A semi-idealized modeling study on the long-lived eastward propagating mesoscale convective system over the Tibetan Plateau

Based on a 16-warm-season statistical study on the mesoscale convective systems (MCSs) that were generated over the Tibetan Plateau (TP), 11 long-lived eastward propagating MCSs of the same type were selected for a composite semi-idealized simulation and a corresponding no-latent-heating sensitivity run by using the Weather Research and Forecasting (WRF) model. Common evolutionary features and associated mechanisms of this type of long-lived eastward propagating MCS were investigated. Main results are as follows: (i) This type of MCS was generated in a favorable background environment which was characterized by a notable upper-tropospheric divergence south of an upper-level jet, a strong warm advection around a middle-level shortwave trough’s central area, and an instable convective stratification below the trough. Development of the MCS featured rapid increase of cyclonic vorticity in the middle and lower troposphere. The convergence-related vertical stretching and tilting were key factors for the cyclonic-vorticity’s production, and convection-related upward cyclonic-vorticity transport contributed to the upward extending of the MCS. (ii) During the vacating stage of the MCS, it first coupled with a quasi-stationary Tibetan Plateau vortex (TPV) over the TP’s eastern section, and then decoupled from the vortex. In the former stage, the MCS contributed to maintaining ascending motions and convergence associated with the TPV, which favored its persistence; whereas, in the latter stage, decoupling weakened the TPV-associated convection significantly. This reduced the upward transport of cyclonic vorticity notably, which, together with the negative tilting effect, finally led to the vortex’s dissipation. (iii) After vacating TP, the MCS first weakened due to the disappearance of strong direct sensible heating from the TP on its bottom, and then, under the favorable conditions associated with the shortwave trough over the eastern section of the TP, the MCS redeveloped rapidly. Convergence-related cyclonic-vorticity production in the middle and lower troposphere and upward transport of cyclonic vorticity due to convection governed the MCS’s redevelopment. (iv) Sensitivity simulation shows that latent heating was a necessary condition for the formation and development of the long-lived eastward propagating MCS. On the one hand, this MCS affected the TP’s eastern section and downstream regions directly by inducing precipitation; and on the other hand, it exerted effects on the precipitation over a wider range in the downstream regions by modulating large-scale circulations over and around the TP.