# Tropical Cyclone Winds from WindSat, AMSR2, and SMAP: Comparison with the HWRF Model
## 1 Introduction  
Passive satellite microwave radiometers, which operate at frequencies in the C-band (4-8 GHz) or higher, have historically had difficulty measuring oceanic surface winds in areas of heavy precipitation [1, 2, 3]. This is due to the fact that raindrops attenuate the observed signal coming from the surface. In addition, it is difficult to distinguish the signals caused by wind roughening of the ocean surface from those due to rain. The attenuation can degrade the satellite retrievals to the point where they are not useable for scientific purposes. This can often lead to gaps in microwave-satellite coverage when precipitation is present, particularly in tropical cyclones, which are often affected by heavy precipitation. However, it is possible to mitigate the problem of rain attenuation for microwave sensors that operate at the C- and X-band (8-12 GHz) frequencies by using a linear combination of these channels that is simultaneously sensitive to wind speed and relatively insensitive to rain [4]. This is due to the fact that the effect of atmospheric scattering is still relatively small at these frequencies [5], and the spectral differences in brightness temperature (TB) due to wind induced surface emissivity between the two channels are relatively small, while the spectral differences due to rain attenuation are relatively large. Recently, an algorithm [4] was developed for the AMSR2, AMSR-E, and WindSat radiometers that takes advantage of their C- and X-band channels to measure winds in tropical cyclones, even in areas of heavy precipitation (see [6, 7, 8, 9] for a further description of theseinstruments). This algorithm was trained using the Soil Moisture Active Passive (SMAP) L-band (1.4 GHz) radiometer, which was launched by NASA in 2015 [10; 11; 12]. SMAP and other L-band radiometers, such as the Soil Moisture and Ocean Salinity (SMOS [13; 14]) radiometer, have several advantages over other microwave sensors when making wind measurements in tropical cyclones. First, due to the low frequency of their observed signal, L-band radiometers are minimally affected by rain, and can retrieve surface winds unobstructed [13; 15]. Secondly, the surface brightness temperature (TB) signal measured by L-band radiometers increases approximately linearly with wind speed and does not saturate at higher winds. This allows L-band radiometers to make accurate wind measurements up to 70 m/s (Category 5 tropical storms) without signal degradation [16; 17; 18; 19; 10; 13]. Because of this, SMAP is an ideal candidate to help train algorithms for both the WindSat and AMSR radiometers, as detailed in [4]. These new algorithms have resulted in the creation of new wind products referred to as 'TC-winds', which were developed and processed by Remote Sensing Systems (RSS) for the AMSR-E, AMSR2, WindSat, and SMAP radiometers. TC-winds are processed in near-real time, with \(\sim\)3 h latency for AMSR2 and SMAP, and are freely distributed to the public. The combined measurements from SMAP and AMSR2 provide very good spatial and temporal coverage in the tropics. As such, these TC-winds constitute an important data set for characterizing tropical cyclone intensity, shape, and structure throughout the course of a storm's lifetime. These satellite wind measurements offer valuable information when assimilated into tropical cyclone forecast models such as the US Navy's Automated Tropical Cyclone Forecasting System (ATCF) [20]. These assimilated data take the form of tropical cyclone fixes, which include the maximum 10 min maximum sustained winds and wind radii for the 34 kt (17 m/s), 50 kt (25 m/s), and 64 kt (33 m/s) winds for each satellite pass over a TC in all tropical ocean basins. Due to their wide range of scientific applications, it is important to assess the quality of the TC-winds by comparing them with reliable external sources.