A Method for Diagnosing the Secondary Circulation with Saturated Moist Entropy Structure in a Mature Tropical Cyclone

Under the adiabatic, axisymmetric and steady assumption, a relationship between the saturated moist entropy structure and the secondary circulation in a tropical cyclone(TC) is derived from the continuity equation. It is found that the isentropic surfaces coincide with the streamlines, and the streamfunction can be expressed with saturated moist entropy. The secondary circulation and the saturated moist entropy structure depend on each other. Thus, a method for diagnosing the secondary circulation with the structure of saturated moist entropy is proposed. The method is verified with a simulated intense idealized TC with a highly axisymmetric structure. The diagnosed secondary circulation reproduces well the moist inflow in the boundary layer and the moist updraft in the eyewall. This method facilitates secondary circulation diagnosis in theoretical or mature TCs that satisfy the adiabatic, axisymmetric and steady approximations.

Comparative analysis of the rapid intensification of two super cyclonic storms in the Arabian Sea

A comparative analysis of the rapid intensification(RI)of super cyclonic storms Chapala(2015)and Kyarr(2019)in the Arabian Sea is conducted using the North Indian Ocean tropical cyclone data,microwave sounding images,the NOAA OISST data and the ERA5 reanalysis data.Results show that the subtropical westerly jet stream and the Southern Hemisphere anticyclonic circulation led to the formation of an obvious double-channel outflow from the northern and southern sides of the two storm centers,and the substantial inflow appeared at the eastern boundary layer of both storms.These promoted the vertical ascent motion and release of the latent heat of condensation.A warm sea surface is a necessary but not dominant factor for the RI of cyclonic storms in the Arabian Sea.During the RI of Chapala and Kyarr,the deep vertical wind shear was less than 10 m s-1;moreover,the mid-level humidity conditions favored the RI of the two cyclonic storms.Chapala had a single warm core,whereas Kyarr had double warm cores in the vertical direction.The impacts of the latent heat of fusion is more obvious for Chapala,and the potential vorticity in its inner core increases from 4.4 PVU to 8.8 PVU,whereas the potential vorticity and vorticity in the inner core of Kyarr do not change significantly.Microwave detection images show that both Chapala and Kyarr were accompanied by the formation of eyewalls during the RI phase,and the radius of maximum wind decreased and the maximum wind speed increased during the eyewall-thinning process.Both Chapala and Kyarr passed through a positive anomaly region of maximum potential intensity during the RI phase,which increases the possibility to develop to higher intensity after genesis.

Thermal Wind Imbalance along the Curved Streamline of the Secondary Circulation in Tropical Cyclones

The thermal wind balance in tropical cyclone(TC)eyewalls has been controversial for decades.This study reveals the relationship between the acceleration and curvature on the TC secondary circulation streamline,providing a way to judge thermal wind balance or imbalance in TCs from a simple but clear perspective.According to the relationship between the curvature and acceleration on the streamline,the vertical and radial components of the acceleration cannot be zero simultaneously on the streamline curve,implying that the thermal wind imbalance corresponds to the curvature of the streamline.On the regular scales of TCs,we discuss the conditions of the thermal wind balance approximation and find that the conditions become more stringent with increasing altitudes.In the TC secondary circulation,as an indication of thermal wind imbalance,gradient wind imbalance can be found in the low-level eyewall since there is usually a large curvature when the inflow in the low-level eyewall turns into updrafts sharply.Additionally,gradient wind imbalance also appears at the top level of TC eyewalls because the stringent conditions are too easily broken there.

CAN THE EXTREME RAINFALL ASSOCIATED WITH TYPHOON MORAKOT (2009) HAPPEN IN HONG KONG?

Typhoon Morakot hit Taiwan in August 2009 and brought torrential rain and high death toll to the region. The registered maximum cumulative rainfall depth approached the world record of the greatest point rainfall. In this paper, the risk of experiencing rain episodes of similar severity in Hong Kong was assessed using the Advanced Research WRF(Weather Research and Forecast) model to simulate a direct hit of Typhoon Morakot to the city. A number of numerical experiments were conducted by transplanting the vortex of Morakot and the associated environmental conditions to the South China Sea to study the amount of rainfall that could fall in Hong Kong. The results revealed that the difference in the topography between Taiwan and Hong Kong alone accounted for more than 60% of the total rainfall registered in Taiwan. The enormous land mass of China to the north of Hong Kong would also weaken Morakot rapidly upon its landfall over the south China coast, causing a shift in its track and redistribution of rainfall, and a further reduction of the rainfall amount that Hong Kong would receive. Despite that, some experiments suggested that Hong Kong could receive nearly 800 mm of rainfall in 24 hours, a figure that would break the historical record of 697.1 mm set in 1889 in Hong Kong.