# Characterization of Tropical Cyclone Intensity Using the HY-2B Scatterometer Wind Data
## 1 Introduction  
Tropical cyclones (TCs) may cause severe damages, notably in coastal areas, through strong winds, heavy rain, and potential storm surges. The TC influence mainly depends on its location and intensity, as well as on the vulnerability of the affected areas. Hence the primary concern in operational TC monitoring and forecasting is to obtain an accurate TC location and intensity information. A variety of observational systems and techniques, including in-situ buoys [1], airborne radiometers [2], and satellite remote sensors [3; 4; 5; 6], have been developed to monitor tropical cyclones, among which the Global Navigation Satellite System Reflectometry (GNSS-R) techniques, as well as spaceborne radiometers and scatterers are particularly useful. For instance, the L-band GNSS-R instruments onboard low-orbit satellites, such as the Cyclone Global Navigation Satellite System (CYGNSS), are able to measure sea surface wind speed under all precipitating conditions [3]. With a constellation of eight micro-satellites, CYGNSS provides relatively large spatial coverage and high temporal resolution. However, due to the low sensitivity of the reflected GNSS signal to high winds, the high winds from GNSS-R measurements are usually of large retrieval errors [7]. The low frequency radiometers, especially the L-band instruments onboard the Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) missions, are sensitive to high and extreme sea surface wind speeds, while little sensitive to rain. This gives SMOS and SMAP distinct advantages over satellite scatterometers carrying co-polarized beams, in terms of retrieving extreme TC winds (>35 m/s). Moreover, recent works show that the C- and X-band radiometer channels, although more sensitive to rain than L-band radiometers, are also useful in determining the maximum TC wind speedwith relatively low bias and root-mean-square errors [8; 9]. Nonetheless, the shortcoming of radiometers in TC monitoring is that they have relatively poor spatial resolution to resolve the steep gradient structure in the vicinity of hurricane eyewalls, notably for those high category storms with relatively small eyewall radii [5]. Satellite scatterometers generally provide high-quality vector winds over the global ocean surface, such that they have been widely used in the study of TC structure and location [10; 11; 12; 13]. Notably the Ku-band pencil-beam scatterometers, such as SeaWinds onboard QuikSCAT [14], OSCAT onboard the Indian Oceansat-2 [15] and SCATSat-1 satellites [16], and HSCAT on the Chinese HY-2 satellite series [17], are with wide observation swaths of ~1800 km, making them more likely to acquire the entire TC structure in a single overpass than the C-band fixed fan-beam scatterometers, e.g., Advanced Scatterometers (ASCAT) onboard the MetOp satellite series [18]. However, due to signal saturation in the (co-polarized) radar backscatter measurements, both C- and Ku-band scatterometer extreme wind speeds show large errors. Moreover, the rain contamination on the Ku-band backscatter leads to an underestimation of the TC intensity estimates [7; 19]. As such, it is difficult to characterize the TC intensity directly using the maximum wind speed from scatterometer-derived wind fields. Particularly, it is difficult to distinguish true high winds from the rain-contaminated Ku-band retrievals [20], making the estimation of TC intensity with the Ku-band scatterometer wind speed unreliable. In addition, scatterometers are generally calibrated against in situ buoy measurements [7], leading to a large underestimation of high and extreme winds when using dropsonde wind data as a reference (i.e., the gold standard for the extreme winds community). A recent study by [PERSON] et al.