Characteristics of Hurricane Bertha (2008) near Bermuda

During the period 12-14 July 2008, Hurricane Bertha stalled over the subtropical Atlantic, the centre only moving approximately 140 km in 48 hours, and the maximum sustained winds decreasing from 41 m/s (Category 1) to 28 m/s (tropical storm). Prior to the period of focus of this study, Bertha (a 'Cape Verde storm') had attained Category 3 intensity just south of the Tropic of Cancer. The thermodynamics of this rapid weakening will be examined, with a view to establishing the influence of a negative feedback mechanism. This mechanism is thought to have involved a reduction of upper ocean heat content via wind-induced upwelling, supported by data from the Bermuda Atlantic Time Series in situ oceanic observations. The resulting weakening of Bertha will be explored from a Maximum Potential Intensity (Emanuel, 1986) perspective, and supporting observational data include dropsonde profiles, Bermuda's radiosonde ascents, and satellite remote sensing imagery. The consequence of the feedback process was a reduction in track and intensity predictability, as demonstrated by an examination of NHC forecast error and model spread. It is hypothesized that the sudden decrease of predictability was directly due to inconsistent resolution of the deep layer mean steering flow by the various operational forecast models. The resulting forecasts and the actual impact of Tropical Storm Bertha on Bermuda will also be discussed.