Principal Investigator: Peter Dodge
Collaborating scientist(s):
Dr. Frank D. Marks, Jr.
Dr. Robert W. Burpee (National Tropical Prediction Center)
Dr. John Gamache
Objective: Analyze airborne Doppler radar data collected in Hurricane Gilbert on 14 September, 1988, to estimate the axisymmetric components of the three-dimensional windfield of this Category 5 hurricane.
Rationale: Hurricane Gilbert of 1988 was the most intense tropical cyclone yet observed in the Atlantic Basin. Data collected by a NOAA research aircraft on 13 September indicates that Gilbert's minimum sea-level pressure (MSLP) was 888 mb. A few hours after this aircraft left the storm, a second NOAA aircraft monitored the hurricane when its MSLP was ~ 895 mb. This aircraft had an airborne Doppler radar and was the first to record Doppler data in a tropical cyclone with a MSLP < 900 mb. The Doppler data allow us to estimate the kinematic windfield in the inner core, and compare it with the winds in other less-intense hurricanes.
Method: Doppler data from two orthogonal legs through the center of the storm were interpolated to a storm-relative coordinate system, in a pseudo-dual Doppler analysis. The three-dimensional wind field was then retrieved from the Doppler observations (see THREE-DIMENSIONAL VARIATIONAL ANALYSIS OF DOPPLER WIND OBSERVATIONS). Because the two legs comprise an hours worth of data, the windfield does not resolve convective features but does provide an estimate of the mean structure of the storm.
Accomplishment: The reflectivity pattern from a single sweep of the lower-fuselage radar shows Gilbert's mesoscale features (FIGURE 1). The inner eyewall is the closed ring > 25 dBZ located 1-20 km from the center. Rainbands 60-90 km from the center coalesced to form an outer eyewall. In the inner core the tangential winds > 50 ms-1 extended to 12 km, ~ 4 km higher than had been observed in previous hurricanes. (FIGURE 2) The secondary circulation had flow into the center up to altitudes of 2.5 km, and quite weak outflow above. The region between the inner and outer eyewalls was characterized by lower reflectivities, reduced flight-level winds and vertical velocities < 1 ms-1. The outer eyewall contained secondary maxima of flight-level wind, radar reflectivity and vertical velocity. Doppler analysis of a small part of the outer eyewall shows that there was a very shallow inflow layer, < 2 km deep. The height of the radar bright-band decreased beyond the outer eyewall, perhaps demarking the limit of subsidence warming. A manuscript is nearing completion that discusses the kinematic structure of the inner core, a portion of the outer eyewall, and the regions beyond, and compares these features with those observed in other hurricanes.
Key reference:
Dodge, P., R. W. Burpee, and F. D. Marks, Jr., 1991: Airborne Doppler radar analyses of the core of Hurricane Gilbert. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Amer. Meteor. Soc., Miami, 551-552.
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