are strong enough to do substantial damage—unreported 2 Recognition of a small registration error in the satellite data phenomena can not be considered in the forecast; second, a brings radar and satellite data into correspondence. Geographic and political boundaries in the satellite data are shifted northward rela- few make their influence felt in the synoptic and airways ob- tive to their true locations by about 37 km (20 n.mi.). servational data, but that influence, with rare exceptions, dis-

Unauthenticated | Downloaded 10/07/21 12:07 PM UTC 1386 Vol. 63, No. 12, December 1982 torts the larger scale distributions while leaving the smaller discussions; Andrew Horvitz, of NESS's Applications Laboratory, scales essentially undefined. provided the satellite data, and the radar photographs were fur- Some improvement in the detection and measurement of nished from archives at Asheville, N.C. Discussions with Fred San- small-scale vortices and some other entities should follow ders led to substantial improvement of the early draft. deployment of Doppler radars for routine operational use. Fortunately, the weather is usually very good to us!

References Acknowledgment. NSSL's mesonetwork stations were designed by John Carter and are maintained by NSSL staff, including Sherman Davies-Jones, R. P., 1982: dynamics. In Thunderstorm Fredrickson, Robert Goldsmith, Leonard Johnson, Jack Reece, and Morphology and Dynamics, edited by E. Kessler, U.S. Government Jerry Wardius. Joan Kimpel prepared the illustrations, Charles Printing Office, Washington, D.C., pp. 297-362. Clark copied the satellite images, and Evelyn Horwitz typed the Ward, N. B., 1972: A note on the effects of pressure gradients on manuscripts. Several members of the staff at the National Weather fluid flow with atmospheric applications. J. Atmos. Sci., 29, Service Forecast Office at Oklahoma City provided data and helpful 982-984. • announcements1

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