Satellite observations of Hurricane Bill (2009): links to African easterly waves and precipitation patterns

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Monday, 5 January 2015
H. Barbosa, Laboratory for Analyzing and Processing Satellite Images, Maceio, Brazil; and V. Nietosvaara and L. M. Silva

Understanding the characteristics of storms that impact the Caribbean region is of importance to emergency planning. On 18 August 2009, Hurricane Bill tracked through this region forming from a tropical wave in the eastern Caribbean and growing to a category 4 hurricane (on the Saffir-Simpson Hurricane Scale) before strenghen over Bermuda. Bill caused $46.2 million in damages and two deaths over a nine day period. The tropical wave moved off the west coast of Africa and embedded within the Intertropical Convergence Zone (ITCZ). This study focuses on possible connections between the African Easterly Waves (AEWs) and the precipitation patterns from hurricane Bill. The essential features are captured by employing both Meteosat Second Generation (MSG) and MSG's Multi-sensor Precipitation Estimate (MPE) images. A mesoscale re-examination of this hurricane is presented in terms of the deep, convective bursts (“overshooting clouds”) occurring during a mature stage of this hurricane just prior to a period of rapid intensification. In addition, we have examined the distinctive spatial signatures of the “overshootings” on the organization and growth of the warm core of this hurricane. Dominant hurricane tracks based on the locations of MSG MPE images is highlighted. Through the study it was discovered that the AEWs has major impact on precipitation patterns of the Hurricane Bill. The intensification of AEW (the positive vorticity at 700 hPa) around eye may turn out to be a strong signature of strenghening of the storm. Analysis of the difference between the water vapor (WV) and the infrared brightness temperature (IR) reveals a remarkable axisymmetric structure through the core of the convection which supports the genesis of the strong rainfall episodes. WV–IR differences larger than +3.0 °C are associated with deep convective clouds (“overshooting clouds”) that have a large amount of ice and strong updrafts. Several overshooting tops cooling below -80 ºC are found in our analysis. The study opens up an avenue for successive validation and refinement of the analyses together with their improved implementation for operational nowcasting.