Relationships between surface winds of Super Typhoon Lekima (2019) and topographic disturbances in eastern China

The impact of the topography on the surface wind field of typhoon is a critical cause of phenomena such as path deflection, intensity reduction, and precipitation increase. However, the direct influence of the topography on tropical cyclone surface wind structures, especially nonisolated continental topography, remains uncertain. The objective of this study was to investigate the relationship between the characteristics of the surface wind field and the topography based on the Weather Research and Forecasting (WRF) model simulation results of Super Typhoon Lekima, and the scale separation method was applied to systematically analyze the impact of the nonisolated continental topography. The main conclusions are as follows:

The topographic disturbances and simulated surface wind field are subdivided into meso-α, meso-β, and meso-γ scale phenomena with horizontal scales ranging from larger than 200 km, 20 km to 200 km, and smaller than 20 km using the Barnes filter, respectively. The distribution of the wind field exhibited a much more clear correlation with the topographic disturbance in meso-β and meso-γ scale compare to meso-α scale fields: the average wind speed (Vavg) over land was larger than that over the ocean, and Vavg over the region with a significant topographic disturbance was larger than that over the region with a smaller topographic disturbance.

The cyclonic circulation was decomposed into meridional and zonal wind components to explore the impact of the topographic disturbance on the wind field with different intensities and wind directions. The wind of meso-α scale exhibited no obvious correlation with the corresponding topographic disturbance. The distribution characteristics of the meso-β scale zonal wind and meridional wind near the tropical cyclone center were similar to those of the meso-α scale phenomena. Within the region where the large-scale zonal wind was westerly, the meso-β scale westerly (easterly) winds always corresponded to positive (negative) topographic disturbances. Conversely, within the region where the large-scale zonal wind was easterly, the meso-β scale easterly (westerly) winds always corresponded to the positive (negative) topographic disturbances. Regarding meridional wind, the meso-β scale northerly (southerly) winds always corresponded to positive (negative) topographic disturbances with the land dominated by northerly winds. The distribution characteristics of the meso-γ scale meridional and zonal winds were similar to those of the meso-β scale winds. The analysis results indicate that the influence of terrain disturbance is mainly reflected in the meso-β and meso-γ scale surface wind fields: positive topographic disturbances imposed an acceleration effect on the large-scale airflow, while negative topographic disturbances exerted a deceleration effect (Figure 1).

Furthermore, the fitted linear relationships between the topographic disturbance and 10-m zonal wind, topographic disturbance and 10-m meridional winds with different background winds are analyzed (Figure 2). Firstly, the magnitude of the correlation (positive or negative) depends on the wind direction of the large-scale airflow. A positive (negative) correlation exists between the zonal wind and the disturbance terrain at the meso-β and meso-γ scales when the meso-α-scale zonal wind is positive (negative). Secondly, the wind velocity of the background wind field also affects the fitted relationship at different simulation times even for the same dominant background wind direction. In general, the higher the absolute value of the large-scale background wind speed is, the higher the disturbance wind speed for the same terrain disturbance. Moreover, the velocity of the disturbance wind is related to the scale of the disturbance terrain. Specifically, disturbance terrain with a smaller scale could cause stronger disturbance airflow. The differences between these fitted relationships may relate to the critical factors determining the effect of terrain on airflow, including the terrain height, width, and wind speed.

Furthermore, based on the understanding of the correlations between topographic disturbances and tropical cyclone wind field characteristics during Super Typhoon Lekima, sensitivity numerical experiments and other methods will be considered to further explore the mechanism of the effect of multiscale complex terrain on the wind field structure of the landing tropical cyclone.