Converts maximum 10-m 1-minute symmetric sustained wind speed to gradient wind speed. The conversion factor depends on whether the storm is over land or water.
calc_gradient_speed(vmax_sfc_sym, over_land)A numeric vector of 1-minute sustained wind speed at 10 meters, with motion asymmetry removed (m / s).
TRUE / FALSE of whether the storm is over land (TRUE) or water (FALSE).
A numeric vector with maximum gradient-level 1-min wind speed (m / s).
This function uses the following conversion: $$V_{max,G} = \frac{V_{max,sym}}{f_r}$$ where:
\(V_{max,G}\): Max gradient-level 1-min sustained wind (m / s)
\(V_{max,sym}\): Max 10-m 1-min sustained wind with motion asymmetry removed (m / s)
\(f_r\): Reduction factor (see below)
The function uses a reduction factor based on Figure 3 in Knaff et al., 2011. If over water and within 100 kilometers of the storm's center, the ratio of gradient wind speed to surface wind speed is assumed to be 0.90. If over land, this reduction factor is reduced by 20% (\(0.9 * 0.8 = 0.72\)).
For this calculation, we assume that the radius of maximum wind for all storms is 100 kilometers or smaller (in the code, we do not calculate \(R_{max}\) until after we estimate gradient wind speed from surface wind speed, so we don't have that storm-specific estimate to use here).
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