The Initial Mesoscale Vortexes Leading to the Formation of Tropical Cyclones in the Western North Pacific

A statistical analysis of the initial vortexes leading to tropical cyclone(TC)formation in the western North Pacific(WNP)is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018.It is found that TCs in the WNP basically originate from three kinds of vortexes,i.e.,a mid-level vortex(MV),a low-level vortex(LV),and a relatively deep vortex with notable vorticity in both the lower and middle troposphere(DV).Among them,LV and DV account for 47.9%and 24.2%of tropical cyclogenesis events,respectively,while only 27.9%of TCs develop from the MV,which is much lower than that which occurs in the North Atlantic and eastern Pacific.Such a difference might be ascribed to the active monsoon systems in the WNP all year round.Due to the nearly upright structure of mid-level convergence in the early pre-genesis stage,TC genesis efficiency is the highest in DV.Compared with MV,LV generally takes a shorter time to intensify to a TC because of the higher humidity and the stronger low-level cyclonic circulation,which is related to air-sea interaction and boundary-layer convergence.Further examination of the relationship between tropical cyclogenesis and large-scale flow patterns indicate that the TC genesis events associated with LV are primarily related to the monsoon shear line,monsoon confluence region,and monsoon gyre,while those associated with MV are frequently connected with easterly waves and wave energy dispersion of preexisting TC.Compared with other flow patterns,tropical cyclones usually form and intensify faster in the monsoon confluence region.

HIGH-RESOLUTION NUMERICAL SIMULATION OF TYPHOON LONGWANG(2005) WITH THE SPECTRUM NUDGING TECHNIQUE

With the Weather Research and Forecasting model(WRFV3.2.1), the application of spectrum nudging techniques in numerical simulation of the genesis and development of typhoon Longwang(2005) is evaluated in this work via four numerical experiments with different nudging techniques. It is found that, due to the ability to capture the large-scale fields and to keep the meso-to small-scale features derived from the model dynamics, the experiment with spectrum nudging technique can simulate the formation, intensification and motion of Longwang properly. The improvement on the numerical simulation of Longwang induced by the spectrum nudging depends on the nudging coefficients.A weak spectrum nudging does not make significant improvement on the simulation of Longwang. Although the experiment with four-dimensional data assimilation, i.e., FDDA, also derives the genesis and movement of Longwang appropriately, it fails to simulate the intensifying process of Longwang properly. The reason is that, as the large-scale features derived from the model are nudged to the observational data, the meso- to small-processes produced by the model dynamics important to the intensification of typhoon are nearly smoothed by FDDA.

Evolution of the Mid-tropospheric Vortex during the Formation of Super Typhoon Megi(2010)

As a follow-up of a previously published article on the contribution of tropical waves,this study explores the evolution of the mid-tropospheric mesoscale cyclonic vortex(MV)during the formation of Typhoon Megi(2010)with a successful cloud-resolving simulation.It is found that the formation and intensification of the MV were related to the deep convection and subsequent stratiform precipitation,while the weakening of the MV was related to the shallow convection.Both the upward transport of vorticity related to the deep convection and the horizontal convergence associated with the stratiform precipitation contributed to the formation and intensification of the MV.Even though the latter was dominant,the former could not be ignored,especially in the early stage of the MV.The MV played dual roles in the formation of Megi.On the one hand,the formation and intensification of MV were primarily associated with the stratiform precipitation,which induced the low-level divergence inhibiting the spin-up of the near-surface cyclonic circulation.On the other hand,the coupled low-level cold core under the MV benefited the accumulation of the convective available potential energy(CAPE),which was favorable for the convective activity.A sensitivity experiment with the evaporative cooling turned off indicated that the development of the MV retarded the genesis process of Megi.

Circulation Features Associated with the Record-breaking Typhoon Silence in August 2014

Climatologically, August is the month with the most tropical cyclone(TC) formation over the western North Pacific(WNP) during the typhoon season. In this study, the reason for abnormal TC activity during August is discussed—especially August 2014, when no TCs formed. The large-scale background of August 2014 is presented, with low-level large-scale easterly anomalies and anticyclonic anomalies dominating over the main TC genesis region, a weak monsoon trough system,and a strong WNP subtropical high(WPSH), leading to significantly reduced low-level convergence, upper-level divergence,and mid-level upward motion. These unfavorable large-scale conditions suppressed convection and cyclogenesis. In August2014, equatorial waves were inactive within the negative phase of the Madden–Julian Oscillation(MJO), with fewer tropical disturbances. Although the low-level vorticity and convection of those disturbances were partly promoted by the convective envelopes of equatorial waves, the integral evolution of disturbances, as well as the equatorial waves, were suppressed when propagating into the negative MJO phase. Moreover, the upper-level potential vorticity(PV) streamers associated with anticyclonic Rossby wave breaking events imported extratropical cold and dry air into the tropics. The peripheral tropospheric dryness and enhanced vertical wind shear by PV streamer intrusion combined with the negative MJO phase were responsible for the absence of TC formation over the WNP in August 2014.

SIMULATION EXPERIMENTS ON THE MM5V3 COUPLED WITH LAND SURFACE MODEL BATS

In this paper,the complex land surface model BATSle was coupled to the PSU/NCAR MMSV3.And the coupled model was used to simulate the heavy rain in summers of 1991 and 1998.The results show that,with the detailed land surface parameterization,the model can (1) simulate the distribution of heavy rain in Yangtze River Basin in summers of 1991 and 1998 more precisely,and improve the results by increasing the simulated precipitation rate;(2)improve the simulation results of surface temperature:(3)improve the simulation results of the circulation and temperature fields at middle and high levels.