Evolution of the Structure of Precipitation in Hurricane Allen (1980)

Reflectivity data from the airborne radar systems on board the three NOAA aircraft were gathered during six consecutive days in Hurricane Allen of 1980. The data have been used to specify the horizontal and vertical precipitation distribution within 111 km radius of the hurricane center. The evolution of the structure and intensity of the precipitation in the storm is described from representative time composite radar maps for seven research flights made during the 6-day period. The eyewall was characterized by a narrow ring (12–15 km wide) of intense reflectivity (42–47 dBZ) surrounding the center of the storm at a radius that varied in time from 12–40 km. The eyewall had steep radial gradients of reflectivity (4–5 dB km−1) and tilted radially outward in height. The rainbands were characterized by areas of enhanced reflectivity embedded in a region of stratiform rainfall that contained a distinct bright band at the height of the 0°C isotherm. The most striking changes in structure during the 6-day period were the rapid contraction in eyewall radius and the development of a secondary ring of intense reflectivity 80–100 km from the storm center. These changes in eye radius appeared to be related to the vortex evolution, as discussed by Willoughby and others. Changes in storm intensity, coincident with the eyewall radius changes, seemed to have little effect on the total storm rainfall or latent heat release. The maximum storm rainfall occurred when the storm had a double eyewall structure. After the period of the double eyewall, the mean rain rate in the eyewall increased as the storm approached maximum intensity. However, coincident with the increase in eyewall rain rate, the eyewall area decreased, resulting in little change in the total storm rainfall. The sequence of time composites provided the first opportunity to describe, quantitatively, the precipitation distribution within 111 km of the center of a mature hurricane that was away from land influences. The rainfall analysis showed that the mean rain rates in the eyewall were a factor of 6 greater than those outside the eyewall (11.3 mm h−1 versus 1.8 mm h−1), but because the eyewall region encompassed such a small area, it only contributed 40% of the total rainfall within a radius of 1° latitude of the storm center. The precipitation distribution around the storm was asymmetric; more rainfall occurred ahead of the storm than behind. In general, the maximum precipitation in the eyewall region was within 15–20° of the storm track. The maximum rainfall in the rainband region was 40–50° to the right of that in the eyewall.