# Tropical Cyclone Winds from WindSat, AMSR2, and SMAP: Comparison with the HWRF Model
## Methods
### Spatial Resampling
This was calculated by averaging all HWRF observations within a given 0.25 degree grid cell and assigning the resulting wind speed to that grid cell. In other words, the weighting function \(W\) was set equal to 1 if the HWRF wind lies within the 0.25 degree grid cell, and 0 if it lay outside. The resulting resampled HWRF wind field resembled the Gaussian average reasonably well. It is feasible to use this averaging method for doing a quick and simple resampling. Figure 4d shows the resulting wind field when a drop-in-the-bucket average was performed within a 40 km box. The resampled HWRF wind field appeared much coarser, and features of the storm were more difficult to resolve when compared to both SMAP and the Gaussian average. A 40 km drop-in-the-bucket average should not be used when matching the spatial scales of the high-resolution HWRF field with the low-resolution satellite field. The physical reason for this is that, even if the actual satellite footprint is 40 km, the signal is not collected uniformly over that footprint diameter, but rather with a Gaussian-shaped gain, as illustrated above. Figure 5 shows an example of the Gaussian weighting method applied to the HWRF winds at times surrounding a SMAP pass for the case of Hurricane Dorian on 30 August 2019. The top two panels display the HWRF wind fields at the original HWRF resolution both before and after the SMAP pass. The bottom left (Figure 5c) and bottom right panels (Figure 5e) show the same HWRF wind fields that have been resampled to the satellite resolution using the Gaussian weighting method, while the bottom middle panel (Figure 5d) shows the winds from the SMAP pass itself. It can be seen by comparing the resampled HWRF wind fields to that of SMAP that the chosen Gaussian resampling method resulted in model winds that closely resembled both the structure and intensity seen by the satellite. For example, in Figure 5, both the resampled HWRF wind fields, as well as SMAP, show a distinct band of winds in the SE quadrant of similar intensity and shape, despite the wind fields being several hours apart. Figure 5: Comparison of SMAP and HWRF winds for Hurricane Dorian on 30 August 2019 for a SMAP pass at 10:51 UTC. Top panels: the HWRF winds before resampling for the HWRF analysis times before (**a**) and after (**b**) the corresponding SMAP pass. Bottom panels: the resampled HWRF winds at analysis times before (**c**) and after (**e**) the SMAP pass (**d**). Another example of an application of the resampling method using Gaussian weighting is seen in Figure 6, which presents the wind fields in the same manner as Figure 5 for an AMSR2 pass over Hurricane Dorian on 4 September 2019 at 7:32 UTC. The AMSR2 pass did not observe Dorian's winds right near the coast, as they were flagged for contamination from land emission. The similarities between the satellite and the resampled HWRF wind fields are evident. The size of the eye, as well as the magnitude of the winds within it, in the resampled HWRF closely resemble those seen by AMSR2. The same applies for the band of strong winds in the NE quadrant of the eyewall. The agreement between AMSR2 and the resampled HWRF was best for the HWRF wind field at 6:00 UTC, which was closest to the AMSR2 overpass. Consistent storm structures and intensities were seen between the models and satellite sensors for most of the 19 storms analyzed in this study. These 19 storms are listed in Table 1. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Storm** & **Basin** & **Dates** \\ \hline Harvey & Atlantic & 18 August 2017–30 August 2017 \\ Irma & Atlantic & 1 September 2017–12 September 2017 \\ Jose & Atlantic & 15 September 2017–22 September 2017 \\ Maria & Atlantic & 17 September 2017–30 September 2017 \\ Lane & Pacific & 18 August 2018; 20 August 2018; 23 August 2018–24 August 2018 \\ Jebi & Pacific & 28 August 2018–4 September 2018 \\ Florence & Atlantic & 1 September 2018–16 September 2018 \\ Mangkhut & Pacific & 7 September 2018–16 September 2018 \\ \hline \hline \end{tabular}
\end{table}