Environmental Influence on Typhoon Bobbie's Precipitation Distribution

Edward B. Rodgers Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Harold F. Pierce Science Systems and Application, Inc., Lanham, Maryland

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Abstract

The distribution and intensity of tropical cyclone precipitation has been known to have a large influence on the intensification and maintenance of the system. Therefore, monitoring the tropical cyclone convective rainband cycle and the large-scale environmental forcing mechanisms that initiate and maintain the tropical cyclone convective rainbands may aid in better understanding and predicting tropical cyclone intensification.

To demonstrate how the evolution of the tropical cyclone precipitation can be monitored, the frequent Special Sensor Microwave/Imager (SSM/I) observations of precipitation from Typhoon Bobbie (June 1992) were used to help better delineate Bobbie's convective rainband cycle. Bobbie's SSM/I-observed convective rainband cycle was then related to the tropical cyclone's intensity change. To obtain a better understanding of how Bobbie's convective rainbands were initiated and maintained, total precipitable water (TPW) over the ocean regions, mean monthly sea surface temperatures (SSTs), and analyses from the European Centre for Medium-Range Weather Forecasts(ECMWF) model were examined. The SSM/I TPW helped to substantiate the ECMWF-analyzed regions of dry and moist air that were interacting with the system's circulation, while the mean monthly SSTs were used to determine whether the western North Pacific, where Bobbie traversed, was warm enough to allow for sufficient energy flux to support convection. The ECMWF model was employed to examine the environmental forcing mechanisms that may have initiated and maintained Bobbie's convective rainbands, such as mean vertical wind shear, environmental tropospheric water vapor flux and divergence, and upper-tropospheric eddy relative angular momentum flux convergence.

Results from the analyses of Typhoon Bobbie suggested the following: 1) The SSM/I observations of Bobbie's precipitation were able to detect and monitor convective rainband cycles that were similar to those observed with land-based and aircraft radar, in situ measurements, and SSM/I observations of western North Atlantic tropical cyclones. 2) The evolution of Bobbie's intensity coincided with the SSM/I-observed convective rainband cycles. 3) The SSM/I observations of the TPW over nonraining ocean regions were able to substantiate the ECMWF-analyzed moist and dry regions that were interacting with Bobbie's circulation. 4) In regions of warm SSTs and weak vertical wind shear, the enhancement of the precipitation in Bobbie's inner-core convective rainbands coincided with the inward convergence of upper-tropospheric eddy relative angular momentum, while the initialization of Bobbie's outer-core convective rainbands appeared to coincide with the large horizontal convergence of moisture. 5) The dissipation of rain in the inner-core convective rainbands appeared to be associated with inward propagation of newly formed outer convective rainbands, strong vertical wind shear (above 10 m s−1), and cool SSTs (below 26°C).

Abstract

The distribution and intensity of tropical cyclone precipitation has been known to have a large influence on the intensification and maintenance of the system. Therefore, monitoring the tropical cyclone convective rainband cycle and the large-scale environmental forcing mechanisms that initiate and maintain the tropical cyclone convective rainbands may aid in better understanding and predicting tropical cyclone intensification.

To demonstrate how the evolution of the tropical cyclone precipitation can be monitored, the frequent Special Sensor Microwave/Imager (SSM/I) observations of precipitation from Typhoon Bobbie (June 1992) were used to help better delineate Bobbie's convective rainband cycle. Bobbie's SSM/I-observed convective rainband cycle was then related to the tropical cyclone's intensity change. To obtain a better understanding of how Bobbie's convective rainbands were initiated and maintained, total precipitable water (TPW) over the ocean regions, mean monthly sea surface temperatures (SSTs), and analyses from the European Centre for Medium-Range Weather Forecasts(ECMWF) model were examined. The SSM/I TPW helped to substantiate the ECMWF-analyzed regions of dry and moist air that were interacting with the system's circulation, while the mean monthly SSTs were used to determine whether the western North Pacific, where Bobbie traversed, was warm enough to allow for sufficient energy flux to support convection. The ECMWF model was employed to examine the environmental forcing mechanisms that may have initiated and maintained Bobbie's convective rainbands, such as mean vertical wind shear, environmental tropospheric water vapor flux and divergence, and upper-tropospheric eddy relative angular momentum flux convergence.

Results from the analyses of Typhoon Bobbie suggested the following: 1) The SSM/I observations of Bobbie's precipitation were able to detect and monitor convective rainband cycles that were similar to those observed with land-based and aircraft radar, in situ measurements, and SSM/I observations of western North Atlantic tropical cyclones. 2) The evolution of Bobbie's intensity coincided with the SSM/I-observed convective rainband cycles. 3) The SSM/I observations of the TPW over nonraining ocean regions were able to substantiate the ECMWF-analyzed moist and dry regions that were interacting with Bobbie's circulation. 4) In regions of warm SSTs and weak vertical wind shear, the enhancement of the precipitation in Bobbie's inner-core convective rainbands coincided with the inward convergence of upper-tropospheric eddy relative angular momentum, while the initialization of Bobbie's outer-core convective rainbands appeared to coincide with the large horizontal convergence of moisture. 5) The dissipation of rain in the inner-core convective rainbands appeared to be associated with inward propagation of newly formed outer convective rainbands, strong vertical wind shear (above 10 m s−1), and cool SSTs (below 26°C).

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