Effects of Extreme SST Cooling on Hurricane Ophelia (2005) Structure: RAINEX Observation and Coupled Model Simulation

Although previous studies have shown that a large area of warm water provides a favorable condition for intensification of hurricanes, storm development and intensity over the warm ocean vary in a broad range. Various factors contribute to the storm intensity including the atmospheric environment conditions and internal dynamics of each individual storm. In the 2005 hurricane season, Hurricanes Ophelia and Katrina both developed over the warm water near the east coast of South Florida. However, they evolved differently with distinct structure and intensity. Katrina became one of the most intense Category 5 hurricanes in the Gulf of Mexico, whereas Ophelia remained a relatively weak Category 1 hurricane over several days near the Gulf Stream. Ophelia developed from a tropical depression to a hurricane over the Gulf Stream. It subsequently slowed down to near stationary for two days by an unusual set of the environmental flow. The slow motion of Ophelia induced a strong ocean response with mixing and upwelling of cold water below the mixed layer. The observed SST reached 21 oC, a 7-8 degree of SST cooling beneath the hurricane. However, Ophelia was able to maintain a weak hurricane and tropical storm status more than three days before re-intensified before approaching the North Carolina coast. RAINEX conducted multi-day aircraft observations with three airborne Doppler radar, GPS dropwindsondes, and AXBTs. It provides one of the most unique set of observations into this unusual storm for most of it lifecycle. The inner eyewall dissipated slowly while the storm-induced SST cooling increased. However, Ophelia was surrounded by warm SST and a relative low shear environment. The converging inflow was able to main the rainbands in the outer region, which eventually formed an outer eyewall to sustain the storm from dissipation for a long period time. The convection and the hurricane vortex in Ophelia were much shallower than most hurricanes observed previously, with radar echo tops near 10-km level or lower, compared to others close to 15-16 km over the same region. The high-resolution (1.3km grid spacing) coupled atmosphere-ocean model was able to capture the storm structure and intensity. In contrast, the uncoupled atmosphere model over-intensified the storm with unrealistic air-sea fluxes due to the lack of upper-ocean cooling. Detailed analyses from the models and RAINEX observations will be presented in this talk.