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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 F. Adler and Edward B. Rodgers

Abstract

Data from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) are used to make calculations of the latent heat release (LHR) and the distribution of rainfall rate in a case study of a tropical cyclone as it grows from a tropical disturbance to a typhoon. The results indicate that the latent heat release characteristics of tropical cyclones can be determined from the microwave data and that such observations are potentially useful in the monitoring of such storms. The LHR (calculated over a circular area of 4° latitude radius) increases during the development and intensification of the storm from a magnitude of 2.7 × 1014 W (in the disturbance stage) to 8.8 × 1014 W (typhoon stage). The later value corresponds to a mean rainfall rate of 2.0 mm h−1. Even during the disturbance stage, the LHR increases significantly. It is also shown that the more intense the cyclone and the greater the LHR, the greater the percentage contribution of the larger rainfall rates to the LHR. In the disturbance stage the percentage contribution of rainfall rates ⩾ 6 mm h−1 is typically 8%; for the typhoon stage, the value is 38%. The distribution of rainfall rate as a function of radial distance from the center indicates that as the cyclone intensifies, the higher rainfall rates tend to concentrate toward the center of the circulation.

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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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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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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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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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Edward B. Rodgers, William S. Olson, V. Mohan Karyampudi, and Harold F. Pierce

Abstract

The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane’s rapid intensification and decay stages (3–5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal’s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal’s middle- and upper-tropospheric region during these convective burst episodes allowed Opal’s eyewall to become more buoyant, enhanced the generation of kinetic energy and, thereby, rapidly intensified the system. The observations also suggest that Opal’s intensity became more responsive to the convective burst episodes (i.e., shorter time lag between LHR and intensity and greater maximum wind increase) as Opal became more intense.

Analyses of SSM/I-retrieved parameters, sea surface temperature observations, and the European Centre for Medium-Range Weather Forecasts (ECMWF) data reveal that the convective rainband (CRB) cycles and sea surface and tropopause temperatures, in addition to large-scale environmental forcing, had a profound influence on Opal’s episodes of convective burst and its subsequent intensity. High sea surface (29.7°C) and low tropopause (192 K) temperatures apparently created a greater potential for Opal’s maximum intensity. Strong horizontal moisture flux convergence within Opal’s outer-core regions (i.e., outside 333-km radius from the center) appeared to help initiate and maintain Opal’s CRBs. These CRBs, in turn, propagated inward to help generate and dissipate the eyewall convective bursts. The first CRB that propagated into Opal’s eyewall region appeared to initiate the first eyewall convective burst. The second CRB propagated to within 111 km of Opal’s center and appeared to dissipate the first CRB, subjecting it to subsidence and the loss of water vapor flux. The ECMWF upper-tropospheric height and wind analyses suggest that Opal interacted with a diffluent trough that initated an outflow channel, and generated high values of upper-tropospheric eddy relative angular momentum flux convergence. The gradient wind adjustment processes associated with Opal’s outflow channel, in turn, may have helped to initiate and maintain the eyewall convective bursts. The ECMWF analyses also suggest that a dry air intrusion within the southwestern quadrant of Opal’s outer-core region, together with strong vertical wind shear, subsequently terminated Opal’s CRB cycle and caused Opal to weaken prior to landfall.

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