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A severe convection process occurred in southern Xinjiang during June 15–17, 2021. Here, the convection initiation mechanism is revealed by analyzing the impacts of three-dimensional divergence (

Located in the middle of Eurasia, Xinjiang is not directly affected by the monsoon system. It features a typical continental arid and semiarid climate (

Deep convective systems are accompanied by intense convergent airflow in lower layers and divergent airflow in higher layers. The contributions from divergence include water vapor transport (

A series of studies on the convection cell structure (

A severe convection process occurred in southern Xinjiang during June 15–17, 2021. Daily precipitation at the Hetian station exceeded 45.5 mm, breaking the local daily precipitation record. Luopu County experienced daily precipitation over 100 mm, reaching a rainstorm level (^{-1} at 1200 UTC on 15 June 2021 (

Time series of the observed surface wind speed at the Hetian automatic weather station (units: m s^{−1}).

The 200-hPa trough line was located in Central Asia, and Xinjiang was in front of the upper trough at 0000 UTC on 15 June 2021. The jet stream center was to the north of Qinghai Province. Southern Xinjiang was on the right side of the upper jet stream entrance, leading to divergence flows in higher layers (

^{–5} s^{-1}), geopotential heights (blue contours, units: gpm) and wind fields (wind barbs, units: m s^{−1}) at 200 hPa, and geopotential heights (blue contours, units: gpm) and wind fields (wind barbs, units: m s^{−1}) at

The convection initiation process was simulated using the WRF (V4.4) model and the NCEP (National Centers for Environmental Prediction) operational assimilation system GSI (Gridpoint Statistical Interpolation, Version 3.7). The background field and lateral boundary of the model were obtained from the NCEP global forecast system (GFS) analysis field and forecast field (0.5°×0.5°). The model cold start time was 1200 UTC on 13 June 2021. The three-dimensional variational scheme was used to assimilate the satellite observations and conventional data in the GDAS (Global Data Assimilation System) every 6 h. After two assimilation cycles, a 48-h forecast was carried out from 0000 UTC on 14 June 2021. The horizontal resolution of the model was 3 km (901×901 grid points) with a total of 61 vertical levels. The model top was fixed at 50 hPa. The WSM6 cloud microphysics scheme (

The observed precipitation data were obtained from the CLDAS (China Meteorological Administration Land Data Assimilation System) hourly merged precipitation grid dataset (0.05°×0.05°). The observed rain belts extended from northwest to southeast along the Kunlun Mountains. The simulated precipitation center (79°E, 37°N) was located west of the observed precipitation center (80°E, 37°N). This discrepancy may have resulted from the initial fields, approximations and parameterizations of the model, but the overall precipitation area was consistent with the observations (

The primary convection cell was in the initiation stage at 0700 UTC on 15 June 2021 (

Cross sections of reflectivity (shadings, units: dBZ) and ^{–4} s^{-1}, dotted line for negative value), ^{–4} s^{-1}), and ^{–4} s^{-1}) along 78.91°E at 0700 UTC on 15 June 2021; ^{–4} s^{-1}), vertical divergence (red line, units: 10^{–4} s^{-1}), and three-dimensional divergence (black line, units: 10^{–4} s^{-1}) integrated from 2 km to 7 km and hourly precipitation (purple dotted line, units: mm). “A” denotes the convection position. The values on the right side of the legend denote the linear correlation coefficient between divergence and simulated precipitation in panel

Taking Eqs

The left-hand side term in Eq.

All the terms in Eq.

Cross sections of the ^{−1} s^{−3}), ^{−1} s^{−3}), ^{−1} s^{−3}), ^{−1} s^{−3}), ^{−1} s^{−3}), and ^{−1} in panels

The vertical pressure gradient force can accelerate or decelerate the atmospheric vertical motion, resulting in the triggering or inhibition of convection. The vertical pressure gradient force can be expressed in Cartesian coordinates as follows:

Taking

The left-hand side term in Eq.

Equation

Cross sections of ^{-3}), ^{−2}) and ^{−1}) after 6 s along 78.91°E at 0700 UTC on 15 June 2021. The labels “B” and “C” denote the positions corresponding to positive changes in VPGF and vertical velocity, respectively.

The local change in the vertical pressure gradient force (

Time series of the domain mean of the local vertical pressure gradient force change (black solid line, units: m s^{−3}) and vertical velocity change (black dotted line, units: m s^{−1}) integrated from

The pressure at the windward slope of the Kunlun Mountains was strengthened by the convergence of

Cross sections of ^{−1}) and streamlines, ^{−1}) and water vapor flux divergence (red contours, units: 10^{–6} g cm^{−2} hPa^{−1} s^{−1}) along 78.91°E at 0700 UTC on 15 June 2021.

According to the analyses above, a schematic of convection initiation in southern Xinjiang was built. As shown in

Schematic of convection initiation in southern Xinjiang.

The upward VPGF provided dynamic uplift conditions for air parcels. The vertical acceleration increased, and the upward motions were strengthened (

A severe convection process occurred in southern Xinjiang during June 15–17, 2021, resulting in heavy precipitation. The WRF model and GSI assimilation system were used here to perform a high-resolution simulation of this convection. The observed precipitation area, magnitude and evolution trend were well-captured by the simulation data. On this basis, the effects of three-dimensional divergence on convection initiation were fully considered. The impacts of three-dimensional divergence on the pressure and vertical pressure gradient force were analyzed to reveal the convection initiation mechanism. The preliminary conclusions are summarized as follows.

The horizontal divergence and vertical divergence had opposite phase distributions, and both had strong centers away from the convection. The three-dimensional divergence, by contrast, agreed well with the convection position and precipitation evolution.

A pressure tendency equation including the forcing effects from three-dimensional divergence was derived based on the mass continuity equation and thermodynamic equation in Cartesian coordinates. The pressure change was dominated by the vertical pressure advection term and three-dimensional divergence forcing term. The negative pressure change tendency was weakened because of convergence motions from three-dimensional divergence, and a positive pressure change appeared in local areas. The mass field was adjusted by three-dimensional divergence, resulting in a local pressure change.

The vertical pressure gradient force can be affected by pressure changes, thus further impacting the vertical velocity. The vertical pressure gradient force equation was derived herein using the mass continuity equation and pressure tendency equation. The vertical gradient of the three-dimensional divergence term was the dominant term affecting the local change in the vertical pressure gradient force, which was determined by the pressure and vertical gradient of

The local change term on left hand side of the derived equations still had some imbalances with the forcing terms on right hand side. This was mainly aroused by the calculation errors, equation approximation and model errors. But the local change of pressure and VPGF was overall consistent with the spatial distribution mode of the main forcing terms on right hand side of the equations.

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

KZ and LR contributed to conception and design of the study. WZ and LT organized the database. LC and HL performed the data assimilation. KZ wrote the first draft of the manuscript. LR, WZ and LT reviewed and modified the manuscript.

This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA17010105), and the National Natural Science Foundation of China (42075008). This work was supported by the National Key Scientific and Technological Infrastructure project “Earth System Numerical Simulation Facility” (EarthLab).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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