In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea（SCS）. A wake of baroclinic near-inertial waves（NIWs） commenced after Nes...In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea（SCS）. A wake of baroclinic near-inertial waves（NIWs） commenced after Nesat passed the array. The associated near-inertial currents are surface-intensified and clockwise-polarized. The vertical range of NIWs reached 300 m, where the vertical range is defined as the maximum depth of the horizontal near-inertial velocity 5 cm/s. The current oscillations have a frequency of 0.709 9 cycles per day（cpd）, which is 0.025 f higher than the local inertial frequency. The NIWs have an e-folding time-scale of 10 d based on the evolution of the near-inertial kinetic energy. The depth-leading phase of near-inertial currents indicates downward group velocity and energy flux. The estimated vertical phase velocity and group velocity are 0.27 and 0.08 cm/s respectively, corresponding to a vertical wavelength of 329 m. A spectral analysis reveals that NIWs act as a crucial process to redistribute the energy injected by Typhoon Nesat. A normal mode and an empirical orthogonal function analysis indicate that the second mode has a dominant variance contribution of 81%, and the corresponding horizontal phase velocity and wavelength are 3.50 m/s and 420 km respectively. The remarkable large horizontal phase velocity is relevant to the rotation of the earth, and a quantitative analysis suggests that the phase velocity of the NIWs with a blue-shift of 0.025 f overwhelms that of internal gravity waves by a factor of 4.6.展开更多
Based on prediction data of T213 and ECMWF and historical information of tropical cyclone,the causes of long lasting and large scale rainstorm brought by Nesat in Guangxi were analyzed.Nesat,of high intensity,carried ...Based on prediction data of T213 and ECMWF and historical information of tropical cyclone,the causes of long lasting and large scale rainstorm brought by Nesat in Guangxi were analyzed.Nesat,of high intensity,carried much vapor and energy and went through Beibu Gulf by subtropical high pressure.After Nesat made landfall,subtropical high pressure maintained high and gradient between typhoon circulation and subtropical high pressure increased;the typhoon was supported by abundant vapors and energies by powerful eastern flows.Furthermore,northern cold airs went downward and met southeast wind behind of the typhoon,resulting in shears and convective precipitation.Therefore,the rains brought by the typhoon itself,jet stream,and cold air contributed to long lasting rainfall under influence of Nesat.展开更多
Fifty-seven days of moored current records are examined, focusing on the sequential passage of Typhoons Nesat and Nalgae separated by 5 days in the northwestern South China Sea. Both typhoons generated strong near-ine...Fifty-seven days of moored current records are examined, focusing on the sequential passage of Typhoons Nesat and Nalgae separated by 5 days in the northwestern South China Sea. Both typhoons generated strong near-inertial waves(NIW) as detected by a moored array, with the near-inertial velocity to the right of the typhoon path significantly larger than to the left. The estimated vertical phase and group velocities of the NIW induced by Typhoon Nesat are 0.2 cm s^(-1) and 0.85 m h^(-1), respectively,corresponding to a vertical wavelength of 350 m. Both the vertical phase and group velocities of the NIW induced by Typhoon Nalgae are lower than those of Typhoon Nesat, with the corresponding vertical wavelength only one-half that of Nesat. The threshold values of induced near-inertial kinetic energy(NIKE) of 5 J m^(-3) reach water depths of 300 and 200 m for Typhoons Nesat and Nalgae, respectively, illustrating that the NIKE induced by Typhoon Nesat dissipated less with depth. Obvious blueshifts in the induced NIW frequencies are also detected. The frequency of NIW induced by Typhoon Nesat significantly increases at water depths of 100–150 m because of Doppler shifting, but decreases significantly at water depths of 100–150 m for Nalgae because of the greater influence of the background vorticity during the passage of Typhoon Nalgae.展开更多
基金The National Natural Science Foundation of China under contract Nos U1133001,41030855 and 41376027the National High Technology Research and Development Program(863 Program)of China under contract No.2013AA09A502
文摘In September 2011, Typhoon Nesat passed over a moored array of instruments recording current and temperature in the northern South China Sea（SCS）. A wake of baroclinic near-inertial waves（NIWs） commenced after Nesat passed the array. The associated near-inertial currents are surface-intensified and clockwise-polarized. The vertical range of NIWs reached 300 m, where the vertical range is defined as the maximum depth of the horizontal near-inertial velocity 5 cm/s. The current oscillations have a frequency of 0.709 9 cycles per day（cpd）, which is 0.025 f higher than the local inertial frequency. The NIWs have an e-folding time-scale of 10 d based on the evolution of the near-inertial kinetic energy. The depth-leading phase of near-inertial currents indicates downward group velocity and energy flux. The estimated vertical phase velocity and group velocity are 0.27 and 0.08 cm/s respectively, corresponding to a vertical wavelength of 329 m. A spectral analysis reveals that NIWs act as a crucial process to redistribute the energy injected by Typhoon Nesat. A normal mode and an empirical orthogonal function analysis indicate that the second mode has a dominant variance contribution of 81%, and the corresponding horizontal phase velocity and wavelength are 3.50 m/s and 420 km respectively. The remarkable large horizontal phase velocity is relevant to the rotation of the earth, and a quantitative analysis suggests that the phase velocity of the NIWs with a blue-shift of 0.025 f overwhelms that of internal gravity waves by a factor of 4.6.
文摘Based on prediction data of T213 and ECMWF and historical information of tropical cyclone,the causes of long lasting and large scale rainstorm brought by Nesat in Guangxi were analyzed.Nesat,of high intensity,carried much vapor and energy and went through Beibu Gulf by subtropical high pressure.After Nesat made landfall,subtropical high pressure maintained high and gradient between typhoon circulation and subtropical high pressure increased;the typhoon was supported by abundant vapors and energies by powerful eastern flows.Furthermore,northern cold airs went downward and met southeast wind behind of the typhoon,resulting in shears and convective precipitation.Therefore,the rains brought by the typhoon itself,jet stream,and cold air contributed to long lasting rainfall under influence of Nesat.
基金supported by the National Natural Science Foundation of China(Grant Nos.41676008&40876005)the National Key Research and Development Program of China(Grant No.2016YFC14001403)the National Program on Global Change and AirSea Interaction(Grant No.GASI-IPOVI-04)
文摘Fifty-seven days of moored current records are examined, focusing on the sequential passage of Typhoons Nesat and Nalgae separated by 5 days in the northwestern South China Sea. Both typhoons generated strong near-inertial waves(NIW) as detected by a moored array, with the near-inertial velocity to the right of the typhoon path significantly larger than to the left. The estimated vertical phase and group velocities of the NIW induced by Typhoon Nesat are 0.2 cm s^(-1) and 0.85 m h^(-1), respectively,corresponding to a vertical wavelength of 350 m. Both the vertical phase and group velocities of the NIW induced by Typhoon Nalgae are lower than those of Typhoon Nesat, with the corresponding vertical wavelength only one-half that of Nesat. The threshold values of induced near-inertial kinetic energy(NIKE) of 5 J m^(-3) reach water depths of 300 and 200 m for Typhoons Nesat and Nalgae, respectively, illustrating that the NIKE induced by Typhoon Nesat dissipated less with depth. Obvious blueshifts in the induced NIW frequencies are also detected. The frequency of NIW induced by Typhoon Nesat significantly increases at water depths of 100–150 m because of Doppler shifting, but decreases significantly at water depths of 100–150 m for Nalgae because of the greater influence of the background vorticity during the passage of Typhoon Nalgae.
