Binary mesovortex structure associated with southwest vortex

Previous work has concluded that the southwest vortex (SWV) is a single mesoscale vortex. Applying the National Centers for Environmental Prediction Final Operational Global Analysis, the Interim European Centre for Medium‐Range Weather Forecasts Re‐Analysis, and the non‐hydrostatic mesoscale Advanced Research Weather Research and Forecasting (WRF) model to a case study, we discovered a new type of SWV associated with another coexisting mesoscale warm and moist vortex. In the case study, meso‐β‐scale vortex‐A was generated at 1800 UTC 17 July, and dissipated around 0500 UTC 18 July 2013, with a lifespan of approximately 11 h. Vortex‐B occurred at 0600 UTC 17 July and moved out of the Sichuan Basin at 0800 UTC 18 July 2013, remaining over the basin for approximately 26 h. Stronger atmospheric upward motion and two mesoscale rainbelts associated with each of the vortices further demonstrate the binary mesoscale vortex structure related to the SWV using the WRF model. The quasi‐geostrophic balance of the two mesoscale cyclonic circulations is responsible for the generation and maintenance of the two closed mesoscale vortices.


Introduction
The Tibetan Plateau (TP) is the largest plateau in the world, with the Sichuan Basin located on its eastern flank. A mesoscale vortex, known as a southwest vortex (SWV) because of its location over southwest China (Tao and Ding, 1981), is commonly found at the eastern and southeastern flanks of the TP (Wang et al., 1993). Unlike the mesoscale convective vortex that is most evident between 500 and 600 hPa over the central United States (Davis and Trier, 2007), the SWV is normally most visible in the lower troposphere centered at 700 or 850 hPa (Lu, 1986;Fu et al., 2013Fu et al., , 2014b. The SWV has been studied extensively. Lu (1986), e.g. showed that in a SWV the lower troposphere and the middle troposphere were dominated by convergence, while the upper troposphere is controlled by divergence (Zhao and Fu, 2007). Huang (1986) found that ascending motion existed in the center and periphery of the SWV. A strong and well-developed SWV can stretch up to 100 hPa, which is extremely deep (Chen et al., 1998). In general, the SWV has complex temperature and humidity characteristics during its formation and development stages. Kuo et al. (1988) found that a high equivalent potential temperature ( e ) occurred in the center of the SWV. These characteristics have many similarities with the vortices found in the United States, as summarized by Menard and Fritsch (1989). Recently, Li et al. (2014) studied the SWV using ensemble-based analyses and forecasts methods, indicating that a more baroclinic environment facilitated the evolution of the SWV. The mechanisms associated with the formation and development of the SWV are largely influenced by the unique thermo-dynamical and dynamical environments of the plateau, including topography (Wu and Chen, 1985;Jiang et al., 2012;Wang and Tan, 2014), latent heat release (Kuo et al., 1986;Wang and Orlanski, 1987;Kuo et al., 1988), and the interaction between SWV and other systems, such as the eastward movement of the plateau vortex (Wang, 1987;Yu et al., 2016).
As a mesoscale vortex, SWV can comprise small-scale weather systems. Tao (1980) found that three mesoscale vortices that were lined up with one shear line were associated with the SWV. However, due to the lack of high spatial and temporal resolution datasets, it is difficult to find multiple mesoscale systems associated with the SWV. This paper identifies and studies a new binary vortex structure related to the SWV. The article is structured as follows: data and methods are described in Section 2; Section 3 presents the results, including the observed and simulated results of the binary mesoscale vortices, and an analysis of the potential mechanism for the formation and development of the binary mesoscale vortices; and Discussion and conclusion are presented in Section 4.

Data and methods
Hourly gauged rainfall data, including the conventional meteorological observations and automated weather The large-scale circulations associated with the SWV were analyzed based on the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis with 1 ∘ × 1 ∘ latitude-longitude grids at 6-h intervals. The Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) with a temporal resolution of 6 h and a spatial resolution of 0.5 ∘ × 0.5 ∘ was also used to analyze the weather circulation.
To investigate the evolution of the SWV, version 3.4.1 of the non-hydrostatic mesoscale Advanced Research Weather Research and Forecasting (WRF) model was applied. A one-way nested run was performed in this simulation, and its initial and lateral conditions were derived from the NCEP FNL Operational Global Analysis. The WRF horizontal grid spacing is 30 km (200 × 150 grid points) in the domain, which has 28 vertical levels. The numerical simulation was initialized at 0000 UTC 17 July 2013 and the results were output hourly with a total simulation length of 36 h. The model parameterization schemes in the domain were as follows. The Rapid and Accurate Radiative Transfer Model scheme (Mlawer et al., 1997) was used for longwave radiation and the Dudhia scheme (Dudhia, 1989) for shortwave radiation. We used the Kain-Fritsch convective scheme (Kain, 2004) for cumulus convection. The Monin-Obukhov scheme (Janjić, 2002) was used for the surface layer physics. The Noah land surface scheme (Ek et al., 2003) was applied along with the Yonsei University planetary boundary layer (PBL) scheme (Hong et al., 2006). The microphysics scheme was used by Thompson et al. (2008). The model grids are shown in Figure 1.

Observations of the binary mesoscale vortices associated with the SWV
A mesoscale SWV was observed over Sichuan Province and an isoline of 3055 gpm occurred in the vortex at 1200 UTC 17 July 2013 ( Figure S1, Supporting information). The vortex had been generated in Sichuan Province at 0600 UTC 17 July 2013 (not shown). Following the criteria of Lu (1986), it was categorized as a typical SWV. In accordance with the SWV in the lower troposphere, a longwave trough was observed at the 500-hPa level behind the lower level SWV. This further demonstrates that the SWV is a shallow system and has baroclinic features.
To study the detailed structure of the mesoscale SWV, the 700-hPa FNL analyses over the region are given in Figure 2. At 1800 UTC 17 July, the shape of the SWV began to change. Mesoscale cyclonic vortices appeared in the northeast (labeled 'A') and south ('B') of Sichuan Province, although the 3080 gpm isoline was not closed in mesoscale cyclonic vortex-A. Based on the criteria of Orlanski (1975), vortex-A was a meso--scale vortex. Although vortex-A satisfied the criteria of meso--scale, the temporal scale of vortex-B was 2 h beyond the guidelines of Orlanski (1975) on the meso--scale; therefore, vortex-B was classified as a mesoscale vortex. The weather patterns presented in Figure 2(b) persisted until 0000 UTC 18 July 2013. The two mesoscale vortices were observed from 1800 UTC 17 July to 0600 UTC 18 July. Compared with the scope of SWV (Bao and Li, 1985), the location of vortex-A was only 1 ∘ N of SWV. Therefore, further study is required on whether vortex-A belonged to SWV. At 0600 UTC 18 July 2013, vortex-A began to move to the northeast, and vortex-B moved south. Similar weather patterns were also present in the ERA-Interim dataset.

Simulation of the binary mesoscale vortex structure
Many studies, including two major TP scientific field experiments in 1979 and 1998 (Xu and Chen, 2006), have examined the formation and development of SWVs (Li et al., 2014). Figure 3 shows the two mesoscale vortices simulated by WRF at 0600, 1800, and 2100 UTC 17 July, and at 0300, 0500, and 0800 UTC 18 July 2013. The WRF model configuration, initial and lateral boundary conditions, and physical parameterization schemes were introduced as described previously. At 0600 UTC 17 July 2013, a parent mesoscale vortex was produced by WRF (Figure 3(a)). Twelve hours later at 1800 UTC 17 July, the parent mesoscale vortex split into two mesoscale vortices ('A' and 'B'). Meso--scale vortex-A was located over the northeast of Sichuan Province, and mesoscale vortex-B was located over the south of the province.
Vortex-A began to form from 1800 UTC 17 July 2013, and then merged to another low system (labeled 'L') at 0500 UTC 18 July 2013.  isolines, they should be considered as two mesoscale vortices associated with the developing SWV, as shown in Figure 3(c). Fu et al. (2014a) proposed that 88.5% of the detected SWVs caused rainfall. In heavy rainfall, an oblique meso--scale vortex appears frequently in the interior of the SWV (Gu et al., 2008). Figure 4 shows the observed and WRF-simulated hourly precipitation. The two rainbelts ('A' and 'B') for the observed and WRF-simulated precipitation are separated in space (Figure 4). The differences between them, including the location and the strength, were beyond the scope of this study. Rainbelt-A was located roughly to the south of vortex-A, and rainbelt-B to the south of vortex-B, which further demonstrates that the two mesoscale vortices (Figure 3) were two separate features.
To investigate the vertical structure of these binary vortices, Figure 5 shows the vertical cross-section of the WRF-simulated potential pseudo-equivalent temperature and vertical motion between the center points of A and B. At 0600 UTC 17 July 2013, there was a strong upward motion center between the vortices; however, the two centers were observed in the typical time of the two mesoscale vortices. Furthermore, two warm and moist centers also existed above vortices A and B. These two warm and moisture centers lasted until 0800 UTC 18 July 2013, although vortices A and B had changed by this time.

Potential mechanism for the generation and maintenance of the binary mesoscale vortices
Based on the geostrophic adjustment theory (Blumen, 1972), mesoscale mass fields tend to adjust to wind fields. At 0600 UTC 17 July 2013 (Figure 3(a)), an extensive area of positive relative vorticity was produced by WRF over Sichuan Province. After a few hours, two closed mesoscale vortices began to emerge as a result of geostrophic adjustment. This situation continued until 0500 UTC 18 July 2013 (Figure 3(e)). Following the movements of the relative vorticity centers, vortex-A merged with another low system, L, in the northeast of Sichuan Province, and vortex-B moved to

Discussion and conclusion
The SWV is often associated with extreme weather, especially with heavy rain. The severity of heavy precipitation caused by SWV is second only to that caused by tropical cyclones in China (Wang et al., 1996). Due in part to the fact that their spatial and temporal scales are too small to be captured by the conventional observational network, the two mesoscale cyclonic vortices were difficult to observe in the relatively coarse data. To satisfy the quasi-geostrophic balance of the two mesoscale cyclonic circulations, the binary mesoscale vortex structure associated with SWV was generated and maintained. Questions still remaining for future work include whether vortex-A belongs to the SWV, whether vortex-B is responsible for the generation and maintenance of vortex-A, and how vortex-B and vortex-A interact with each other.