Inflow Patterns of Thunderstorms As Shown by Winds Aloft *

90 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

Inflow Patterns of Thunderstorms as Shown by Winds Aloft *

HORACE R. BYERS and EDWIN C. HULL

B. S. Weather Bureau Thunderstorm Project f

ABSTRACT

Balloons ascending simultaneously at various points around thunderstorms are followed by radar or radio-direction finding (rawins and rawinsondes) to determine horizontal inflow or outflow of the air. It is found that in the early stages of development of a thunderstorm cell, inflow or horizontal convergence is present at the ground as well as all heights reached by the cloud. In a mature thunderstorm cell, the observations show outflow or horizontal divergence under the cloud base and convergence at all heights between about 4,000 and 23,000 feet, with divergence again in the uppermost levels. Relationships to rainfall and to cloud entrainment of environmental air are shown.

NE of the most important phenomena [6] has shown that ordinary cumulus clouds "en- which the Thunderstorm Project f was train" air from the environment. His preliminary O designed to measure was the inflow or thermodynamic treatment has been expressed in outflow of air at various altitudes in and around relatively complete graphical form leading to a thunderstorms [4]. This was accomplished by new conception of convection and thermal sta- establishing ten balloon stations with radar or bility in a paper by Austin [1]. Fluid convection radio-direction-finding equipment concentrated in models designed and operated by Ference, Phillips a small area. By simultaneous releases of the and Fultz [7] show in the laboratory the devel- balloons in the vicinity of or inside thunder- opment of frictionally-driven circulations bringing storms, the horizontal divergence or convergence surrounding fluid into the warm ascending and of the winds at the various levels could be com- cold descending currents in a manner similar to puted by measuring the rate of shrinkage or that of jet streams as described by Tolmien [5] expansion of the areas outlined by the balloon and others. positions. Thus, for example, three balloons The data on the inflow and outflow of air would form the vertices of a triangle. If at any around thunderstorms have been studied from height the area of this triangle were decreasing, the observations made in the Thunderstorm Proj- horizontal convergence would be indicated; di- ect operations in the St. Cloud, Florida, area vergence would be shown by an expansion of area. during the summer of 1946 [8, 9]. The obser- In the circulation of a thunderstorm, these vations consist of rawin and rawinsonde runs horizontal inflows and outflows are only part of made by as many as ten stations within an area the complete picture, involving also the vertical of 120 square miles. The releases were made currents which are well known as being especially simultaneously into a thunderstorm cloud or into characteristic of these storms. In a recent paper its nearby area. The data show the wind direc- [3], by Byers and Braham, the complete three- tion and velocity for each 1000-foot level as well dimensional representation was shown, based upon as the location of the balloon and the time at vertical currents measured from airplanes as well which it reached each level. During all observa- as the horizontal circulation obtained from the tions, the large CPS-1 (MEW) search radar balloons. However, it is believed that the hori- was operated and photographs of the 'scope were zontal measurements are of such significance that made showing the outline and location of the they deserve separate treatment and should be storm echo. The surface data of rainfall, wind, made available in relatively complete form for temperature, pressure and relative humidity were meteorologists to examine. Other recent studies available from the micronetwork of 55 recording have emphasized the importance of and the inter- stations. est of meteorologists in this problem. Stommel Storms and upper-wind runs were selected in which there were simultaneous wind readings 151 Published report No. 4 of the Thunderstorm Project. from three or more stations on two or more f A joint project of the Air Force, Navy, National sides of the radar storm echo. Whenever pos- Advisory Committee for Aeronautics and the Weather Bureau. sible, situations were chosen that included a single

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FIG. 1. Diagram showing wind field about a cumulonimbus cloud, illustrat- ing deformation of wind field by presence of cloud. or isolated thundercloud, in order that all the had been made for a particular level within an effects noted could be ascribed to the single cloud eight-minute period were included. The radar and the results could be more easily interpreted. echoes showing the position of the storms at the Wind vectors for each 1000-foot level were time were plotted on the same chart. plotted with 1 inch equal to 20 mph on a chart It was immediately evident that the normal having a scale of inch to the mile. The origin smooth pattern of the wind field is destroyed in of the vector was plotted at the actual position of the vicinity of the storm (FIG. 1). The winds the balloon rather than at the position of the at a distance from the cloud continue on a con- station making the run. All wind readings that sistent and orderly course, while the winds near

FIG. 2. Map of St. Cloud area showing triangles spanning radar cloud echo, used for computations of convergence and divergence aloft. Lettered stations released rawins, numbered stations released rawinsondes. Triangles used in computations are identified by the station letters and numbers from which the balloons were released.

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FIG. 3. Distribution of convergence and divergence with height as measured for the various triangles which span storm No. 53. (Positive values indicate divergence.) or within the storm have a greatly different direction and speed. This change in the wind field in the region of the storm cloud varies with time and with height and with the age and intensity of the storm. In order to compute and demon- strate the amount and direction of this change for all levels of the wind run, the following method was used. Triangles were drawn with vertices at the ori- gins of the various wind vectors and computations of convergence or divergence were made for the triangles which most nearly spanned the storm (FIG. 2). The computations were made by a graphical method proposed by Dr. John C. Bel- lamy, using the nomogram designed by him [2]. When applied in suitable situations, this method determines the average velocity convergence or divergence over the area enclosed by the triangle expressed as a percentage change in area per unit time where decrease in area corresponds to convergence and an increase in area to divergence. FIG. 4. Generalized convergence-divergence aloft curve Throughout this paper the divergence in the hori- for storms in which rainfall has begun at the surface.

Unauthenticated | Downloaded 09/25/21 10:57 AM UTC VOL. 30, No. 3, MARCH, 1949 93 zontal velocity field is expressed as a percentage rapidly with the maximum rates occurring be- change in area for 12 minutes. A 10% increase tween 15,000 feet and 20,000 feet. In cases in area per 12 minutes is equivalent to divergence where the balloon runs are adequate, the con- of 1.39 X 10-4 sec"1. vergence is shown to continue as high as 22,000 Eight swarm ascents were computed and illus- feet. Above 22,000 feet divergence was found to trated. The graphs of the convergence and di- be present. A generalized pattern for this type vergence aloft for the various storms were of two of storm is shown in FIGURE 4. general types. Five storms, including No. 53 A previous study has shown that the surface (FIG. 3), had very similar patterns with diver- divergence is related to the intensity of the sur- gence present in the lowest levels and marked face rainfall and that rainfall in excess of 0.30 convergence aloft. There is a notable consistency inch per 5 minutes is always accompanied by among similarly-situated triangles in the same divergence in the winds at the surface (FIG. 5). storm and also from storm to storm. The divergence continues at the surface under the The divergence usually extends to 3000 feet storm, at the same time that there is convergence but may extend to 5000 feet. In all of the storms, aloft, as long as the intensity of the surface rain- the rates of convergence are approximately equal fall continues high (FIG. 6). from 4000 feet to 15,000 feet and then increase The remaining three ascents computed were

FIG. 5. Scatter diagram showing relation of values of surface divergence to rainfall intensities. Points corresponding to rates of rainfall less than .03 inches per 5 minutes, were omitted from this diagram.

Unauthenticated | Downloaded 09/25/21 10:57 AM UTC 94 BULLETIN AMERICAN METEOROLOGICAL SOCIETY made into growing cumulus before heavy rain had begun at the surface. They gave values of convergence-divergence aloft which are less consistent among themselves, but they differed mark- edly from the other five in that they did not show the divergence in the lower levels; instead, they showed convergence at all levels including the surface (FIG. 7). The convergence at the surface continued throughout most of the ascent (FIG. 8). The composite curve for these storms (FIG. 9) is similar above 4000 feet to the generalized curve of the other five but the degree of convergence is not as great and the excessive maximum rate at 15,000-20,000 feet is not present. Triangles made up of winds entirely on one side of the cloud exhibit a more erratic and often opposite pattern from simultaneous triangles which span the storm. Those containing one or more winds within the storm also show an erratic and

FIG. 7. Distribution of convergence and divergence with height as measured for the various triangles which span storm No. 69.

inconsistent pattern but this would be expected since the winds within the storm are only the more violent part of the general thunderstorm circulation. If convergence persists from the surface upward, with no evidence of divergence, as shown by FIGURE 8, we conclude that the convergence must accompany an upward rush of air within the cloud. On the other hand, if the convergence overlies divergence of considerable depth and intensity, we conclude that the divergence and at least part of the overlying convergence are asso- ciated with downdrafts. That the cloud is a convergent region whether it contains updrafts, downdrafts or both updrafts and downdrafts is shown in FIGURES 6 and 8. In FIGURE 6 convergence is present from 4000 feet to 13,000 feet about a cloud containing downdrafts in the lower levels. The presence of the downdrafts is shown by the continued surface divergence throughout the period of the balloon ascent. FIGURE 8 shows convergence about the cloud at all levels and continuing convergence at the surface. These FIG. 6. Comparison of simultaneous convergence- conditions necessitate an updraft at all levels. divergence at surface and aloft for a typical triangle As a check on the reality of the values and spanning storm No. 53. The lower curves show that patterns of convergence obtained from these surface divergence (scale increasing downward) was high during rainfall. The upper curve, for a triangle com- thunderstorm studies, the same computations putation between stations A, 20 and 40, shows that con- were made for stable conditions. Data from the vergence aloft accompanied this condition. (Compare 1947 season in Ohio were used, since no swarm times indicated on curve in upper diagram with time ascents were made in stable thermal stratifications scale on lower.) in Florida. The results showed appreciable values

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of convergence or divergence but with no dis- cernible pattern repeating itself from case to case. It seemed that there was a certain amount of randomness in the behavior of the paths of the balloons in a stable atmosphere, although the variations were small. Isolated values as high as =±=20% change in area in 12 minutes were noted, but most of the values were less than 10%. The consistent patterns observed in thunderstorms therefore appear to be quite real and very nearly correct in actual value.

CONCLUSIONS Computations of convergence-divergence aloft made possible by extensive wind measurements over a small area lead to the following conclusions :— 1. There is a marked change in the wind field in an area of strong convection because air is drawn into the clouds in response to the vertical currents within. It was seen that this inflow takes place regardless of whether the currents are updrafts or downdrafts. 2. Throughout its active life, the entire cloud from its base upward, is a convergent region. In its early stages the inflow rate is approxi- ' _ . r mately equal at all levels. As downdrafts de- FIG. 8. Comparison of simultaneous convergence- . . , ...... 1 divergence at surface and aloft for a typical triangle vel0P ™n the Ci0ua> mver^e aPPear* be" spanning storm No. 69. Convergence indicated at sur- neath from the surface to 4000 feet- Above face as well as aloft, but surface divergence develops this level the convergence intensifies and a maxi- after heavy rain. mum of convergence develops between 15,000 and 20,000 feet. Thus it appears that the life cycle of a thunderstorm should include an early stage in which only updrafts are present. These updrafts are sur- rounded by an inflow area at all levels. As the storm develops to the point of producing a heavy shower, a downdraft reaches the surface and a resultant increase in convergence aloft is found. Inasmuch as the amount of convergence aloft exceeds the amount of divergence near the surface, the updrafts must continue in some part of the cloud, even after the downdraft develops, and divergence must again be present at the top. 3. Winds-aloft measurements made in the vicinity of strong convective activity are worthless as a measure of the general wind field and the winds are representative only of the fact that the balloon is in the vicinity of convective activity. If wind runs are to have any validity, they must be taken before or after periods of strong convection. A statistical study of the sampling errors of both FIG. 9. Generalized convergence-divergence aloft curve wind and temperature-humidity measurements at for storms taken before rainfall has begun at the sur- times of convective activity has been undertaken face. as a part of the Thunderstorm Project.

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REFERENCES Fluid Dynamics. Oxford Univ. Press. 2 vols. 702 pp. [1] Austin, J. M., 1947: The Effect of Lateral Mixing [6] Stommel, H., 1947: Entrainment of Air into a upon the Growth of Cumulus Clouds. (Unpub- Cumulus Cloud. J. Meteor., vol. 4, 91-94. lished manuscript.) [7] Univ. of Chicago, Dept. of Meteor., 1947: Report to [2] Bellamy, J. C., 1948: Objective Calculations of the U. S. Weather Bureau on Research Performed Divergence, Vertical Velocity and Vorticity. Bull. by the Univ. of Chicago Dept. of Meteor, in Con- nection with the Thunderstorm Project—October, Am. Meteor. Soc., vol. 28, pp. 45-49. 1946 to June, 1947. [3] Byers, H. R., and Braham, R. R., Jr., 1948: Thun- [8] U. S. Weather Bureau Thunderstorm Project, 1946: derstorm Structure and Circulation. Jn. of Meteor., Operation and Activity of the Thunderstorm Proj- vol. 5, 71-86. ect to September 20, 1946. Report No. 1 to Chief, [4] Byers, H. R., Holzman, B. G., and Maynard, R. H., U. S. Weather Bureau. 1946: A Project on Thunderstorm Microstructure. [9] White, F. D., 1946: The Thunderstorm Project: Bull. Am. Meteor. Soc., vol. 27, 143-146. Operational Report on Phase 1. Bull. Am. [5] Goldstein, S., Ed., 1938: Modern Developments in Meteor. Soc., vol. 27, 549-550.

United Air Lines Weather Frequency Tables

The Weather Service Division of the UAL Operating Base at Denver issues for its meteorological and other operating personnel monthly summaries of average frequencies of various weather conditions at its terminals. Data for one month for all terminals on one leg of a major route are conveniently grouped on a single sheet, along with a short "general" sketch in words. The following "weather types" (as they are labelled) are covered in the tables: total hours with weather worse than 500-ft ceiling and visibility one mile; hours of ceiling 0 to 300 ft, 0-600 and 0-1,000 ft; hours of visibility 0 to %th mile, 0-%, and 0-2 mi.; hours with thunderstorm; hours with precipitation; days with precip. .01 in. or more; total precip. for the month; greatest precip. in one day; total snowfall for month; average per cent of possible sun- shine; prevailing surface wind direction; average hourly wind velocity; average temperature for month; average daily highest temp.; average daily lowest temp.; highest temp, ever recorded; lowest temp, ever recorded. Also included are charts of the U. S. showing average isohyets for the month, average number of days with thunderstorms, the mean surface temperature of the adjacent oceans, and tracks of tropical hurricanes (Aug. and Sept.). If this sort of material is of interest to airline forecasters and operators, it could easily be more widely distributed in this particular form since these are more or less standard items in the summaries of most weather services. The "general" remarks appended to each table of the UAL summaries are succinct and very apropos; an example: (for the month of August, on the Red Bluff to Bakersfield airway) "The excellent flying weather of summer continues through August in the interior valleys of California with clear skies interrupted only by the small fair- weather type of cumulus clouds during the afternoons. Flights with frequent intermediate stops are exposed to choppy air from about 1100 to 1900 PST as surface temperatures climb into the high nineties or above 100° day after day. All terminals in the interior provide excellent alter- nate field coverage for the fog-shrouded stations along the coast."—R. G. S.

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