## 4 Results and Discussion
### Impact of Different Scatterometer Product Resolutions
The presence of the rear-inflow jet ensured a wind maximum that started at or below 500 m above ground level, as friction near the surface prevented a rise in the wind speed to reach this maximum (logarithmic wind profile). At the eastern side of the domain and away from the precipitation (Figure 6a; R1), wind retrievals indicated southwesterly flow. Further south of the respective region (below 28.8\({}^{\circ}\)N) the wind direction shifted abruptly to northwest. However, the ECMWF model winds used in the ambiguity selection process indicated southheasterly flow over the whole domain (Figure 7). The coastal buoy observations of wind speed and direction for all stations were also examined before and after ASCAT overpasses to ensure that the buoys were operating normally. The observations indicated abnormal behavior of the buoys during this time and were suspicious, therefore, the ambiguity removal solutions were considered here as likely correct wind vectors. Scatterometers retrieve winds near the surface; i.e., equivalent neutral winds, therefore the wind gradient should have a distinctive signature, especially at the boundaries of the horizontally expanding cold air. The changes in the wind field induced by the outflow boundary were depicted in the wind gradient field, where the mag  
Figure 4: ASCAT-A and -B 25-km wind products on 28 May 2016 at 02:51 UTC and 03:42 UTC, respectively. The configuration is the same as in Figure 3.  
nitude increased from near zero value in the open ocean to \(3\times 10^{-4}\) s\({}^{-1}\) within the system (Figure 6a). The two thresholds used in the GF algorithm for the 50-km retrievals were \(6.8\times 10^{-5}\) s\({}^{-1}\) and \(1.2\times 10^{-4}\) s\({}^{-1}\), respectively. A unique GF (Figure 6a, black polygon) was identified that extended about 200 km in horizontal distance and located in front of the system. At the overpass of MetOp-B satellite, the system had propagated further away to the open ocean, towards the southeast, and it was characterized by intense precipitation with maximum rain rate of 100 mm h\({}^{-1}\) (Figure 6b). Over convective core of the system high values of wind gradient existed, reaching a maximum of 6 \(\times\) 10\({}^{-4}\) s\({}^{-1}\), associated with wind variability over intense rain rates. At the eastern side of the system (Figure 6b, R1) the wind had a similar response to ASCAT-A. In front of the system the wind gradient was characterized by higher values than at the overpass of ASCAT-A, indicating a more vigorous outflow boundary, which might have emanated from intersecting boundaries of nearby convective cores. The gradient wind feature (Figure 6b, black polygon) expanded outwards but in a more confined shape than ASCAT-A, probably denoting the presence of a well-defined outflow boundary. The thresholds used in the GF process were the same as for ASCAT-A since both ASCAT instruments are well calibrated with each other and have similar performance [68; 69]. A sensitivity test was performed on the two thresholds to capture differences in the population of the gradient wind features, owing to smaller-scale wind features that could be associated with the MCS and its evolution. The primary threshold removes the background noise and the secondary accounts for the bias due to lower wind gradients. The primary (secondary) threshold used in the GF method varied from \(1-9\times 10^{-5}\) (\(10^{-4}\)). The results indicated that there was no change in the number of features, thus providing additional confirmation that wind vectors at 50-km spatial resolution cannot resolve smaller than mesoscale wind features. The coastal wind product at 25-km spatial resolution provides retrievals as close as 50 km from the coast (Figure 6c,d). At the overpass of ASCAT-A (Figure 6c) many of the coastal wind vectors were contaminated by rain, therefore the suggested wind field provided a more realistic solution. Once again, wind speed within the precipitation system reached a maximum of 24 m s\({}^{-1}\), whereas over the open ocean the maximum was 10 m s\({}^{-1}\).