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Liang Wu
,
Zhiping Wen
,
Ronghui Huang
, and
Renguang Wu

Abstract

The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

Full access
Guixing Chen
,
Weiming Sha
,
Toshiki Iwasaki
, and
Zhiping Wen

Abstract

Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.

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Wenxin Zeng
,
Guixing Chen
,
Yu Du
, and
Zhiping Wen

Abstract

A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

Free access
Wenxin Zeng
,
Guixing Chen
,
Lanqiang Bai
,
Qian Liu
, and
Zhiping Wen

Abstract

Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of type-2 episodes. The NLLJs strengthened the mesoscale low-level ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced mei-yu rainband was reestablished in Central China during the type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall.

Open access