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 de- termine 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*.

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