Rainfall Patterns Observed by Digitized Radar During the Landfall of Hurricane Frederic (1979)

In September 1979, two research teams traveled to the coastal area in the path of Hurricane Frederic to record observations of the storm’s rainbands with mobile radar recorders. The researchers were in position at the National Weather Service offices at Slidell, Louisiana and Pensacola, Florida, a few hours before Frederic’s outer convective bands reached the Gulf Coast. Although the recorder taken to Pensacola was damaged in transit, the recording system at Slidell collected digital data for 26 h as Frederic moved ashore at 6–7 m s−1, approximately 125 km to the cast of Slidell. Calculations of storm-total rainfall indicate that local rainfall maxima tended to occur in two general areas: 1) parallel to the coast near the point of landfall, with a northward extension approximately along Frederic’s track, and 2) on a long band oriented from south-southeast to north-northwest ∼50 kin to the west of the storm track. The storm-total rainfall maximum along the coast was explained by a rapid increase in the intensity and area coverage of deep convection in mesoscale rainbands in the north eyewall that occurred as the north eyewall interacted with the coastline. Raingage-radar comparisons indicate that the storm-total rainfall estimated by the radar is probably within a factor of 2 of the true. Maximum rainfall totals measured by gages and determined by radar were ∼250 mm. Frederic’s maximum accumulated rainfall was slightly below average, relative to other hurricanes that have made landfall along the Gulf Coast of the United States. During the four hours that the most intense convection in the north eyewall was near the coast, maximum hourly rainfall rates were 50–75 min. In this four-hour period, 3% of the land area within 100 km of the center had hourly rainfall > 50 mm h−1, and 39% of the same area had rain rates of 25–50 mm h-1. Land areas with the greatest wind damage were highly correlated with the location of radar-observed reflectivities ≥ 41 dB(Z). Analyses of time series of radar reflectivities and 5 min peak wind gusts at the surface indicate that the maximum surface winds near the glut occurred a few kilometers inside the radar eye.