While Typhoon Megi (2010) shared many common features of tropical cyclones (TCs) that crossed Luzon Island in northern Philippines, it experienced a significant inner-core size increase with little eyewall contraction during its rapid intensification (RI) phase. This is unusual since the majority of TCs experience an eyewall contraction with little or a slow inner-core size increase during RI. The inner-core size increase during RI of Typhoon Megi was simulated reasonably well using the Advanced Research Weather Research and Forecasting (ARW-WRF) model with both dynamical initialization and large-scale spectral nudging. In this paper processes responsible for the inner-core size increase of Typhoon Megi during its RI phase were analyzed based on a control simulation. It is shown that the inner-core size increase was primarily related to the binary interaction of Megi with a large-scale low-level depression in which Megi was embedded. The shearing/merging of the largescale depression with Megi and the subsequent axisymmetrization led to the strengthening of the outer circulation of Megi. Both the moist condition in the low-level depression and the binary interaction contributed to active spiral rainbands. Diabatic heating in spiral rainbands enhanced low-level inflow, which brought absolute angular momentum inward, increasing tangential wind speed outside the eyewall, thus leading to the outward expansion of tangential wind and the increase of the inner-core size of Megi.

This study explores, for the first time, the asymmetric distribution of precipitation in tropical cyclones (TCs) making landfall along east China coast using reflectivity data collected from coastal Doppler radars at mainland China and Taiwan. Six TCs (Saomai, Khanun, Wipha, Matsa, Rananim and Krosa) from 2004 to 2007 are examined. The temporal and spatial evolution of these TCs’ inner and outer core asymmetric precipitation patterns before and after landfall is investigated. The radius of inner-core region is a function of the size of a TC apart from a fixed radius (100 km) adopted in previous studies.
All six TCs possessed distinct asymmetric precipitation patterns between the inner- and outer- core regions. The amplitude of asymmetry decreases with the increasing TC intensity and it displays an ascending (descending) trend in the inner (outer) core. In the inner-core region, the heavy rainfall with reflectivity factor above 40 dBZ tends to locate at the downshear side before landfall. Four cases have precipitation maxima on the downshear left side, in agreement with previous studies. As TCs approaching land (~ 2 hr before landfall), their precipitation maxima generally shift to the front quadrant of the motion partly due to the interaction of TC with the land surface. In the outer-core region, the precipitation maxima occur in the front quadrant of the motion in five of the six cases before landfall. After landfall, the precipitation maxima shift from the right-front quadrant clockwisely to the right-rear quadrant of the motion collocated well with the mountainous areas along the coast, which indicates the impact of topography forcing on the precipitation distribution. This study illustrated how the precipitation asymmetry in the inner- and outer-core at different stages of TC landfall is affected by storm motion, vertical wind shear and topography.

Severe typhoon Vicente (1208) was the first tropical cyclone that necessitated the issuance of No.10 Hurricane Signal in Hong Kong since Typhoon York back in 1999. Hurricane force winds were recorded over the southwestern part of Hong Kong during the passage of Vicente. In the evening on 23 July 2012, “convective hot towers” appeared on the eyewall of Vicente and were captured on both radar imagery and lightning location map. The corresponding cloud top overshot 15 km up to the top of the troposphere, accompanied by cloud-to-ground lightning. Such observations signified that the associated updraft turned violent at the locations of convective hot towers. Shortly afterwards, Vicente intensified rapidly to a severe typhoon over the South China Sea to the south-southwest of Hong Kong around midnight, reaching its peak intensity with an estimated maximum sustained wind of 155 km/h near its centre. It is the first time in the century-long tropical cyclone history of Hong Kong that the signature of “convective hot towers” in a rapidly intensifying typhoon can be observed in details by multiple remotesensing platforms. This paper serves to document such rare observational evidence and share the main results as a checklist for reference by operational forecasters when monitoring proximate typhoons for signs of rapid intensification.

Satellite precipitation estimation has become a major source of data for global moisture and regional environmental monitoring. This preliminary study first reviews the current status of such applications especially for tropical cyclone landfalls, and the science behind rainfall estimation based on microwave emission. One of the most popular integrated rain retrieval product, the NASA TRMM 3B42 data, is validated by ground-based observations for ten landfalling tropical cyclones during 2007-2010 in the Taiwan area. While there is a general trend of underestimation by most satellite rainfall products compared with ground observations, the rainfall distributions within tropical cyclones are quite well revealed. This gives a reasonably good volumetric total rainfall within the cyclones, and thus sub-synoptic-scale rainfall footprints. However, there are large case-to-case variations for satellite rainfall estimates to capture individual convective episodes. In addition, land effects including those from topography are still the major difficulties. As a consequence of these factors, mesoscale rainfall footprints can deviate quite largely from those observed by ground measurements during tropical cyclone landfalls. Potential pathways to improve the current satellite rainfall products are discussed, which include development of statistical correction methodologies that consider different rainfall mechanisms; consideration of topographic effect and disaggregation of current rainfall products.

Analysis of field data was performed with respect to variations in the intensity and traveling of tropical cyclones resulting from a possible effect of the gravity anomaly. The obtained results suggest that this kind of influence may exist, even though it seems to be weaker than some of other known factors. According to the observed data, a decreased gravity is conducive to tropical cyclone intensification.