###### Abstract  
The spaceborne synthetic aperture radar (SAR) cross-polarization signal remains sensitive to sea surface wind speed with high signal-to-noise ratio under tropical cyclone (TC) conditions. It has the capability of observing TC intensity and size information over the ocean with large coverage and high spatial resolution. In this paper, TC wind distribution characteristics were studied based on SAR images. We collected 41 Sentinel-1A/B cross-polarization images covering TC eye, which were acquired between 2016 and 2020. For each case, sea surface wind speeds were retrieved by the modified MS1A model in a spatial resolution of 1 km. After deriving the value and location of maximum wind speed, wind fields were simulated symmetrically within a 200 km radius. Two new methodologies were proposed to calculate the decay index and the symmetry index based on the retrieved and simulated wind fields. Characteristics of the two indices were analyzed with respect to maximum wind. In addition, the maximum and averaged wind speeds of the right, back and left side of the motion direction were compared with TC intensity and storm motion speed. Statistical results indicate that right-side wind speed is the strongest for maximum and average, the wind difference between the left and right side is dependent on storm motion speed.  
t 29 September 2021 4 November 2021  
by co-polarization (vertical vertical (VV) or horizontal horizontal (HH)) signals [16]. However, the normalized radar cross sections (NRCS) of cross-polarization (vertical horizontal (VH) or horizontal vertical (HV)) increase with increasing wind speeds even at high wind conditions (>25 m/s). The comparison between VV- and VH-polarization NRCS is shown in Figure 1. The VV NRCS is simulated by the CMOD5 model [17]. The VH NRCS is simulated by the modified MS1A (MMS1A) model [18]. In addition, many studies show that cross-polarized NRCS is independent of wind direction, indicating that wind retrieval from cross-polarization SAR images does not need a priori input [19; 20; 21].  
Moreover, advantages of SAR include wide spatial coverage, high spatial resolution and operating under day, night and all-weather conditions, which make up for the deficiency of spaceborne scatterometer and flight observations and contribute to the investigations of TC wind distribution [22; 23; 24]. Based on SAR retrievals, many studies regarding storm morphology focused on intensity, wind direction, size parameters, decay and asymmetry of wind distribution. For example, [PERSON] et al. input a set of hurricane morphology parameters and hurricane motion speed to the inflow angle model to derive the hurricane wind direction [8]. According to the asymmetric and symmetric wind fields fitted with SAR retrievals, [PERSON] et al. proposed a method to compute the asymmetric index and reported that a TC wind field becomes more symmetric when its intensity increases [25]. [PERSON] et al. proposed a symmetric TC wind field model by the fitting function of tropical cyclone fullness (TCF) [7] and maximum wind speed, radial wind curve and the Rankine vortex model [18].  
In this paper, new simple methodologies were proposed to calculate TC decay and symmetry indices using SAR images. Wind distribution characteristics were studied based  
Figure 1: The comparison between VV- and VH- polarization NRCS simulated by the CMOD5 model and the MMS1A model in different wind speeds and incident angles of 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\). \(\phi\) is the relative wind direction.  
on the combination of best tracks and high-resolution SAR retrievals. The remainder of this paper is organized as follows: Section 2 introduces the Sentinel-1 and best track data used in this work. The calculation methodologies of decay and symmetry indices are described in Section 3. The characteristics of TC wind distribution are analyzed in Section 4. Finally, conclusions and perspectives are given in Section 5.