A new multiple-Doppler-radar wind analysis algorithm and its application in resolving the flow structure of Typhoon Morakot (2009) over Taiwan's complex terrain

A new variational-based method is developed to retrieve the three-dimensional wind field over complex terrain using data from multiple Doppler radars. Experimental results indicate that this method can recover the winds along the radar baseline. The retrieved winds satisfy the vorticity equation, thus a direct vorticity budget diagnosis can be conducted without producing a residue term, which also leads to a more accurate retrieval of the thermodynamic parameters. This method can simultaneously analyze data from any number of radars, thus is suitable for the wind analysis using networking radars. Most importantly, the flow field immediately above the mountain slopes can be successfully reconstructed by incorporating the Immersed Boundary Method into the computation. This method is employed to analyze the flow structure of Typhoon Morakot (2009) after landfall. For this case, a noticable feature of the prevailing westerly flow is that the wind speed increases along the mountain slope, reaching a maximum (~ 40 m s-1) above the mountain crest, which can be reasonably explained by a shallow water model. Further analysis reveals that the upstream atmosphere is statically unstable, which implies that the lifting of the incoming convective cells by the topography will easily trigger precipitation. Thus, most of the rainfall is confined at the windward side of the mountains even under the strong wind condition.