Calculates the surface wind speed based on an estimated gradient wind speed at a point and the radius from the storm center to the grid point.
gradient_to_surface(wind_gl_aa, cdist)A numerical value with estimated gradient-level wind speed (m / s) at a grid point.
A numerical value with radius from the storm center to the grid point, in kilometers.
A numeric vector with the estimated symmetric surface wind speed at the grid point, in meters / second.
The reduction factor is based on Figure 3 of Knaff et al., 2003. Over water, it is estimated to be 0.9 up to a radius of 100 km, 0.75 for a radius of 700 km or more, and decrease linearly between a radius of 100 km and 700 km. Points over land should use a reduction factor that is 20% lower. Because all of the counties are over land, the function makes this adjustment for all grid points.
Knaff JA, DeMaria M, Molenar DA, Sampson CR, and Seybold MG. 2011. An automated, objective, multiple-satellite-platform tropical cyclone surface wind speed analysis. Journal of Applied Meteorology and Climatology 50(10):2149-2166