The strength of a tropical cyclone (TC), broadly defined, is the wind speed beyond, but adjacent to, the eye wall. Strength is important for both basic and applied reasons: partial conservation of the angular momentum in the inflow of winds in the strength region affects TC intensity and accurate forecasts of strength at landfall are vital to warn the appropriate length of coast.
To estimate strength we employ a large data set established by the Hurricane Research Division within NOAA/AOML. This set consists of aircraft passes from the eye to as much as 150 km radius with 0.5 km horizontal resolution. Our focus is on 1454 passes in the 850 - 900 mb layer collected within 22 TCs from 1979 to 1995. These TCs include tropical storms to category 4 hurricanes. Since the passes reach to just 150 km radius we have a truncated view of strength compared to the area-weighted average wind speed in the 1.0 to 2.5 degrees latitude radial band surrounding the cyclone core used by Weatherford and Gray (1988). We define strength to be the mean tangential wind speed from 65 to 140 km for each pass. This is beyond the outer edge of the eye wall in virtually all TCs and is where we have an abundance of observations.
The best track data are employed to determine the motion of each TC, the stage in the life cycle when the sampling occurred, and the location of the pass relative to the circulation center. Both satellite and aircraft data are used to assess intensity of the system defined as minimum central pressure or maximum sustained winds in the eye wall. Each pass is classified according to TC category, deepening and filling rates, and motion of the system. We also identify the radius of maximum winds for each pass. Passes will be composited from several TCs and will also be examined on a case study basis. Questions we will explore include:
(1) How asymmetric is strength ?
(2) How are intensity and strength correlated ?
(3) What is the wind speed at 50, 100, and 150 km radius ?
(4) Does the modified Rankine vortex model fit the data ?
(5) Do strength changes precede intensity variations ?
We intend to discuss our findings in the context of the work done by Weatherford and Gray (1988), who examined northwest Pacific typhoons with data collected on reconnaissance flights.