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R. Cecil Gentry

Project STORMFURY is a project of Commerce and Defense for experimenting at modification of hurricanes. Justification for the work is based on recent discoveries about hurricanes and the high potential benefit to cost ratio of the experiments. Evidence is presented that two approaches for modification should be considered: 1) seeding of clouds, and 2) inhibiting evaporation from the ocean. The scientific aspects and logistic problems of both approaches are reviewed. The Program and goals of Project STORMFURY are discussed.

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R. Cecil Gentry

Abstract

Study of the hurricanes of the last 22 years reveals that nearly every tropical cyclone of full hurricane intensity whose center crosses the United States coast between Brownsville, Texas and Long Island, New York has associated tornadoes. Approximately 60% of the tropical cyclones of only storm intensity when crossing the coast also have tornadoes reported. The climatology of hurricane-tornadoes is brought up to date through 1982.

Tornadoes form in areas of the hurricane where the tipping term and convergence terms of the vorticity equation are the largest. This usually happens in air that has had a relatively short trajectory over land. That is, they form far enough inland for the lower layers of air to be slowed by friction with the ground but close enough to the ocean for the upper layers of air to still be moving at approximately the speed they had while above the ocean. This means that most of the tornadoes either form near the center of the hurricane (from the outer edge of the eyewall outward) or in an area between north and east-southeast of the hurricane center.

The blackbody temperatures of the cloud tops which were analyzed for several hurricane-tornadoes that formed in the years 1974, 1975 and 1979 did not furnish strong precursor signals of tornado formation, but did follow one of two patterns. Either the temperatures were very low or the tornado formed in areas of strong temperature gradients.

Tornadoes with tropical cyclones can occur any hour of the day or night, but occur most frequently between 1200 and 1800 LST. Most are relatively weak but can reach at least the F3 level of intensity. Most form in association with the outer rainbands of the hurricane, but in the period 1973–80 more than 20% were with the inner bands or near the outer edge of the eyewall.

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Edward Rodgers and R. Cecil Gentry

Abstract

Rapid-scan visible images from the Visible Infrared Spin Scan Radiometer (VISSR) sensor on board SMS-2 and GOES-1 have been used to derive high-resolution upper and lower tropospheric environmental wind fields around three western Atlantic tropical cyclones (Caroline, August 1975; Anita, August and September 1977; and Ella, September 1978). These wind fields were used to derive the local change of the net relative angular momentum (RAM), upper and lower tropospheric areal mean relative vorticity and their difference, and the storm's transverse circulation. The local change of the storm's net RAM was investigated for the purpose of predicting future storm intensification while the areal mean relative vorticity and transverse circulation were investigated to better understand how the storm's environmental circulation was affecting its intensification.

The three cases studied suggested that storm intensification could be predicted from the analyses of the storm's local change of net RAM and that there is an ∼6 h lag between the local change in the net RAM and storm intensification. In addition, it was found that changes in the local change of net RAM wore being affected primarily by the net horizontal transport of relative angular momentum caused either by the convergence of cyclonic vorticity in the lower troposphere or by the divergence of anticyclonic vorticity in the upper troposphere. For the three cases studied, the upper tropospheric environmental circulation helped to influence the local change of net RAM and, therefore, changes of storm intensity by hindering or enhancing the storm's outflow and by weakening or strengthening the environmental anticyclonic circulation.

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ROBERT H. SOURBEER and R. CECIL GENTRY

Abstract

The storm situation of January 21, 1957, is studied and the vorticity and horizontal divergence patterns are computed from analyzed synoptic maps at low and high elevations of the troposphere. Contour and streamline charts for the period are presented to show that consideration of many of the synoptic parameters ordinarily used in analysis and forecasting would not lead one to expect such heavy rainfall. Computations of divergence are compared with the rainfall charts in an effort to determine the cause of the heavy rainfall which varied in amount up to 21½ inches within a 24-hour period. The divergence patterns move horizontally with time in such a manner that a high-level divergence area becomes superimposed over a low-level convergence area at the time of heavy rain.

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R. Cecil Gentry, Tetsuya T. Fujita, and Robert C. Sheets

Abstract

Hurricane Gladys, 17 October 1968, is studied with data collected by Aollo 7 manned spacecraft, ESSA's especially instrumented aircraft, weather search radar, the ATS-III and ESSA 7 satellites, and conventional weather networks. This is the feast time data from all of these observing tools have been used to study the structure and dynamics of a hurricane. Techniques used in computing and integrating the various types of data are described and illustrated.

A dominant feature of this immature hurricane was a large cloud which provided a major link between the low- and high-level circulations of the storm. Evidence is presented to suggest this type of cloud and its attendant circulation are features representative of tropical cyclones passing from the tropical storm to the hurricane stage.

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Joseph Steranka, Edward B. Rodgers, and R. Cecil Gentry

Abstract

Satellite-measured equivalent blackbody temperatures of Atlantic Ocean tropical cyclones are used to describe the associated convection and cloud patterns. Average equivalent blackbody temperatures were developed from 538 geostationary satellite observations of 23 tropical cyclones. The average values were stratified into tropical storm or hurricane intensity category, then normalized to local standard time and composited to provide a 24 h diurnal time series. The composited values represent the mean cloud top temperature within data rings around the tropical cyclone centers.

The cloud top temperatures when compared to a mean tropical atmosphere suggest that the mean top of the dense cloud canopy of hurricanes is near 10.6 km and extends horizontally to 321 km radius from the center. The mean top of the dense canopy of tropical storms is near 9.7 km and extends horizontally to 278 km from the center. The mean top of the deep convection near the center of hurricanes is near 13 km and in tropical storms is near 12 km. A Fourier series analysis of the 24 h time series shows diurnal and semidiurnal cloud patterns which are statistically significant at the 0.0005 and 0.01 levels, respectively. The cloud cycles are in phase with the convection and cloud activity found in tropical systems by other investigators.

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Joseph Steranka, Edward B. Rodgers, and R. Cecil Gentry

Abstract

The relationship between the mean temperature of the top of the cloud canopies and the future maximum winds of Atlantic Ocean tropical cyclones is analyzed. The area-average cloud top temperatures from 309 observations of 12 tropical cyclones which occurred during 1974–79 were compiled from infrared measurements made by Geostationary Operational Environment Satellites. Maximum winds were obtained from best track records.

The satellite measurements showed that prolonged surges of intense convection developed in the near region surrounding the depression centers before the maximum winds initially increased. Subsequent weakening of the convection occurred but was frequently followed by new surges of intense convection. It was found that when these prolonged surges of intense convection 1asted for 9 or more hours, and the filtered (6-h running mean) area-average cloud top temperature within 222 km of the tropical cyclone centers was 238 K or less, that the maximum winds of the tropical cyclones increased by 5 m s−1 or more within 24 h later, 71 % of the time. However, when intense convection was not present, similar maximum wind increases occurred only 37% of the time.

The future maximum winds were compared with both the filtered area-average cloud top temperatures measured during the strong convective surges and the storm's intensities at the filtered temperature times using multiple linear regression. The correlation was found to be 0.771 for moderate/strong storms (storm intensity of 26 m s−1 or more) and 0.610 for weak storms (stores intensity of less than 26 m s−1). The relationships are statistically significant at the 0.0005 and 0.05 levels, respectively, and the lag time is near 24 h. The standard error of the regression is 5.7 and 6.2 m s−1, respectively. Statistical tests made to determine the quality of expected performance suggest that predictive equations will yield maximum wind intensities within 3 and 4 m s−1, respectively, of the standard error of the regression 95% of the time. In an independent test, the standard deviation of the error of the predicted maximum winds of moderate/strong storms was 8 m s−1, or well within the expected bounds.

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R. Cecil Gentry, Edward Rodgers, Joseph Steranka, and William E. Shenk

Abstract

A relationship between maximum winds and satellite-measured equivalent blackbody temperatures near tropical cyclones is investigated with data from both the Atlantic and western North Pacific areas. This investigation revealed not only a significant correlation between satellite-derived equivalent blackbody temperatures and maximum winds but also a strong lag relationship between these temperatures and maximum winds. From this latter relationship a regression technique was developed to forecast 24 h changes of the maximum winds for weak (maximum winds ≤ 65 kt) and strong (maximum winds > 65 kt) tropical cyclones by utilizing the equivalent blackbody temperatures around the storm alone, and together with changes in maximum winds during the preceding 24 h and the current maximum winds. Testing of these equations with independent data showed that the mean errors of forecasts made by the equations are lower than the errors in forecasts made by persistence techniques.

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GORDON E. DUNN, R. CECIL GENTRY, and BILLY M. LEWIS

Abstract

The National Hurricane Center and the National Hurricane Research Laboratory joined forces in an effort to improve techniques for forecasting hurricane motion in the spring of 1959 when the latter moved its headquarters from West Palm Beach to Miami into offices adjacent to those occupied by the principal hurricane forecast office in the United States. Results now available from verification of forecasts made during the period 1954 through 1966 show that there has been a significant improvement in the accuracy of hurricane forecasts during the period of increased cooperation between the research and operational forecasting groups. This improvement is indicated by a reduction in the mean error of hurricane forecasts of approximately 10 and 12 percent, respectively, for the two principal hurricane forecast areas near the eastern coasts of the United States.

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Edward Rodgers, R. Cecil Gentry, William Shenk, and Vincent Oliver

Abstract

During the 1975, 1976 and 1977 North Atlantic hurricane seasons, NOAA's National Environmental Satellite Service (NESS) and NASA's Goddard Space Flight Center (GSFC) conducted a cooperative program to determine the best resolution and frequency now available from satellite images for deriving winds to study and forecast tropical cyclones. Rapid-scan images were obtained in 1975 at 7.5 min interval from SMS 2 for Hurricane Eloise on 22 September and of tropical cyclone Caroline on 28, 29 and 30 August; in 1976 at 3 min intervals from GOES 1 for tropical storms Belle on 5 August and Holly on 25 October; and in 1977 at 3 min intervals Corn GOES 1 for tropical cyclone Anita on 30 and 31 August and 1 September. Cloud motions were derived from these images using the Atmospheric and Oceanographic Information Processing System (AOIPS) at GSFC. Winds that were derived from the movement of upper (∼200 mb) and lower tropospheric (∼900 mb) level clouds using rapid scan data were compared with the 15 and 30 min interval data. This was done using visible images having 1, 2, 4 and 8 km resolution for the areas within 650 km of the storm center for the 1975 and 1976 tropical cyclones. Greater than 10 (5) times as many clouds could be tracked to obtain winds at both levels using 3 and 7.5 min rapid-scan images as when using 30 min (15 min) interval images. In addition, by using the frequent images, it was possible to track a few bright areas within the central dense overcast which appeared to be moving with the winds at low levels. For Hurricanes Eloise and Caroline the winds that were derived by tracking these bright areas within the central dense overcast had speeds differing in the mean by only 2.5 m s −1 from the wind speed measured by aircraft flying at ∼0.5 km above the surface in the same quadrant 4 h later. Full-resolution visible images (1 km) were needed to track slow moving low-level cloud elements, since on a degraded resolution image, subpixel movement would introduce additive inaccuracies to the wind measurements. Rapid-scan full-resolution GOES 1 data for tropical cyclone Anita (1977) provided representative wind fields only outside the central dense overcast at the lower tropospheric level. For this area aircraft-measured wind speeds differed in the mean again by only 2.5 m s−1.

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