Response of Internal Waves to 2005 Typhoon Damrey over the Northwestern Shelf of the South China Sea

Continuous observation of sea water temperature and current was made at Wenchang Station(19°35′N,112°E) in 2005. The data collected indicate vigorous internal waves of both short periods and tidal and near-inertial periods. The temperature and current time series during 18-30 September were examined to describe the upper ocean internal wave field response to Typhoon Damrey(0518) . The strong wind associated with the typhoon,which passed over the sea area about 45 km south of Wenchang Station on 25 September,deepened the mixed layer depth remarkably. It decreased the mixed layer temperature while increasing the deep layer temperature,and intensified the near-inertial and high-frequency fluctuations of temperature and current. Power spectra of temperature and current time series indicate significant deviations from those obtained by using the deep ocean internal wave models characterized by a power law. The frequency spectra were dominated by three energetic bands:around the inertial frequency(7.75× 10-6 Hz) ,tidal frequencies(1.0×10-5 to 2.4×10-5 Hz) ,and between 1.4×10-4 and 8.3×10-4 Hz. Dividing the field data into three phases(before,during and after the typhoon) ,we found that the typhoon enhanced the kinetic energy in nearly all the frequency bands,especially in the surface water. The passage of Damrey made a major contribution to the horizontal kinetic energy of the total surface current variances. The vertical energy density distribution,with its peak value at the surface,was an indication that the energy injected by the strong wind into the surface current could penetrate downward to the thermocline.

ANALYSIS OF CAUSATION OF ASYMMETRIC PRECIPITATION ASSOCIATED WITH SEVERE TYPHOON DAMREY

Severe typhoon Damrey moved across Hainan Island from 00:00 UTC 25 September to 00:00 UTC 27 September in 2005 and gave rise to a significant rain process during its 48-h passage.The precipitation intensity on the southern part of the island is stronger than that on the northern,showing obvious asymmetric distribution.Using Tropical Rainfall Measuring Mission(TRMM) data,the associated mesoscale characteristics of the precipitation were analyzed and the formation of asymmetric rainfall distribution was investigated in the context of a subsynoptic scale disturbance,vertical wind shear and orographic factors.The results are shown as follows.(1) The subsynoptic scale system provided favorable dynamic conditions to the genesis of mesoscale rain clusters and rainbands.(2) The southern Hainan Island was located to the left of the leeward direction of downshear all the time,being favorable to the development of convection and leading to the asymmetric rainfall distribution.(3) Mountain terrain in the southern Hainan Island stimulated the genesis,combination and development of convective cells,promoting the formation of mesoscale precipitation systems and ultimately resulting in rainfall increase in the southern island.

Observations and analysis of environment and acoustic field changed by the passage of typhoon Damrey in the Yellow Sea in 2012

In order to investigate the environment and acoustic filed change induced by typhoon in shallow sea, we conducted two ex- periments just before and after the passage of typhoon Damrey, which is the strongest to affect the area north of the Yangtze River since 1949, in the Yellow sea in 2012. The data show that the temperature of the whole water column increases dramati- cally except the sea surface layer after the passage of Damrey while the salinity decreases obviously. The thermocline deepens and weakens, which leads to a change of internal wave activity. The transmission losses （TL） of the two experiments show that the environment change induced by typhoon can increase the TL as large as 8 dB at a distance of 9.2 km and depth of 15 m. The scintillation index （SI） of the sound intensity is simulated to estimate the change of the effect of internal wave activity on acoustic field showing that the SI decreases to a half after the typhoon＇s passage. The normal mode structures of the two experiments are also significanOy different due to the thermocline changes. In addition, the signal arrives earlier after the ty- phoon＇s passage due to the water temperature increase.

The Analysis of the Asymmetric Precipitation Caused by the No.10 Tropical Cyclone “Damrey” in 2012 Based on Observation and Numerical Simulation

The No.10 tropical cyclone “Damrey” in 2012 is the first landing typhoon on the north of the Yangtze River after 1949. After its landing, Damrey showed an obvious asymmetric structure and precipitation. Using the ERA-interim reanalysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF), the study simulated the whole process of Damrey from pre-landing to extinct by using WRF model. Based on the model result and FY-2E satellite data and observation data, the study analysis the causes of the asymmetric structure of Damrey. It is found that the descending motion is strong on the west and south sides of the typhoon, they blocked the southwest water vapor transport. So the development of convective cloud system was hindered, and the wind shear on the west and south sides on the typhoon was stronger than on the east and north. It caused the result of the precipitation on the east and north sides of typhoon much more than on the west and south. Q-vector, upper level jets and other factors are also analyzed in this study.