The Tropical Cyclone Eyewall Mesovortex: A Physical Mechanism Explaining Extreme Peak Gust Occurrence in TC Olivia, 4 April 1996 on Barrow Island, Australia

Abstract

Abstract A possible world record peak 3-sec wind gust measurement of 220 kt (253 mph; 408 kph) was recorded on Barrow Island, Australia during the passage of Tropical Cyclone Olivia on April 10, 1996. This measurement was one of five extreme 3-sec gusts during a series of 5-min time periods. Gusts of 199, 220 and 202 kt were measured followed by a series of four lower values (minimum of 114 kt) which were then followed by two more extreme gusts of 187 and 161 kt in the 5-min time intervals. The elapsed time between gust maxima was 30 min, representing a scale of 8 nm (15 km) compared to the eye diameter of 40 nm (75 km). The 5-min average winds showed maxima and a minimum at the same time periods as the gusts. The pattern and scales suggests that a mesovortex was imbedded in the already strong eyewall mean winds (5-min mean maximum wind = 95 kt; 1-min mean maximum winds = 106 kt). The extreme gusts represented extreme gust factors of 2.27-2.75, nearly twice the average gust factor throughout the storm of 1.33. This clearly suggests that some process other than mechanical turbulence is important during this period. Radar images from the Bureau of Meteorology radar at Learmonth showed that this event took place at the maximum reflectivity gradient at the inner edge of the eyewall as it passed over the Barrow Island observing station. Eyewall mesovortices occur only at the inner eyewall edge. The observations also show it occurred just after the minimum pressure of 930 mb was observed, as the mean winds were increasing rapidly. In the previous hour, BoM instruments on nearby Varanus Island showed the minimum pressure to be 927 mb, also suggesting Barrow was at the edge of the eye. Varanus measured an extreme gust of 144 kt as a weaker section of the eyewall passed. Additional evidence for the mesovortex theory was seen in the relative humidity measurements. A sudden dip occurred in the near-saturated relative humidity at Barrow at the time of the lull between the two sets of extreme gusts. This may have been caused by a sudden subsidence such as would be expected to occur within the core of a mesovortex. These observations are consistent with the 180 kt flight level wind measured by research aircraft flying through an eyewall mesovortex in Hurricane Hugo (1989) northeast of Barbados and the 190 kt peak gust winds estimated from damage surveys in Hurricane Andrew (1992) in Florida and Tropical Cyclone Tracy (1974) in Darwin, Australia. Introduction During the landfall of intense tropical cyclones with surface 1-minute sustained winds in excess of 130 kt, it has frequently been difficult to obtain actual wind measurements since most instruments are distroyed. The landfall of Cyclone Tracy in December, 1974 in Darwin, Australia and Hurricane Andrew in August, 1992 in South Florida, USA created unprecedented distruction 1,2,3,4. Wind estimates were possible only by indirect means such as damage surveys 5, satellite estimation, ground-based radar data and composites of sparse available surface observations and high-level aircraft observations. Inferences concerning the physical mechansims responsible for the anomalous devastation in these storms has been suggested 2. The primary mechanism proposed has centered on the growth of intense convection in the hurricane's eyewall, convection generated by enhance surface convergence of air at the land-sea boundary. Intense convection in the strong shear region of the eyewall's inner edge is further hypothesized to spin up a convective scale mesovortex at the edge of the eyewall which grows quickly with time, on order of ten minutes, the convective cell lifetime.