# Tropical Cyclone Winds from WindSat, AMSR2, and SMAP: Comparison with the HWRF Model
## 2 Data Sets
### Hurricane Weather Research and Forecasting (HWRF) Model  
Our objective was to compare the storm observations from the TC-winds to another reliable external data source. To this end, we used the Hurricane Weather Research and Forecasting (HWRF) model. This operational, high-resolution model is used by agencies such as the Joint Typhoon Warning Center (JWTC) and the National Weather Service (NWS) Pacific Region (PR) for real-time tropical cyclone forecasting and forecast guidance  
Figure 1: Scalar TC-wind fields for SMAP (**a**) and vector wind fields for WindSat (**b**) along with rain rate as seen by WindSat (**c**) for satellite passes over TC Amphan on 19 May 2020.  
([[https://www.emc.ncep.noaa.gov/gc_wmb/vxt/HWRF/](https://www.emc.ncep.noaa.gov/gc_wmb/vxt/HWRF/)]([https://www.emc.ncep.noaa.gov/gc_wmb/vxt/HWRF/](https://www.emc.ncep.noaa.gov/gc_wmb/vxt/HWRF/)); [[https://ftpprd.ncep.noaa.gov/data/nccf/com/hur/prod/](https://ftpprd.ncep.noaa.gov/data/nccf/com/hur/prod/)]([https://ftpprd.ncep.noaa.gov/data/nccf/com/hur/prod/](https://ftpprd.ncep.noaa.gov/data/nccf/com/hur/prod/)); accessed on 10 May 2021) [25]). There are a large number of HWRF model realizations of TC surface winds available from the past several years that can be used for the TC-wind comparison. Since the HWRF model is operational and does not have a publicly accessible data archive, these data were provided by request from both the NOAA Environmental Modeling Center (EMC) and the University of Michigan ([PERSON]; personal communication). For our study, we obtained HWRF winds for 0-h analyses at 00Z, 06Z, 12Z, and 18Z for 19 storms between 2017 and 2020 (see Appendix A for information on how to download similar real-time data), and used them in a comparison with the TC-winds satellite observations. Note that these 0-h analyses, which are produced every 6 h, are not forecasts themselves, but rather initializations of the HWRF model. Each 0-h analysis contains a constructed vortex that is initialized using data assimilated from the National Center for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS) 6-h forecasts, as well as conventional in situ observations, satellite observations, and Doppler radar radial velocities where available [25]. We chose to use the HWRF 0-h analyses as opposed to the HWRF forecasts (which are produced every 3 h starting at each 0-h analysis time) because the 0-h analyses are less dependent on the HWRF simulation of storm motion/features. Thus, they can be expected to be more representative of the structure and intensity of a tropical cyclone at a given analysis time.  
Additionally, the HWRF model contains three different gridded domains: one large parent domain, and two nested inner domains. All three domains encompass the location of the storm at each analysis time, with each subsequent nested domain from the parent domain downward becoming successively smaller in area. In our study, we used data from innermost domain and part of the surrounding domain. These data extended 9 \(\times\) 9 degrees and had a spatial resolution of approximately 1.5 km. The use of this fine-scale domain allowed us to experiment with various methods of spatial resampling in order to find the one that best captured the storms' features as observed by the satellite and avoided smoothing them out too much. An in-depth discussion of this spatial resampling is presented in the following section.