Structure of precipitating systems over Taiwan's complex terrain during Typhoon Morakot (2009) as revealed by weather radar and rain gauge observations

SummaryThis study documents from an observational perspective the structure of precipitation systems over the complex topography of Taiwan as Typhoon Morakot (2009) impinged on the island on 8 August 2009. An advanced multiple-Doppler radar synthesis technique particularly designed for dealing with non-flat surfaces is applied to analyze the three-dimensional wind fields over the ocean and terrain. In the northern and southern portion of the analysis domain where the mountain slope is relatively gentle and steep, respectively, the radar reflectivity measurements indicate that the precipitation systems exhibit very distinct features, namely, horizontal translation in the north and abrupt intensification in the south. While still far from the southern mountainous region, a north-south oscillation of an east-west-oriented band of strong radar reflectivity (>40 dBZ) with a horizontal span of 20 km is observed. Along the mountain slopes, the band of strong radar reflectivity has a much wider north-south extent. Both the radar and rain gauge observations show that the major precipitation is primarily confined to the windward side of the mountains. An analysis of the saturated Brunt-Väisälä frequency 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 moisture will be consumed before the air reaches the leeward side of the mountains. The long duration and the wide range of heavy precipitation in the mountainous regions resulted in a record-breaking average (over the gauges) rainfall amount of 2000 mm over 4 days.The prevailing winds approaching the mountains are from the west. The cross-barrier wind speed has a maximum (∼40 m s−1) above the mountain crest that can be reasonably explained by a simplified shallow water model.The capability of applying the weather radar to provide a reliable quantitative estimate of the rainfall over a large area with high temporal and spatial resolution is demonstrated using dual-polarimetric radar data. The potential applications of the knowledge of the wind and precipitation characteristics in hydrology and other fields are addressed in this manuscript.

► Behavior of the Typhoon Morakot (2009) rainbands over Taiwan is documented. ► Unstable air that is continuously lifted by topographic forcing results in torrential rainfall. ► Rainfall amounts on the lee side are only 30% of those on the windward side of the mountains. ► A wind maximum that takes place above the mountain crest can be explained theoretically. ► Rainfall estimation using the dual-polarimetric radar gives satisfactory results in mountainous terrain.