Some Effects of Vertical Wind Shear on Thunderstorm Structure *

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HORACE R. BYERS and Louis J. BATTAN U. S. Weather Bureau, Thunderstorm Project

ABSTRACT

Observations of thunderclouds obtained with a 3-cm height-finding radar set are used to obtain a description of the vertical shear of thunderclouds. Several photographs are given which show the shearing of the radar clouds. A scattergram of wind shear plotted against echo shear is presented and shows that the two variables are related, with the former exceeding the latter in almost all cases. Scatter-diagrams are given which verify that strong vertical wind shear tends to restrict the growth of thunderstorms. A series of radar cross sections illustrates the displacement of the upper part of a thundercloud which is subjected to wind shear, and the growth of another cloud column from the lower part of the thundercloud.

1. INTRODUCTION 2. SOURCE OF DATA

HE primary aim of the Thunderstorm Prior to the utilization of radar for meteorologi- Project has been to obtain a precise de- cal observation, it was difficult to obtain the type Tscription of the thunderstorm. In a recent of cloud observational data which would permit paper Byers and Braham have given a detailed dis- the measurement of the dimensions of most cussion of the structure and circulation of a typical thunderstorms. When isolated cumuliform clouds thunderstorm [1]. The proposed model, however, develop during the daytime, it is possible to take assumes that there is no vertical wind shear. photographs of a particular cloud; however, when there are many low clouds or the visibility is poor, When this simplifying assumption is not made, a photographic techniques using visible light become description of the thunderstorm becomes more unfeasible. This type of data permits qualitative complex. It is the purpose of this paper to demon- determinations of cloud behavior, but quantitative strate some of the effects of wind shear on the calculations are often difiicult and in many cases structure of the thunderstorm. impossible. Since the thundercloud is not a rigid body but By using a radar set with a range-height- rather consists of a volume of small particles (ex- indicator (RHI), it is possible to obtain direct cept when hail is present), having little mass and measurements of the horizontal and vertical extent subject to displacement in the wind field, it is of the thunderstorms as detected by the equip- reasonable to expect that if, for example, the wind ment [9, p. 3-6]. The characteristics of radar as flow in the upper levels is from the north while an instrument for observing clouds are such that that in the lower layers is from the south, the there is closer correspondence between the radar upper part of the cloud will move towards the cloud and the actual cloud during the developing south relative to the lower part and may eventually rather than during the dissipating stages of the blow off if it does not evaporate before this occurs. thunderstorm. This results from the fact that dur- Meteorologists have long recognized that pro- ing the former, a large quantity of water droplets nounced vertical shear often inhibits the growth and frozen particles are being carried to the higher of cumulus into thunderstorms. When the wind portions of the cloud, as well as being held aloft shear varies continuously in magnitude and is con- within the cloud, while in the latter case the ab- stant in direction from the base to the top of the sence of strong updrafts permits the precipitation thunderstorm, the axis of the cloud will have a of the large hydrometeors to the lower levels, leav- tendency to bend or tilt in the direction of the shear ing only the smaller particles in the upper part of the cloud. The progressive depletion of the larger vector [2, p. 7]. If the water content and size scatterers results in a radar echo which tends to distribution of hydrometeors are such as to permit "fade out" at the edges rather than cut off sharply. radar detection of the cloud for a relatively long Since it is improbable that the reflectivity of the period of time, the shearing of the storm can be surface layers of the clouds is sufficiently high to seen on a radar 'scope. produce a detectable echo on the scope of conven- tional "S"- and "X"-band radar sets, the radar * Published Report No. 8 of the Thunderstorm Project.

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Thoroughout this study the effects of rain attenuation were considered only in a most qualitative fashion. Although it is realized that in some cases the magnitude of the attenuation was large, the unavailability of power measurements and records of the settings of the radar controls made the determination of accurate corrections impossible.

3. QUALITATIVE DEMONSTRATION OF RADAR CLOUD SHEAR When the wind shear is strong along the azimuth of an RHI cross section through a thunderstorm, the effects of the shear become very evident. FIGURES 2a and 2b are typical examples of such observations. The cross section of FIGURE FIG. 1. Drawing of data available on each photograph 2a was towards an azimuth of 219° while that of of the 'scope of Radar Set AN/TPS-10. The vertical FIGURE 2b was towards 353°; they were photo- lines are 10 miles apart; the nearly horizontal lines are graphed within nine minutes of one another. No 5000 feet apart. upper wind data were available in the immediate vicinity of the thunderstorms; however, the winds cloucl will be smaller than the actual cloud in al- aloft report from Huntington, West Virginia * most all cases even during the developing stages.f (2200Z) indicated that between the levels of 10,- The data used in this study were extracted from 000 feet and 25,000 feet the vertical wind shear the photographic records of the range-height- was towards 170° at 15 mph. Although the re- indicator (RHI) 'scope of the Radar Set AN/ lease was made six to seven hours later than the TPS-10 16]. As the name implies the 'scope pre- radar observations, it is felt that the synoptic situa- sents slant range (straight line distance between tion was such as to permit the use of this wind for radar set and the target) horizontally against a qualitative comparison. It can be seen that the height vertically (FIG. 1). To facilitate the meas- bending of the storms is in the proper direction urement, an overlay was made on which were if the components of the shear along the respective drawn lines of equal horizontal distance from the azimuths are calculated. radar site. From the available RHI 'scope photographs, it was possible to obtain a number of vertical cross t See References [3], [4] and [5] for detailed discus- sions of radar-weather theory. * Approximately 110 miles southeast of the radar site.

FIG. 2. Photographs of the RHI 'scope showing the tilting of radar clouds which are located at azimuths from radar site which differ by about 135 degrees. (Fig. 2a, left; Fig. 2b, right.)

Unauthenticated | Downloaded 10/10/21 04:26 PM UTC 17 0 BULLETIN AMERICAN METEOROLOGICAL SOCIETY sections through the same thunderstorm. Since azimuth as well as in feet to make it directly com- the vertical "slices" were at intervals of three to parable to the vertical axis. The azimuth of 174° four degrees of azimuth and about two seconds in was selected as the reference from which to lay time, it was possible, by assembling the data ob- off the scale in feet since it was close to the center tained during one azimuthal scan of the radar an- of the storm. tenna through the storm, to construct a three- This thunderstorm occurred on August 25, 1947 dimensional model of the radar cloud. FIGURE 3 and was located about 30 miles from the radar site. presents a series of approximately east-west cross The vertical wind shear computed between the sections obtained by extracting the necessary data levels of 15 and 30 thousand feet at about the from the three-dimensional drawings. The dashed same time as the cloud observations indicated a lines are isolines of equal radar cloud thickness (3 component towards the west of 6 mph. The ef- miles) measured into the plane of the paper. From fects of the wind shear in blowing off the top of the FIGURES given it is impossible to determine the the thunderstorm are strikingly illustrated. An- extent of any portion of the radar cloud on either other interesting feature which is worthy of men- side of the central east-west vertical plane. The tion is that in this case the dissipation of the radar horizontal axis has been labeled in degrees of cloud proceeded most rapidly at the middle and

FIG. 3. Series of approximately east-west cross-sections through a radar thundercloud. The azimuth scale is in degrees from north relative to the position of the radar site. The time is indicated in hours, minutes, and seconds.

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probably be explained by similar wind shear con- siderations [7, p. 67].

4. MEASUREMENTS OF VERTICAL CLOUD SHEAR In order to show the relationship between radar cloud and wind shear, the vector difference between the wind velocity at two selected altitudes and the rate of horizontal displacement of the radar cloud at the upper altitude relative to the lower were measured for a number of storms and corresponding values were compared. The tech- nique used for calculating such rates of displacement was the following:

1. A radar cloud was chosen which extended to about 30,000 or 35,000 feet, had well defined outlines and, if possible, was of strong intensity.

2. At time the horizontal distance Rn from the radar site to the axis of the selected cross section FIG. 4. Photograph of PPI 'scope with an off-center at altitudes H1 and Ho was measured and the dif- presentation taken on August 14, 1947. Radial lines are ference between Rh2 and Rhl, Ra was determined. 10 degrees apart; concentric curves are 10 miles apart. to Note the high intensity and sharp outline of northwestern 5. At time the horizontal distance to the axis part of the large echo, while the southwestern end is more of the same vertical cross section of the cloud at diffuse and of weaker intensity. altitudes H1 and Ho was measured and Rh was determined. lower levels. The minimum of echo width at about 4. The magnitude of the rate of displacement of the 15,000-foot level was due in part to the mal- the cloud at altitude Ho relative to H1 along the function of the radar set. selected azimuth, Vc, was then calculated by The effects of wind shear have often been seen using: t on the PPI scope of the V-Beam, a 10-cm control radar set [6]. This piece of equipment has a wide vertical beam width which extended higher than most thunderstorms. FIGURE 4 is a sample of the During the early stages of the thunderstorm photographs taken on August 14, 1947, a day on when the RHI echo had a relatively small diameter which there was relatively strong wind shear to- (1 to 2 miles) and was intense and sharply de- wards the southwest between the levels of 10,000 fined, the measurements of horizontal distance and 35,000 feet, the latter being below the maxi- were considered as very reliable but in the later mum heights to which fully developed thunder- stages when the echo began spreading out and be- storms extended. The upper part of the storms came weak and diffuse in character, it was difficult subjected to this displacement force were carried to delineate its outlines and measurements were towards the southwest. Since the displaced por- regarded as uncertain. Another factor which con- tion of the cloud intercepted only a fraction of the tributed to uncertainty in some cases resulted from beam, and since the larger hydrometeors were the fact that the echoes constantly changed in size falling out of this part of the cloud and probably evaporating, it would be expected that the south- and shape due to other phenomena than wind shear. western edge of the echo would be less intense than This may have been significant when the latter the rest of the echo. The photograph, in fact, does effect was small relative to the former. Another indicate this distribution of intensity. The crew difficulty was introduced because successive cross- of an airplane which flew through one of these sections through the same thunderstorm were not storms observed a shelf of clouds on the south- always through exactly the same part of the western side. On one traverse the airplane flying thunderstorm. at 15,000 feet reported light snow falling from this t Strictly speaking, the radar cloud shear is given by deck of clouds extending out from the storm. Vc/ (Hz — Ht) ; however, in this paper the vector differ- The not infrequent number of similar reports of ence of the velocities at two altitudes is referred to as the rain, snow, and hail outside the thundercloud can "shear between the two levels." This practice is not un- common among meteorologists.

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The vector difference of the wind velocity be- determined. The average was used because in tween the altitudes and H2 was computed from most cases each wind record yielded different re- hodographs using the winds measured by the sults, and it was felt that the average would be winds aloft stations of the Thunderstorm Project more representative than any single value. at approximately the same time as the radar cloud FIGURE 5 is a scatter diagram of the calculated observations (within one hour in all cases except radar cloud velocity at altitude H2 relative to H1 one where the time difference was about two along a particular azimuth, plotted against the hours).* If a thunderstorm, as indicated by the component of the vector difference of the wind PPI 'scope of the control radar, was located over velocities at H2 and H1 along the same azimuth. or close to a station which made a balloon release The data were obtained using 12 storms and select- at the proper time, the wind record from that sta- ing two levels between which reasonably reliable tion was not used since it has been found that the shear measurements could be made. In those circulation set up by the storm results in a distor- cases where the rate of change of echo geometry tion of the normal wind field in its vicinity [8]. and intensity were not too high, it was possible to The magnitude of the component of the vector dif- use relatively long time periods. The numbers ference of the wind velocities between the two plotted above the dots give the altitudes between selected altitudes in the same direction as the which measurements were made; those below the velocity of the cloud at the higher altitude relative dot give the time in minutes between the two ob- to the lower along a particular azimuth was calcu- servations used in the computation. The distribu- lated from each wind record, and a mean value was tion of the points indicates that a definite relationship exists between the wind shear and the result- * All storms studied were located within 30 miles of center of wind network with one exception which was ing radar cloud shear between the same levels. It about 70 miles away. is also evident that in almost every case the wind shear is greater than the cloud shear. The reason for this lies at least partly in the fact that the cloud tends to build on the side of the cloud upstream along the shear vector, since the updrafts within the cloud attempt to rise vertically while the ex- isting cloud is carried downstream. Another contributing factor suggested by one of the Project analysts after a study of the diurnal variation of the vertical wind distribution is that the updraft transports air of smaller horizontal momentum from the lower to the upper levels, thereby re- ducing the horizontal velocity of the upper parts of the thundercloud.

5. WIND SHEAR AND THE THUNDERSTORM'S VERTICAL EXTENT

It has been suggested that when strong wind shear exists, no thunderstorms can develop. It is difficult to state with assurance the values of wind shear required to prevent thunderstorm formation ; however, the available data indicate that its magnitude has an effect on the maximum heights reached by thunderstorms. FIGURE 6a is a scatter diagram of wind shear between the levels of 5,000 and 30,000 feet against the maximum height attained by any radar thunderstorm during the same day. FIGURE 6b presents the wind shear against FIG. 5. Scatter diagram of vector difference of cloud the mean maximum height reached by any of the velocities at altitudes H2 and Hh along a particular echoes on the same day. The wind shear values azimuth plotted against the component of the vector differ- were obtained by taking the available winds aloft ence of the wind velocities at altitude H2 and Hx, along the same azimuth. The correlation coefficient is 0.90, and records for a particular day of the Project sta- the equation of the regression line is Y — 6.13 + 1.07X. tions ; obtaining the vector average of the wind

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in all cases. For this reason the mean heights are probably more significant. 2. The number of wind observations was not the same on every day considered. 3. On some days the vertical wind shear between intermediate levels was of the same order of magnitude but different in direction from that between the altitudes of 5,000 and 30,000 feet. 4. The meteorological factors influencing convection were not the same every day.

If thermodynamic conditions are suitable for strong convection, the answer to the question as to whether thunderstorms will occur under certain wind shear conditions depends upon the relative magnitude of many factors, the most important of which appear to be the following:

a. The horizontal cross-sectional area of the updraft within the storm, b. The magnitude of the updraft velocity, and c. The magnitude of the wind shear.

If the first two factors are small while the last is large, the tops of the clouds will be blown off as soon as they start to penetrate into the zone of strong vertical shear. This phenomenon is often FIG. 6. Scatter diagram of the mean of the vector dif- observed during the early part of the convection ference of the wind velocity between the levels of 5 and 30 period when the cumulus clouds are small in size thousand feet, plotted against (a) the maximum height and have relatively weak updrafts within them. attained by any radar cloud during the convection period, However, when the updraft has a large cross- and (b) the mean maximum height of all echoes which sectional area and is of a strong magnitude, it can extended over 25,000 feet on each day. The encircled dots are those which extended off the top of the 'scope and cause the cloud to extend a considerable distance were corrected by adding 1000 feet. Most appeared to into the shear zone before the upper part of the extend considerably higher. Height value of underlined cloud is displaced to such an extent relative to the dot subject to question. lower that it is cut off from its source of energy, the updraft. If all other factors were constant, shear between 5,000 and 30,000 feet of the stations then the higher the rate of growth of the cloud which made a simultaneous release; and then tak- relative to the wind shear, the greater would be the ing an arithmetic mean of the several vector aver- maximum height attained by the storm. Also, if all other factors were constant, the greater the ages available during the thunderstorm period. cross-sectional area of the updraft, the longer These levels were selected so that the lowest was would be the time before the updraft is completely above the mean cloud base and the upper was be- cut off from the upper part of the cloud and con- low the maximum heights attained. The figures sequently the higher it would eventually extend. indicate that even with an average wind shear in Once the updraft has been removed, the hydro- excess of 20 mph between 5,000 and 30,000 feet, meteors which were suspended, or in the process thunderstorms did occur. They also show a tend- of being carried upward, will begin to fall to the ency for the strong wind shear to restrict the ground and the displaced part of the storm will maximum height reached by thunderstorms.* The begin to dissipate. scattering may be attributed to several causes: It has been assumed in this discussion that no 1. The length of time of the radar observations strong inversions, dry layers or other convection- and the area of radar coverage were not the same suppressing conditions are present to prevent the * It was noted from a study of the noise level on the RHI 'scope growth of the thunderstorm; otherwise, the maxi- that the values of mean and maximum vertical extent of echoes were influenced by settings of the gain control of the radar set. It appears, mum height attained by the clouds is largely de- however, that the differences of the radar sensitivity did not entirely account for the day to day variations of radar-cloud heights. termined by factors other than wind shear.

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6. VERTICAL SHEAR AND THUNDERSTORM parently the same lower part of the cloud and EVOLUTION forms a new top. FIGURE 7 presents a series of As has been pointed out in earlier paragraphs, RHI cross sections along an azimuth of about under certain conditions it is possible for the upper 180° through a storm which occurred 011 August part of the thunderstorm to be tilted a considerable 17, 1947 which illustrates this idea. The vertical distance from the vertical after it has been exposed wind shear between 15,000 and 40,000 feet at about to wind shear for a substantial period of time. It the same time had a component of 21 mph towards has been observed 011 some occasions that after the 180° and was strongest between 30,000 and 35,000 original top of the storm has been displaced, a new feet. The progressive displacement of the upper radar cloud column builds up from what is ap- part of the cloud can be followed from one time to

FIG. 7. Series of photographs of the RHI 'scope showing vertical cross-section through a particular thundercloud.

Unauthenticated | Downloaded 10/10/21 04:26 PM UTC VOL. 30.. No. 5, MAY, 1949 175 the next. At 140951 (hours-minutes-seconds) the Workman, Holzer, and Pelsor [10, p. 26] pro- new core can be seen to extend to about 36,000 posed that the ice crystals from the displaced top feet as a strong echo. At this time the original top of the cloud would fall among supercooled water is dissipating. At 141045 the existence of two droplets and result in the formation of another up- main upper columns or cores is very apparent; the draft when heat of fusion is released during the newly developed one which has been sustained by growth of the ice crystals at the expense of the the updraft being the more intense. By 141216 water. Although this process is possible, it could the original upper part of the thunderstorm echo not be detected among the cases analyzed for this has dissipated somewhat and at the same time has study. subsided. One would expect that a pilot flying In addition to contributing to the initiation of the beneath the displaced portion of the cloud could downdraft, the vertical wind shear may also be the encounter precipitation even though flying outside driving mechanism whereby the hydrometeors the cloud at his level provided the time of flight being carried aloft by the updrafts are displaced was before the dissipation had progressed too far. towards the downdraft region of the cloud from That this actually occurs has been verified by which they fall as precipitation [2]. In this regard pilots' reports of all forms of precipitation fall- it should be realized that the direction of the wind ing from shelves of clouds extending out from shear vector obviously need not be in the same thunderstorms. direction as the direction of cloud movement. The above is a good example of a process which has been observed to occur when suitable condi- 7. ACKNOWLEDGMENTS tions prevailed. On some occasions the process The authors wish to thank Roscoe R. Braham, has not been as well defined; nevertheless, the Harry Moses and Chester W. Newton for their evolution of the thunderstorm can be followed. It helpful suggestions during the preparation of this is possible under the proper conditions for a paper. thunderstorm to have more than two different tops before complete dissipation takes place. 8. REFERENCES An important consequence of this process sug- gests itself, that it may be instrumental in the [1] Byers, H. R., and Braham, R. R., 1948: Thunder- storm Circulation and Structure, /. Meteor., Vol. initiation of the downdraft in a thunderstorm. The 5, No. 3. theory which is most widely accepted at the pres- [2] Bjerknes, J., 1940: Memorandum on Local Summer ent time proposes that the descending flow is Storms, U. S. Engineers Office, San Francisco, Calif. started when the large suspended hydrometeors [3] Atlas, D., 1947: Preliminary Report on New Tech- fall through a weak portion in the updraft and niques in Quantitative Radar Analysis of Rain- frictionally drag the surrounding air along. When storms, USAF Air Materiel Command. [4] Wexler, R., and Swingle, D. M., 1947: Radar Storm vertical wind shear is present, it is easily seen how Detection, Bull. Am. Meteor. Soc., Vol. 28, No. 4, the hydrometeors in the upper part of the cloud 159-167. may be displaced away from the updraft, after [5] M.I.T. Department of Meteorology, Weather Radar Research, 1946: First Technical Report Under which they will begin to fall under the influence of Signal Corps Contract. the gravitational force. The falling particles may [6] U. S. Weather Bureau Thunderstorm Project, 1947: drag the surrounding air by viscous forces and Operation and Activity of the Thunderstorm Project April 1-October 1, 1947. Report No. 3 thereby initiate the downdraft. If they fall out- to Chief of U. S. Weather Bureau. side the cloud, evaporative cooling and consequent [7] Hydrometeorological Section, Office of Hydrologic increase of density would contribute to the suste- Director, 1947: Thunderstorm Rainfall, Hydro- meteorological Report No. 5. nance and strengthening of the downdraft. The [8] Byers, H. R., and Hull, E. C., 1948: Inflow Pat- downward momentum may be transferred in all terns of Thunderstorms as Shown by Winds Aloft, directions, resulting in a reduction of the strength Bull. Am. Meteor. Soc., Vol. 30, No. 3, 90-96. [9] U. S. Weather Bureau Thunderstorm Project, 1948: of the adjacent updraft thereby permitting the A Report on Thunderstorm Conditions Affecting heavier liquid and frozen particles to fall through Flight Operations, Weather Bureau Technical this weakened updraft. As this process continues, Paper No. 7. r [10] Workman, E. J., Holzer, R. E., and Pelsor, G. T., it is possible for the downdraft to propagate across 1942: The Electrical Structure of Thunderstorms, the thunderstorm. NACA Tech. Note No. 864.

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