Structure of the Upper Troposphere and Lower Stratosphere in the Vicinity of Hurricane Isbell, 1964

551. 515. 2 Structure of the Upper Troposphere and Lower Stratosphere in the Vicinity of Hurricane Isbell, 1964

by R. Cecil Gentry

National Hurricane Research Laboratory, Environmental Science Services Administration, Miami, Florida, U.S.A. (Received Octobor 10, 1967)

Abstract

Extensive reconnaissance of mature Hurricane Isbell was made October 14, 1964. Data collected at 850 mb, 700 mb, 550 mb, 200 mb, 150 mb, and in the lower stratosphere reveal that the storm had a warm core (up to 15°C above normal in the eye) from sea level up to 115 mb and a cold core between 115 mb and 90 mb. Convection in the storm was very intense and clouds rose into the stratosphere, with isolated clouds projecting 2. 5 km above the tropopause. The lower stratosphere (up to 4000 ft above the tropopause) had temperatures ranging from 1. 8°C to 7. 6°C below normal. Even in the stratosphere, which had a very stable lapse rate, there were temperature gradients as large as 5°C per 10 n miles in the vicinity of the hurricane center. The cyclonic wind circulation decreased from a maximum of 116 kt at low levels to near zero at 115 mb, and increased, relatively, in the layer near the tropopause. Data were insufficient, however, to verify whether the very weak wind field measured at the tropopause and in the lower stratosphere was actually cyclonic. The tropopause was abnormally high and cold above the hurricane with the height varying inversely with distance from the storm, at least beyond the vicinity of the eye wall. Data were insufficient to define the slope of the tropopause over the eye and eye wall.

1. Introduction

Moreinformation is needed concerning the structure of temperature, wind, and pressure fields of hurricanes in the upper troposphere and lower stratosphere. To support the current efforts to develop satisfactory numerical—dynamical models of hurricanes, we need answers to these questions : 1) Does the low pressure at sea level in the hurricane result from changes in the air column temperature in the troposphere or in the stratosphere ? 2) Does the tropopause dip above the hurricane to bring a warmlargermassof relatively warstratospheric air over the center ? 3) Does the tro-

* Partly worked under the U.S.—Japan Cooperative Science Program. 294R. C. GentryVol. XVIII No. 4

popause height increase as the hurricane approaches and bring relatively cool upper tropospheric air over the center ? Various models formulated have simulated the hurricane structure in differing ways. Data are needed to test these models to determine which simulate nature's effects.

RIEHL suggested that the tropopause might be higher above the center of a hurricane than in the surrounding areas (RIEHL 1954, p. 318) . He and others hypothe- sized that there is some level above a hurricane, probably between 150 and 80 mb, where the vortical components of the wind and pressure fields are zero, and where the slopes of the pressure surfaces are functions only of the surrounding larger scale cir- culations (MALKUSand RIEHL 1960; MILLER, 1964; RIEHL, 1954) . This implies that changes in sea-level pressures in hurricanes are a function of temperature changes produced primarily by vertical currents within the troposphere, and by the latent heat released as the moisture condenses in the ascending currents. Our investigation was designed to obtain some detailed information about atmospheric structure near the tropopause in the area of a hurricane, and to answer some of the questions previously listed. In the Atlantic area, only four radiosonde observations to high levels have been made in the eyes of hurricanes (Fig. 1) . Three observations made in Florida did not reach the tropopause (RIEHL, 1948 ; SIMPSON, 1947 ; SUGG 1967) . The one made at Bermuda in Hurricane Arlene in August 1963, reached the tropopause. STEAR (1965) reported that temperatures in the eye of this storm were higher than in the ambient atmosphere up to 150 mb and were lower from 120 mb to 90 mb. The tropopause was higher above the eye of the storm than beyond the storm circulation. Arlene had already recurved into the westerlies and was a weak tomoderate hurricane. Maximum winds of 85 kt were measured at Bermuda. At Hong Kong, a sounding was obtained to high levels in the eye of weak typhoon. Alice in May 1961. Mr. GORDONBELL* of the Royal Obs' ervatory reported that temperatures in the eye at levels below the 120 mb surface were much warmer than those in the mean May sounding for Hong Kong. At the tropopause the eye sounding had slightly lower temperatures than the mean sounding, but at levels above the 85 mb the temperatures were approximately the same as in the mean sounding. ARAKAWAprepared a cross section for a typhoon that passed near Tokyo in 1949 (ARAKAWA1950). One sounding was made near the eye wall, but none was made in the eye. The data suggested that the tropopause was higher over the storm's convective area than at greater distances. A recent source of high-level data over hurricanes has been from an instrumented. U-2 aircraft operated for the United States Air Force Cambridge Research Labora- tories. PENN (1965) made a study of the ozone and temperature measurement taken over Hurricane Ginny, October 1963. Ginny, however, was then barely of hurricane intensity. The above accounts show that no measurements are recorded in the literature of the structure of a hurricane of at least moderate intensity in the layer including the tropopause and lower stratosphere while the storm was still in tropical areas. A chance to improve this situation was provided by a cooperative project of the Air

* Personal Communication . Force Cambridge Research Laboratories, ESSA's Research Flight Facility, and ESSA's National Hurricane Research Laboratorythis project provided research reconnaissance flights into Hurricane Isbell on October 14, 1964. Data from these flights and from rawinsonde flights made at neighboring weather stations give considerable information on the structure of this hurricane between 150 and 85 mb as well as lower levels.

Tropical storm Isbell started developing into a hurricane south of Cuba on Octo- ber 12, 1964 (Fig. 2) . DUNN et al. (1965) prepared a general description of this 296R. C. GentryVol. XVIII No. 4 storm. It slowly moved north while intensifying. During the night of October 13-14, Isbell accelerated as it crossed Cuba and intensified rapidly while moving into the Florida Straits where it was located by the reconnaissance flights on October 14. By 1200 GMT, October 14, the moderately intense hurricane had a central pressure of 964 mb and was 70 n miles west-southwest of Key West, Florida.

The hurricane was of moderate intensity and changing rather slowly in structure and intensity between 1830 and 1925 GMT, October 14, while the principal aircraft data used in this investigation were obtained. The minimum sea level pressure, and maximum winds measured in the storm between 5, 000 and 15, 000 feet varied on October 14 as follows : 1200 GMT, 964 mb 1755 GMT, 968 mb and 116 kts ; 1915 GMT, 968 mb and 112 kts ; and 2108 GMT, 970 mb and 110 kts. Fig. 10 shows the wind field at 850 mb. By 1730 GMT the hurricane had a well-formed radar eye. Although the storm structure did vary with time, pictures in Fig. 3 (a, b, and c) show that the structure of the rainbands and eye wall resembled that of a mature hurricane at least through 2000 GMT. Afterwards the storm began dissipating and by 2230 GMT the eye wall image was so poor that it was difficult to track the hurricane center with radar. Nevertheless, the maximum winds and the pressure at sea level in the center of the storm remained nearly constant during the reconnaissance period, and the structure and distributions of the strongly convective radar bands indicated a a mature hurricane during this time.

Experienced observers reported more than usually severe turbulence at the flight levels between 5, 000 and 45, 000 feet in the northern quadrants of the storm. This 1967Structure of Hurricane Isbell, 1964297 298R. C. GentryVol. XVIII No. 4

hurricane also had several tornadoes associated with it later that evening as it passed over southern Florida.

The U-2 aircraft of the Air Force Cambridge Research Laboratories made reconnaissance of the lower stratosphere in the vicinity of Isbell between 1830 and 1925 GMT. It flew south from Fort Myers, Florida, across the hurricane to a point about 60 n miles south of the storm center, then to a point 50 n miles southeast of the center, and there the plane turned northwest to re-cross the storm (Fig. 2) . The aircraft departed Fort Myers at 52 mb, and slowly descended enroute to fly near 93 mb (about 2,000 feet above the tropopause) most of the time that it was near the storm center. On the westward track, the pilot picked a spot surrounded by the highest clouds and assumed that this was the eye. There he descended through clouds into the troposphere to 116 mb. (That there was an unusually large amount of cloudiness in the eye between 35,000 and 50,000 ft all day was verified by data from the planes flying at levels between 5,000 and 45,000 ft.) During the aircraft's descent, turbulence became stronger than the pilot considered safe for operation, so he rapidly ascended again. After leaving the storm vicinity the plane climbed to about 50 mb to return to land.

The pilot, Captain ROSBORG,reported that the cirrus cloud's tops north and near the storm were at 54, 000 feet, and that some cumulus tops protruded through this cirrus deck up to 61, 000 ft*. The tropopause was 52, 600 ft. (All elevations are given in pressure altitude).

Temperatures, D-values, and winds recorded on the U-2 flight have been analyzed. The temperature data are presented in considerable detail and brief descriptions are given of the pressure and wind fields, based on what can be deduced from the U-2 data and rawinsonde measurements made at nearby stations. Data collected by the Research Flight Facility aircraft operating at several levels in the troposphere are used to supplement those collected by the U-2.

2. Temperatures

Fig. 4 compares temperatures measured on the various aircraft flights with those measured on rawinsonde flights at Miami at 1200 GMT, October 14, and at 0000 GMT on October 15, when the storm was respectively 170 and 45 n miles from Miami. Note that in the stratosphere there is a remarkable agreement between the temperatures measured by the U-2 aircraft and those measured by the rawinsonde temperature elements. PENN reports that the temperature elements used on the U-2 is believed to provide temperature readings accurate to within O. 4°C to 95% pro- bability PENN (1965). Certainly the data collected on this flight appear reasonable when checked for consistency and repeatability, and when compared with rawinsonde data.

At lower levels the Research Flight Facility aircraft measured temperatures in the eye and eye wall within the ranges indicated by the stippled bars in Fig. 4 at 850 mb,

* In a personal communication Captain Rossoao said these heights were measured—not estimated . 700-mb, 550-mb, 200-mb, and 150-mb levels. The higher temperatures at the upper levels were measured in the eye, but at 850 mb and 700 mb some of the temperatures in the eye wall were as high as those in the eye. Temperatures believed represent- ative of the averages of those in the eye are used to simulate an eye sounding (Fig. 4). If this temperature-height curve is projected to the tropopause along the equivalent potential temperature curve, the indicated temperatures in the upper troposphere would be greater than those measured by the U-2.

The U-2 pilot did not have visual contact with the eye. Since he encountered heavy turbulence and the equivalent potential temperatures at his level agree more closely with those in the eye wall rather than in the eye, as determined from the 300R. C. GentryVol. XVIII No. 4 data measured by the aircraft at 200- and 150-mb levels, the pilot probably descended just west or southwest of the eye. The extremely low temperatures measured by the U-2 aircraft at the tropopause and in the upper troposphere are especially interesting. At the tropopause, which was located at 102 mb, the temperature was —85°C. Both the tropopause height and temperature were consistent with those measured by the sounding at Miami (100 mb, —85. 6°C), when the storm passed just northwest of that station 5 hr later at 0000 GMT.

Temperatures measured on the U-2 flight are plotted on an expanded scale in Fig. 5. These can be used for a qualitative evaluation of their accuracy by noting how closely the temperatures on the descent (dashed curve) and ascent (solid curve) agree with each other. The " hatched " area represents the range of temperatures measured while the aircraft was flying (at 93 mb) in the " X " pattern over the storm. The temperatures on the descent and ascent differ somewhat, but this difference should be attributed to space and time variations rather than to any errors. Support for this statement is furnished by the close agreement between temperatures measured during the brief descent and ascent in the troposphere, where there was little space or time variation (see inset in Fig. 5). During this latter portion of the descent, the temperatures first increase, then decrease, and finally increase as the aircraft descends into the troposphere. This relatively complex pattern was verified by the sequence of temperatures measured on the ascent, which followed closely after the descent. 1967Structure of Hurricane Isbell, 1964301

A suggestion that the difference in temperatures mentioned in the preceding paragraph can be attributed to space variations is furnished by the data in Fig. 6, which is a horizontal plot-out of the temperature data in the hurricane's vicinity. All of the data are plotted in a coordinate system moving with the hurricane, which was going northeast at 15 kt. Because the temperatures were measured over a wide pressure range, they were plotted in this illustration as departures from the October mean tropical sounding, as determined by JORDAN (1958). The temperatures in the lower portion of Fig. 6 were measured over or near the hurricane, but those in the upper portion were measured while the aircraft approached and departed the hurricane region. The aircraft was in the stratosphere well above the cloud level on both these later legs and the temperatures should not, have been affected by the hurricane. The variations are believed those that were naturally occurring in the stratosphere at that time. The data in the figure's lower portion also illustrate variations with time and space, but they were strongly influenced by the hurricane's circulation ; therefore a discussion of their locations relative to the storm center follows.

The exact position of the hurricane center is in doubt. By post-storm analysis of data from all sources, it was located at the intersection of the zero lines on the horizontal and vertical scales. This is 10 to 20 n miles north of the point where the U-2 tracks intersected (see lines of dots at 30 S and 3 W in Fig. 6) . This is the point where the air traffic controller sent the pilot for the center. (It is not known what information the former used, but presumably he had the latest center fix from other aircraft and probably some current information from radar) . The pilot evident- ly thought the center was close to where he descended into the troposphere (ellipse near 30 S and 25 W). The radar pictures taken at Key West when the U-2 was in the storm indicate that the eye was 7 to 10 n miles in diameter. Visual and radar observations made at 150 nib by an observer in the Research Flight Facility indicate that the diameter of the eye at that level was 18 n miles. We cannot, therefore, ac- count for the discrepancies by arguing that all of the positions are inside the same large eye ; the eye was too small for that. We must, therefore, accept the uncertain- ty about the precise location of the hurricane center ; but it is probably inside the area bounded by the line of large scallops in Fig. 6.

The only temperature data believed to be significantly affected by the storm's circulation are those in the lower portion of Fig. 6 (inside the line of small scallops). When approaching and departing the hurricane, the pilot flew far above the hurricane clouds. Even after he reached the storm vicinity he stayed above the clouds, with the exception of one brief descent into the troposphere. When he was north of the storm, he was above the layer of significant storm influence. The one possible exception is the " warm " area at 8°N and 3°E which he traversed while descending from 62,000 to 59,000 feet.* When he went south of the storm, however, he descended into the layer where there had been storm influence only a short time before. The rainbands north of the storm (see Fig. 3a, 3b, 3c) were the most turbulent and probably extended the highest. As indicated earlier the pilot reported the cirrus cloud

* The temperatures measured in this area (Fig . 6) can be analyzed to show a warm air mass near the intersection of the 2 zero lines, one of the positions discussed for the hurricane center. The data were not analyzed in this manner because the ozone measurements did not indicate strong vertical currents (PENN1966), and the aircraft was still above the clouds. Fig. 6. Departure from mean tropical conditions of temperatures measured by U-2 during flight over Hurricane Isbell. tops were at 54,000 feet with a few cumulus towers going to 61,000 feet.

These were mostly north and east of the center. Since the storm was moving northeast at 15 kt, this area of high cloud towers traversed the area farther south just before the aircraft arrived. In the area enclosed by the line of small scallops, the plane was flying at levels below 55,000 feet, near 54,000 feet much of the time. 1967Structure of Hurricane Isbell, 1964303

Thus, when south of the storm he was in an area where cloud towers had projected into his flight level within the preceding 1 to 3 hours.

South of the storm, where the temperatures were probably affected by passage of the storm's circulation and its convective cells, all of the temperatures measured were below normal for the altitude and season. All were measured in the stratosphere from 1,500 feet to 2,500 feet above the tropopause. Even in this area and when the aircraft was maintaining relatively constant pressure altitude, there was considerable temperature variation. The anomalies varied from —1.8°C to —7.6°C. There were alternate warm and cold pools or bands encountered on two or more headings. There was an especially strong temperature gradient near the intersection of the north-south and east-west flight tracks, near where the aircraft controller thought the eye was located. On the wastbound leg, temperatures rose from —82°C to —75°C and then decreased to —80°C within a distance of about 20 n miles, while the aircraft was maintaining a relatively constant pressure altitude. Although the changes were not as dramatic, a warm pool was encountered at the same location on the north—south leg a few minutes earlier, with corresponding temperature measurements consistent with those of the east-west leg. In general, the negative temperature anomalies in the lower stratosphere were much greater south and southeast of the surface position of the hurricane center than those near the center itself. The anomalies north of the center within 3,000 feet of the tropopause were not available for comparison because the aircraft was at a higher level.

In the lower stratosphere the temperature lapse rate was such that adiabatic changes would cause subsiding air to be warmer and ascending air to be colder than other air at the same level. All of the temperatures measured near the tropopause were below the October normals (JORDAN, 1958) for the area, so one can reasonably hypothesize that the relatively warm air was either over the eye or over the space between the tropospheric rainbands. That is, the warm air was in areas where the vertical velocities were near zero or possibly had small negative values. By contrast the areas of greatest negative temperature anomalies should mark the locations where convection had been most intense, and where some of the cloud towers had penetrated the stratosphere. Air ascending from the troposphere and expanding adiabatically would be much cooler than the stratospheric air (Fig. 4). For example, if air in the eye wall at 150 mb were to expand adiabatically to 96 mb (the top of the cirrus deck) its temperature would be —90°C, 16°C below normal. At the top of the cumulus towers the anomaly should be much greater. Presumably, in only a small percentage of the area would the towers penetrate the stratosphere, therefore, cold air thus in- troduced would be mixed with the ambient air to produce the observed temperatures.

That the temperatures in Isbell in the upper troposphere and lower stratosphere were abnormally cold is illustrated by tephigrams arranged in time section format for soundings made at Miami, Florida, for 18 hr before to 12 hr after the time (00 GMT, 15 October 1964) the storm was closest to that station (Fig. 7) . The mean tropical temperatures for October (JORDAN, 1958) are represented by the line repeated in each graph. The " dotted " areas represent below-normal temperatures. The tropopause was highest and coldest when the hurricane was nearest the station. Then the storm center was only 45 n miles from Miami, and the poorly defined southern eye wall was very close. The tropopause was at 100 mb with a temperature of --85.6°C at 0000 GMT. The U-2 found the tropopause between 100 and 103 mb with a temperature of —85°C.

The data in Fig. 7 indicate that in Hurricane Isbell, the tropopause sloped up- ward toward the storm at least to within 40 n miles of the center. We cannot specify the tropopause slope over the eye wall and the eye because the eye's location relative to the U-2's descent through the tropopause is uncertain. From the Miami soundings and the U-2 data, there is no doubt of the upper troposphere being much colder than the surroundings or the normal at the tropopause. In the lower and middle troposphere, on the other hand, some of the temperatures in the eye and eye wall were more than 15°C higher than the normal. Above 80 mb there was little difference between the temperatures over the storm and those at greater distances.

The tropopause temperatures over Hurricane Isbell were lower than any others that occurred at Miami during October 1964. The tropopause pressure and temperature at Miami for all soundings taken at 0000 GMT and 1200 GMT in October 1964 are graphed in Fig. 8. The Miami temperature during Isbell was 3°C lower than at any other time during October. Although the pressure at the tropopause during the passage of Isbell was not the lowest ; it was much lower than the average for the month (111 mb) . Fig. 8. Temperature and pressure at tropopause over Miami, Florida, from soundings at 0000 and 1200 GMT, October 1964.

3. Pressure distribution

Since the storm was warm core at low levels and cold core in the upper troposphere, the intense cyclonic vortex at lower levels should have weakened with height up to about 115—mb level the cyclonic circulation should have increased, at least in the relative sense, for a few thousand feet. There are insufficient data to specify that the cyclonic circulation still existed at 100 mb, although there is evidence from data collected on flights at lower levels that the circulation was cyclonic up to at least 150 mb. 306R. C. GentryVol. XVIII No. 4

Hydrostatic checks using the " D-value " gradient at 550 mb and mean temperature gradient above that level (estimated from temperatures measured at 550 mb, 200 mb, and 150 mb) indicate that the height of the 115-mb surface should have been approximately the same at a radius of 60n miles as at the center of the hurricane.

Fig. 9. Time-cross section for Miami, Florida. Isolines are height anomalies in meters. Hurricane Isbell was closest to Miami at 0000 GMT, October 15, 1964.

The pressure distribution on a larger scale can be partially deduced from data in Fig. 9, which is a time section constructed from the Miami soundings. The isolines are height anomalies from the mean tropical atmosphere for October (JORDAN, 1958) and values are in meters. Notice that these height anomalies that were below normal decreased with altitude up to about 250 mb, and then the height anomalies which were above normal increased above 250 mb. One of the items that motivated data collection by using a U-2 was to determine if there were some level where the contour fields were horizontal. Apparently there is none, but the pressure surfaces do become quasi-horizontal at 250 mb and at 70 mb in the areas near but outside the high energy portions of the storm.

The D--values collected by the U-2 have certain inconsistencies that make it difficult to define the height contours in the immediate vicinity of the storm.

4, The winds

The general wind field around Isbell, as revealed by the ra wind reports, had a 1967Structure of Hurricane, Isbell, 1964307

large vertical shear between 100 and 50 mb. Most of the nearby rawinsonde flights measured winds from the east-northeast to northeast at 50 mb and from the south- southwest to west-southwest at 100 mb. This shear in the vertical makes it difficult to deduce by interpolation from rawinsonde data the horizontal wind field above the hurricane at the level where the U-2 was flying. In addition, some uncertainties about the wind measurements taken by the aircraft make it unwise to use these data except in the qualitative sense, noting changes along legs of the track while the aircraft was flying continuously at the same elevation and in the same direction. Because of the uncertainties, the aircraft winds are not reproduced even though they formed a fairly consistant pattern after being adjusted by a post-flight calibration procedure, such as the one described by HAWKINSet at (1962) . This " adjusted " wind field showed very weak winds (0-15 kt) in the lower stratosphere (about 93 mb) above the sea level position of the hurricane center. The winds indicated a very weak cyclonic center near the hurricane eye at 93 mb, but they are not considered reliable enough to substantiate the existence of such a circulation. The information obtained from the aircraft data is considered reliable Fig. 10. Windfield at 850 nib for Hurricane Isbell, October 14, 1964: a) streamlines, and b) isotachs. Dotted lines indicate path of aircraft.

enough to support, the following statements : 1) the winds above the hurricane in the first 3,000 feet- 'Of the stratosphere were weak relative to those in the lower troposphere (Fig. 10), 2) when integrated over an area of the same diameter as the strong cyclonic winds in the middle troposphere, the mean wind in the lower stratosphere had approximately the same speed and direction as the winds analyzed by interpolation between rawinsonde stations and 3) the lower stratospheric winds above the intense convective areas of the hurricane were probably light and variable and may have defined a weak cyclonic circulation.

5. Vertical motions

The ozone measurements made on the same flight indicate there was little or no subsidence in the upper portions of the hurricane eye according to PENN (1966). 1967Structure of Hurricane, Isbell, 1964309

The thick cloud layer in the upper troposphere filling the eye for several thousand feet supports this deduction.

The high, cold tropopause, however, is believed to be the result of the intense convection in the troposphere. No measurements were made of the vertical velocities, but indirect evidence and computations made for other hurricanes indicate that they could be quite strong (GRAY, 1965 ; MALKUS, 1960) . Some clouds, according to the pilot's reports, penetrated the stratosphere and rose about 2.5 km above the level of zero bouyancy for the ascending current. SAUNDERS(1962) developed a model for penetrative convection in stable, stratified fluids and found, " For every 1 km of penetration it is deduced that convective motions have an average vertical intensity of 10 m/sec : this prediction is shown to be in good agreement with the limited observational data...... (Made by the author) of the penetration of tall cumulus clouds into the subtropical stratosphere." If his results are accepted, this would imply that some of the vertical currents in Isbell exceeded 20 misec. Although this is a large value, the severe turbulence, thunderstorms, and tornadoes occurring in this hurricane confirm that the vertical velocities were greater than in many hurricanes.

6. Conclusions

The data collected on October 14 in Hurricane Isbell, a moderate hurricane, show that the storm had a very warm core extending to about 150 mb. In the upper troposphere the core was abnormally cold, and at the tropopause had a temperature of —85°C. The tropopause was higher over the intense convective regions of the hurricane than it was farther from the center. The temperature, pressure, and wind fields all indicate the storm was warm core below 115 mb and cold core between 90 and 115 mb. This latter layer was in the upper troposphere and lower stratosphere. Horizontal temperature gradients above the hurricane and more than 6,000 feet above the tropopause were weak. These data all suggest that there was a relative increase in the cyclonic vortex above about 115 mb on up to about 90 mb, and possibly the circulation was actually cyclonic at those levels. The data, however, were not sufficient to eliminate the possibility that the circulation was a very weak anticyclo- nic one. The horizontal pressure gradient was probably weakest in the area around the hurricane (outside the area of strong cyclonic flow) at about 250-mb and 70-mb levels. The pressure measurements made by the aircraft in the stratosphere within 100 n miles of the storm center were not accurate enough to specify the gradient in that area, but the temperature distribution suggests that the pressure gradients were a minimum near the 115-mb level and again somewhere above the 80-mb level.

References

ARAKAWA,H., 1950: Vertical structure of a mature typhoon. Mon, Weatb, Rev., 78, 197-200. DUNN,G,E., and Staff 1965: The hurricane season of 1964. ibid, 93, 175-187. GRAY,W.M., 1965: Calculation of cumulus vertical draft velocities in hurricanes for aircraft observations. J. Applied Met., 4, 463-474. HAWKINS,H.F., F.E. CHRISTENSEN,S. C. FRANCEand Staff Members of National Hurricane Research Project, Miami, Florida, 1962: Inventory, use, and availability of National Hurricane Research Project meteorological data gathered by aircraft. National Hurricane Research Project Report, 52, 48 p. JoRoAN, CI-, 1958: Mean soundings for the West Indies area. J. Met., 15, 91-97. MALKUS,J,S., 1960: Recent developments in studies of penetrative convection and an applica- tion to hurricane cumulonimbus towers. (in " Cumulus Dynamics", Pergamon Press, N.Y., p. 80.) MALKUS,J.S. and H. R1EIIL, 1960: On the dynamics and energy transformation in steady-state hurricanes. Tellus, 12, 1-20. MILLER, B.I,, 1964: A study of the filling of hurricane Donna (1960) over land . Mon. Weath. Rev., 92, 389-406. PENN, S., 1965: Ozone and temperature structure in a hurricane. J, Applied Met., 4, 212-216. ------, 1966: Temperature and Ozone variations near tropopause level over Hurricane Isbell, October 1964. ibid., 5, 407-410. RiEur, H., 1948: A radiosonde observation in the eye of a hurricane. Quart. J. Roy. Met. Soc., 74, 194-196. , 1954:------Tropical Meteorology, McGraw-Hill, N.Y., 392 p. SAUNDERS,P.M., 1962: Penetrative convection in stably stratified fluids. Tellus, 14, 177-194. SIMPSON,R.H., 1947: A note on the movement and structure of the Florida Hurricane of October 1946. Mon Weath. Rev., 75, 53-58. STEAR, J.R., 1965: Sounding in the eye of Hurricane Arlene to 108760 ft. ibid, 93, 380-382. SUGG„ XL., 1967: The hurricane season of 1966. ibid, 95, 131-142.

ハリケーン Isbell(1964)の周辺における上部対流圏と下部成層圏の構造

R.C.ジェントリー

最盛期のハリケーンIsbe11の広範囲な観測飛行が1964年10月14日に行なわれた。850mb,700mb, 550mb,200 mb,150 mbおよび下部成層圏で集められた資料は,このあらしが海面から115 mbまでは warm core(眼の中では正常値よりも15°C以上暖かい)を,そして115 mbと90 mbの間ではcold coreを形成したことを示している。あらしの中の対流活動は非常に激しく,雲は成層圏にまで侵入したが, 分離した雲のあるものはトロポポーズの2、5km上に突き出た。下部成層圏(トロポポーズから4000 ft以上の高さで)の温度は正常値よりも1.8°C乃至7.6°Cほど低かった。ハリケーンの中心付近では成層圏の中でさえ,非常に安定な気温減率であって,10海里につき5°Cであった。低気圧性の風の循環は下層の最大116ノットから115mbにおけるゼロ近くまで減少し,トロポポーズの近くの層ではやや増加した。しかしながら,トロポポーズと下部成層圏で測定された非常に弱い風の場が真に低気圧性であったかどうかを判定するには資料がじゅうぶんでない。

トロポポーズは,少なくとも限の壁の周辺以遠では中心からの距離に反比例して高度が変化したが,ハリケーンの上では異常に高くそして冷たかった。眼と眼の壁の上のトロポポーズの傾きを定めるには資料が不足であった。

(訳:土屋巌)