Focus on Forecasting

focus on forecasting Mesocyclone Passes over NSSL Surface Station Edwin Kessler National Severe Storms Laboratory, NOAA 1313 Halley Circle, Norman, Okla. 73069 Abstract author's home near Criner, Okla., in support of a study of wind energy. An apparent mesocyclone 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 winds were light and variable, widely scattered mild Criner, Okla. Peak gusts were 18 m s-1 (35 kt); within 3 min the surface 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 thunderstorm whose anemometer 16 m above ground, on the same tower that peak intensity occurred about three hours before the cyclone 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 clouds moving rap- available for operation as a mesonetwork in Oklahoma dur- idly from the southeast, and there were a few small rain- 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 Unauthenticated | Downloaded 10/07/21 12:07 PM UTC Bulletin American Meteorological Society 1381 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 microbarograph in a standard shelter 20 m southeast of the wind generator 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 pressure maximum is higher than the second maximum, which occurred shortly after the minimum pressure. Also, the maximum 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 mesocyclone 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 temperature 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. Unauthenticated | Downloaded 10/07/21 12:07 PM UTC 1382 Vol. 63, No. 12, December 1982 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 pressure. Figure 1 indicates that the period between the wind maxima, when (by definition) the "core" was over the station, 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 cloud overhead, or, at least, a negative potential gradient 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 thunderstorms 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 currents 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)— Unauthenticated | Downloaded 10/07/21 12:07 PM UTC Bulletin American Meteorological Society 1383 FIG.

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