Zhiyong Meng

Motivated by an observed asymmetry in boundary layer convergence lines (BLCLs) between the Kubuqi and Ulan Buh desert borders of the Hetao Oasis in northern China, this work aims to document the differences in BLCL frequency and convective initiation and their potential drivers. Using radar, surface Automatic Weather Station, and reanalysis data in June–August from 2012 to 2022, we found that the Kubuqi border had more frequent, earlier‐forming, and more convective BLCLs, whereas the Ulan Buh border had fewer, shorter‐lived, and more tortuous ones. These contrasts were associated with sharper vegetation and thermal gradients, higher Bowen ratio, and a more favorable background wind on the Kubuqi side. Our findings demonstrate that BLCL behavior is highly sensitive to surface, flow, and terrain heterogeneity, even around a single oasis.

The present work examines a severe, long-lived bow echo in South China during 12–13 April 2016 and investigates the favorable factors for its strength and longevity using a series of 20 cloud-resolving ensemble experiments. Analysis of observational data indicated that this system developed near a surface front under unstable and favorable conditions with dynamic uplifting by approaching troughs at 500–700 hPa. After formation, it propagated rapidly toward the east–southeast across South China and made landfall in Southern Taiwan. The ensemble used four different datasets as initial and boundary conditions and started at five different initial times, whereby comparing the better-performing members with worse ones, four key factors promoting its strength and longevity were identified: (1) A stronger and moister low-level southwesterly flow to the south of the front to enhance convergence and moisture flux at the leading edge—where a stronger inflow with higher equivalent potential temperature (θ_e) values could feed into the bow echo—leading to a stronger and taller updraft and overall more abundant hydrometeors and rainfall; (2) stronger northwesterly to westerly winds near 700 hPa and thus stronger low-level vertical wind shear, resulting in a stronger rear inflow jet (RIJ), bookend vortices behind the bow apex, and, eventually, a faster propagation speed; (3) a deeper low to the northeast of the bow echo near 850 hPa, where its circulation also helped to bring in low-θ_e air from farther away and enhance the RIJ and cold pool; and (4) a convective initiation location farther to the east in a more favorable environment, with higher θ_e and a faster speed to remain in such a better environment. Helped by the above factors, the bow echo in the present case could reach the observed severity and long duration (~15 h) through interactions and reinforcement among its structural components, including the tilted updraft/downdraft, the low-level inflow and stratiform region, the RIJ and bookend vortices, and the cold pool and gust front.

Hainan Island and Leizhou Peninsula in South China are characterized by abundant deep convection initiation (DCI), relatively complex orography, and strong sea breezes. This study uses the high‐resolution (0.02° × 0.02°, 10‐min interval) Himawari‐8 satellite data to identify the isolated DCI events and employs statistical methods to relate the DCI occurrence to local orography and wind conditions. The statistics are based on 10,731 DCI events that occurred during the daytime of 412 locally forced DCI days in the warm seasons of 2016–2022. A 1‐month WRF simulation, driven by the averaged reanalysis fields of these locally forced DCI days, is conducted to derive the local winds in the study area. The spatial distribution of DCI shows hotspots over mountains with high elevation and large relief amplitude, as well as over certain coastal plains experiencing strong sea breezes. DCI probability generally increases with elevation, relief amplitude, and speed of sea breezes, exhibiting significant linear correlations. The hotspots of DCI events tend to appear over mountains in the late morning and subsequently extend to coastal plains with strong sea breezes in the afternoon. The quantitative relationships established in this study provide useful insights for improving the forecasting of deep convection over tropical islands.

The global expansion of green renewable energy in recent decades has led to a substantial increase in the number of wind farms. While the long-term impacts of wind farms on the atmosphere have been the subject of some research, their short-term effects on weather phenomena, such as convection initiation (CI), remain insufficiently understood. This study addresses this gap by exploring the influence of wind farms on CI, contributing to a broader understanding of anthropogenic impacts on natural atmospheric processes. We explored the general features of CI events from June−August during 2021–2023 associated with desert wind farms in Inner Mongolia, China. A total of 72 CI events were identified with 24% associated with the boundary layer convergence line (BLCL) generated due to desert-oasis vegetation contrast. The impact of the wind farms on the CI process embedded in stratiform cloud on 12 July 2022 from DEsert-oasis COnvergence line and Deep convection Experiment was examined through simulations using Weather Research and Forecasting (WRF) model. Results showed that wind farms can accelerate CI by intensifying the BLCL and shift it southward. It was found that wind farms generated upward-propagating gravity waves, which modulated water vapor transport and led to a reduction in moisture content within the stratiform cloud layer, thereby decreasing cloud coverage and thus increasing solar radiation and surface temperature. The modified surface temperature distribution then shifted the BLCL southward and intensifying it, which eventually accelerated CI.

Tornadic tropical cyclones (TCs) in China occur at a similar frequency to those in the U.S., yet the number of TC tornadoes in China is only 16.5% of that in the U.S. This study reveals that one reason is that tornadic TCs in China have shorter track lengths, fewer recurving tracks, and more southerly locations when producing tornadoes. The other reason is TCs' less tornado‐conducive environments in China. TC environments in China are more dynamically favorable but less thermodynamically conducive to tornadogenesis relative to the U.S. The composite impact is that the regions with the most favorable thermodynamic and kinematic conditions mismatch in China, with the former in the southwest quadrant and the latter in the northeast quadrant of TCs, whereas they overlap in the northeast quadrant in the U.S. and thus lead to a larger composite parameter. This mismatch in China primarily stems from tornadic TCs in southern China.

This work presents comprehensive observational evidence on how sharp vegetation contrasts between irrigated oases and deserts can affect local and regional weather. Hetao irrigation district, a 769,333‐ha wide World Heritage Irrigation Structure in Inner Mongolia, China, illustrates how humanity's biggest hydraulic engineering project becomes a storm generator through enhancing vegetation contrast with its adjacent extensive deserts. Based on radar data collected during warm seasons from 2012 to 2023, this study explores the evolution, diurnal variation, organization, and movement of convective systems (CSs) initiated by boundary layer convergence lines that formed due to vegetation contrast between the irrigated oasis and adjacent Kubuqi and Ulan Buh deserts in different synoptic patterns in the Hetao area. A total of 228 CSs formed primarily along vegetation contrast lines in arid areas under synoptic patterns featuring adequate moisture and substantial instability, with ∼35% exhibiting linear organization and ∼20% propagating far downstream. These features were also compared across the borders between the Oasis and the Kubuqi Desert versus the Ulan Buh Desert, which have different sharpness of vegetation heterogeneity. Due to the sharper vegetation contrast, CSs over the Kubuqi Desert were nearly 11.7 times more prevalent with a higher prevalence of stronger linear organizational mode, greater intensity, and more frequent downstream propagation than those over the Ulan Buh Desert with a less sharp vegetation gradient, where stronger cellular organizational mode was dominant. These findings hold important implications for the nowcasting of severe weather in the semiarid Inner Mongolia and potentially in other desert‐oasis border regions worldwide.

This study investigates the formation mechanism of the multiple parallel rainbands’ (MPRB) organizational mode in a mesoscale convective system (MCS) that occurred on 12 August 2017, in the Beibu Gulf—a region with the highest frequency of MPRBs in China. The analysis is conducted using the Weather Research and Forecasting (WRF) model through 1‐km high‐resolution simulation and terrain modification experiments. The results indicated that the MPRB formed through backbuilding processes, primarily over horizontal convective rolls (HCRs), elevated by low mountains along the coastline. The results showed that mountains from 200 m in valleys to 600 m at peaks did not disrupt HCR formation. Instead, mountains of these heights helped elevate HCRs, thereby facilitating convection initiation. However, after increasing the mountain heights above valley heights by 50%, the higher terrain and larger distance between valleys and peaks decreased boundary layer wind and 0−1 km vertical wind shear, preventing HCR formation and thereby suppressing the development of the MPRB. In contrast, lowering the terrain height reduced the HCR height, thereby weakening or even eliminating MPRB even though it enhanced the organization of HCRs. HCR formation in this case was attributed to convective instability with high vertical wind shear and rich moisture. Coastal mountains functioned as barriers, blocking the flow of moisture from the southern sea, which was also essential for HCR formation and convection initiation.

Tornadoes in urban areas pose a great threat to local residents and buildings. The tornado's fine‐scale dynamic structures in turn can be disturbed by these buildings. This study employs an advanced modeling system of mesoscale‐to‐large eddy simulation (LES) to explicitly resolve both the tornado‐like vortex (TLV) and building‐induced flow disturbances. Our modeling reasonably reproduced a TLV passing over building clusters, while the evolutionary characteristics of TLV are comparable to that of the video‐captured tornado funnels of an actual event in South China. The structures of the TLV are significantly disturbed as it approaches high‐rise building clusters. After arriving at the downstream of the building cluster, the disturbed TLV regains a well‐defined structure. Such a reconstruction of TLV structures is characterized by the vertical alignment of the original aloft TLV structures and low‐level building‐induced lee vortex. As the TLV detaches the building cluster, it leaves open space for a wind corridor, a band area of high wind speeds along the direction of ambient flow in the open area among building clusters. This corridor delivers momentum that, when coupled with the reconstruction of TLV, favors local strong winds. Such a type of local strong winds are highly related to the heights of building clusters but are somewhat less sensitive to the detailed TLV track relative to building clusters, as indicated by numerical sensitivity experiments. These results provide insight into possible wind speed changes when tornadoes encounter buildings, which may have valuable implications for urban wind hazard mitigation.

The heterogeneous land surface spanning the Yellow River irrigated oasis and the adjacent Kubuqi and Ulan Buh Desert (Hetao area) in Inner Mongolia, China, has been noted to frequently generate planetary boundary layer convergence line (BLCL), providing an important source of low-level lifting for convection initiation (CI). As the first field experiment to collect comprehensive observations of vegetation-contrast-resulting thermal circulations that consistently generate BLCLs and lead to CI, the Desert–Oasis Convergence Line and Deep Convection Experiment (DECODE) was conducted from 5 July to 9 August 2022 in the Hetao area. Two oasis and four desert observation sites were set up in the region that exhibits the highest frequency of BLCL and CI occurrences, equipped with a suite of advanced instruments probing land–atmosphere interactions, planetary boundary layer processes, and evolution of BLCLs and their associated CI, including Doppler lidars, microwave radiometers, soil temperature and moisture sensors, eddy covariance systems, portable radiosondes, C-band polarimetric Doppler radar, aircraft, and Geostationary High-speed Imager onboard FY-4B satellite. DECODE captured 29 BLCLs (16 with CI), 66 gust fronts, 12 horizontal convective rolls, and one tornado. The observations unveiled full thermal circulations spanning the desert–oasis boundary characterized by a horizontal width of ∼25 km, a convergence height of ∼1 km above ground level (AGL), and divergence from 2 to ∼3.5 km AGL, with vertical wind speeds of up to 2 m s ⁻¹ . Future publications stemming from DECODE will delve into a spectrum of scientific inquiries, including but not limited to land surface and boundary layer processes, BLCL dynamics, CI mechanisms, convective organization, predictability, and model evaluation. Significance Statement: The purpose of Desert–Oasis Convergence Line and Deep Convection Experiment (DECODE) is to collect intensive observations around desert–oasis divide to better understand the physical processes of boundary layer convergence lines (BLCLs), their associated thermal circulations, convection initiation (CI), and organization over heterogeneous land surfaces. This is important because deep convection initiation poses a major challenge for weather forecasting over heterogeneous land surfaces due to limited understanding of land–atmosphere interactions. In addition, the targeting divide between a desert and an irrigated oasis facilitates a unique situation where human activities can change the weather. This campaign captured 29 BLCLs (16 with CI), which provided a rich dataset for studies not only on convection dynamics and microphysics but also on land–atmosphere interactions, boundary layer processes, and even interdisciplinary studies such as wind farm impact and convective dust storms.