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Lewis J. Allison, Joseph Steranka, G. Thomas Cherrix, and Ernest Hilsenrath

Mid-tropospheric circulation features under essentially clear sky conditions have been noted in the data of the 6.7 μm channel of the Nimbus 4 Temperature-Humidity Infrared Radiometer (THIR) while at the same time these features were undetected by the 11.5 μm channel of the same instrument. The characteristic response of the 6.7 μm channel to atmospheric water vapor emission is primarily from the 250-mb (10.5 km) to 500-mb (5.5 km) levels with a peak contribution at 350 mb (8 km). Dry and moist patterns seen in the 6.7 μm data on 21 February 1971 have been integrated into a 400-mb moisture analysis over the United States. This analysis provided more detailed and timely information than was conventionally available about the advection of dry air aloft prior to development of the Mississippi Tornado of February 1971. The derivation of middle to upper atmosphere flow patterns from the Nimbus 4 THIR, 6.7 μm data under cloud-free conditions has a direct application on a global scale for the GARP and World Weather Watch Programs.

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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, John Stout, Joseph Steranka, and Simon Chang

Abstract

The Nimbus-7 Total Ozone Mapping spectrometer (TOMS) was used to map the distribution of total Ozone in and around western Atlantic tropical cyclones from 1979 to 1982. It was found that the TOMS-observed total Ozone distribution within the subtropics during the tropical cyclone seasonal correlated well with the tropopause topoghraphy, similar to earlier middle-latitudinal observations. This relationship made it possible to use TOMS to monitor the propagation of upper-tropospheric subtropical transient waves and the mutual adjustment between the tropical cyclone and the upper-tropospheric waves during their interaction. These total ozone patterns reflected the three-dimensional upper-tropospheric transport processes that were conducive for storm intensification and weakening. It was also found from satellite observations and numerical model simulations that modification of the environmental distribution of total ozone by the tropical cyclones was primarily caused by the secondary circulation associated with the tropical cyclone's outflow jet and the intrusion of stratospheric air in the eyes of tropical cyclones.

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Edward Rodgers, John Stout, Joseph Steranka, and Simon Chang

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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, Lewis J. Allison, and Vincent V. Salomonson

Abstract

The Nimbus 4 Temperature-Humidity Infrared Radiometer (THIR) monitors radiation in the 6.5–7.2 μm water vapor absorption region with a 23-km spatial resolution at the sub-satelite point. Radiation monitored in this spectral region results primarily from emission in the 250–500 mb region of the upper troposphere. The THIR 6.7μm observations are readily available in photofacsimile imagery form which shows very distinctive patterns associated with spatial variations in atmospheric water vapor.

These radiometric observations have been combined in several instances with moisture values measured in the upper troposphere by the standard radiosonde network. In each instance, the result is a much more consistent analysis showing increased spatial detail that agrees with the radiometric observations and does not compromise the conventional data. The improved moisture analyses show relatively dry and moist tongues that are very difficult or impossible to infer from the conventional data alone. The patterns in the moisture fields can be tracked over 12- and 24-hr periods. In addition, by keeping in mind the advective properties of the moisture field, success has been achieved in improving streamline analyses at the 400-mb level over data-sparse regions on a global scale.

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Edward B. Rodgers, Simon W. Chang, John Stout, Joseph Steranka, and Jainn-Jong Shi

Abstract

The mutual adjustment between upper-tropospheric troughs and the structure of western Atlantic Tropical Cyclones Florence (1988) and Irene (1981) are analyzed using satellite and in situ data. Satellite-observed tracers (e.g., cirrus clouds, water vapor, and ozone) art used to monitor the circulation within the tropical cyclones' environment. The tropical cyclones' convection is inferred from satellite flown passive microwave and infrared sensors. In addition, numerical model simulations are used to analyze and interpret these satellite observations. The study suggests that the initiation and maintenance of intense convective outbreaks in these tropical cyclones during their mature stage are related to the channeling and strengthening of their outflow by upper-tropospheric troughs. The convection can be enhanced in response to the outflow jet-induced import of eddy relative angular momentum and ascending motion associated with the thermally direct circulation. The channeling and strengthening of the outflow occurs when the upper-tropospheric troughs are located in a favorable position relative to the tropical cyclones. Both Florence and Irene intensify after the onset of these intense convective episodes. Satellite observations also suggest that the cessation in the convection of the two tropical cyclones occurs when the upper-tropospheric troughs move near or over the tropical cyclones, resulting in the weakening of their outflow and the entrainment of dry upper-tropospheric air into their inner core.

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P. E. Wilkniss, R. E. Larson, D. J. Bressan, and Joseph Steranka

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