Near-Surface Moisture Variability in the Coastal Environments Revealed from Weather Radars

Convection initiation and storm intensity are sensitive to the near-surface moisture variation. However, current conventional observations are not readily available to capture such highly variability of moisture. The conventional weather radar can provide high resolution refractivity retrievals of air, in addition to the standard reflectivity, radial velocity and dual-polarimetric variables. The radar-retrieved refractivity fields provide detailed depictions of the near-surface moisture distribution at the meso-gamma scale. Radar-retrieved refractivity had been applied to the boundary layer structures in the U.S continental environments. This study presents a novel examination of the spatiotemporal characteristics of moisture variability in a moisture-rich summertime tropical coastal region in Taiwan over 4 weeks by using radar-retrieved refractivity.

The physiography in Taiwan lends itself to a variety of flow features and corresponding moisture behavior, neither have been well studied. Refractivity analyses demonstrate how a highly variable moisture field is impacted by the complex interaction between the synoptic-scale winds, local diurnal circulations, terrain, land use, and storm activities.

Under weak synoptic forcing conditions, the daytime refractivity (moisture) field develops differently under local surface wind directions determined by the synoptic-scale prevailing wind and the sea-breeze fronts, which further affects the occurrence of the inland afternoon storms. Furthermore, the refractivity fields illustrate higher-resolution moisture distribution compared to limited surface station point measurements by showing the lagged daytime sea-breeze front between the urban and rural areas and the detailed nighttime heterogeneous moisture distribution related to land-use and rivers. In convective environments, the refractivity fields illustrate how the near-surface moisture pattern varies with the storm types. During some long-lasting precipitation events, low refractivity frequently occurs at the foothills and further affects the propagating and intensity of the incoming convection. The cases here show a promising potential application of radar refractivity technique for enhancing our knowledge of moisture in moist coastal environments and for nowcasting.