## 2 Cruise Experiment  
The experiment was performed in the ECS with two cruises. The first cruise was conducted along the north coast of the ECS from 31 November to 4 December 2018, while the second cruise was conducted along the south coasts of the ECS from 28 April to 6 May 2019 (Figure 1). Overall, 41 stations were sampled in these two cruises experiments, of which 21 stations were covered by the first cruise and 25 stations by the second cruise. Since the two cruises were performed in two seasons, they covered a wide range of water properties (_SSS_ 19-34 psu, _SST_ 10-20 \({}^{\circ}\)C) and sea surface wind conditions (0-11 m/s), which caused a large variation of measured brightness temperature. During the cruises, the following instruments were deployed: a dually polarimetric L-band radiometer designed and produced by Radiometrics Corporation (Figure 2a), and a stabilized platform designed and produced by Watson Corporation to keep radiometer stable during the whole cruise (Figure 2b). Moreover, to obtain the auxiliary data matched with brightness temperature measurements, two CTD (Conductivity Temperature Depth) sensors from SeaBird Electronics and RBR Corporation and a meteorological station from RM Young anemometer were used to measure _SSS_, _SST_ and wind speed. The details of these instruments/experiments are briefly given below. (1) L-band Radiometer PR-1475:  
The PR-1475 is a dually polarimetric L-band radiometer designed and produced by Radiometrics Corporation. The antenna is a potter horn, which is a narrow band low sidelobe antenna, with a half-power beam width of 30\({}^{\circ}\) and measured side lobe levels at \(-\)23 dB. The potter horn design characteristically has sidelobes down more than 30 dB. The radiometric sensitivity is 0.1 K to 1 K depending on the integration time from 10 to 2500 msec and bandwidth of the radiometer is 25 MHz. Figure 1: Location map showing the in-situ sampling stations for the two cruises. TWWC: Taiwan Warm Current; CDW: Changjiang Diluted Water, with the solid line for the summer and spring seasons and the dash line for the winter and autumn seasons. (2) Stabilized platform:  
The stabilized platform is a particular model of the Watson product intended to hold the PR-1475 payload aligned to downward direction on a moving vehicle. This system includes a mounting bracket suited to hold the payload for rotation about its center of gravity, a mounting post made for 35 degrees of physical motion clearance and a custom counterweight designed to compensate for the PR-1475 payload. (3) Salinity and temperature sensors:  
Two different CTD sensors (SBE 37-SI MicroCAT from Sea-Bird Instruments and XR-620 from RBR Corporation) were placed on the stern and the prow of the ship. The SBE 37-SI was placed at about 1 m below the sea surface on the stern of the ship. The XR-620 was deployed to obtain the profiles of salinity and temperature at sampling stations, and it was programmed for a sampling rate of 6 Hz. Since the sampling rate was higher compared to the deployment speed of XR-620, the measured salinity in the first effective record can be recognized as sea surface salinity. The radiometer was mounted on the stabilized platform, which was about 6 m high above the sea surface and provided dual polarization measurements of brightness temperature at 5 fixed incidence angles (16\({}^{\circ}\), 26\({}^{\circ}\), 36\({}^{\circ}\), 45\({}^{\circ}\) and 60\({}^{\circ}\)), and the circular-shaped field of view on the sea surface has the radius in the range of 3 m to 6 m according to different incidence angles (for example, the radius of field of view is about 3 m with incidence angle at 16\({}^{\circ}\)). During the cruises, some stations were measured in daytime, which were hampered by sunlight. In order to avoid radiation coming from the Sun, the antenna of the radiometer was pointed away (backwards) from the sun and the radiometer's field of view was in the shadow of the ship.