Impacts of Convective Activity over the Tibetan Plateau on Plateau Vortex, Southwest Vortex, and Downstream Precipitation

Abstract

In summer, convective activity over the Tibetan Plateau (TP) is vigorous, with some of it moving eastward and vacating the plateau [defined as the eastward-moving type (EMT)]. Although the EMT only accounts for a small proportion, it is closely related to heavy precipitation east of the TP. This study investigates EMT impacts based on a series of composite semi-idealized simulations and piecewise potential vorticity (PV) inversion. The main results are as follows. (i) An EMT begins to affect downstream precipitation before it vacates the TP. A weaker EMT tends to cause the main downstream rainband to reduce in intensity and move southward. (ii) The EMT contributes to the formation of an eastward-moving plateau vortex (PLV) by enhancing convergence-induced stretching. Over the TP, the PLV mainly enhances/maintains the EMT, whereas during the vacating stage, the PLV dissipates (since convergence decreases rapidly when sensible heating from the TP reduces), which substantially reduces the intensity of the EMT. (iii) After PLV dissipation, a southwest vortex (SWV) forms around the Sichuan basin mainly due to convergence-induced stretching, convection-related tilting, and background transport. Piecewise PV inversion indicates that an EMT can directly contribute to SWV formation via lowering geopotential height and enhancing cyclonic wind perturbations around the Sichuan basin (even before its vacating stage), while neither of them governs the SWV formation. Sensitivity runs show that an EMT is not necessary for SWV formation, but can modify the SWV formation time and location, as well as its displacement, which significantly affects downstream precipitation.

Formation of a kind of heavy‐precipitation‐producing mesoscale vortex around the Sichuan Basin: An along‐track vorticity budget analysis

“…The semi-idealized simulation reproduced the salient features of the eight events used in composite, particularly (a) the evolution of the eastward-moving convective system that originated over the TP, (b) the SWV formation around the SCB and (c) the precipitation in the downstream regions (Fu et al, 2019). Therefore, it is reasonable to take the simulation results as being representative of this type of SWV formation.…”

“…To show the common features of SWVs that form under the influences of the eastward-propagating convective systems that originate over the TP, we conduct a semiidealized simulation based on a composite of typical SWVs in the summer of 2016 (Fu et al, 2019). One of the most remarkable characteristics among these eight events is that an SWV forms during the moving-out stage of a strong convective system that is generated over the TP.…”

“…The model configuration and simulation design are the same as those used by Fu et al (2019): they use a one-way triple-nested (36, 12 and 4 km) convection-permitting version of the Weather Research and Forecasting (WRF) model (Skamarock et al, Skamarock et al, 2008) that consists of 331 × 241 (d01), 541 × 331 (d02) and 1,141 × 721 (d03) grid points (not shown). The WRF single-moment sixclass microphysics scheme (Hong and Lim, 2006) and the Yonsei University planetary boundary layer scheme (Noh et al, 2001) are used in all domains, while the Kain-Fritsch cumulus parameterization scheme (Kain, 2004) is used only in d01 and d02.…”

“…The simulation is initiated at 0000 UTC, and runs for 60 hr, a period that includes (a) the entire moving-out stage of the convective system originated from the TP and (b) the formation stage of the SWV. Moredetailed model configurations and the design of the semiidealized simulation can be found in Fu et al (2019).…”

Key statistical characteristics of the mesoscale convective systems generated over the Tibetan Plateau and their relationship to precipitation and southwest vortices

“…(c) Which factors govern the evolution of longer‐lived EMTs? In addition, some EMTs can induce southwest vortices (SWVs) upon vacating the TP (Zhao et al ., 2004; Fu et al ., 2010; Fu et al ., 2019). However, the statistical relationship between SWVs and EMTs remains to be determined.…”