The genesis of two mesovortices (MVs) within a real-data, convection-resolving simulation of the 8 May 2009 central U.S. bow echo system is studied. Both MVs form near the bow apex but differ distinctively in intensity, lifetime, and damage potential. The stronger and longer-lived mesovortex, MVa, stays near the bow apex where the system-scale rear-inflow jet (RIJ) is present. The descending RIJ produces strong downdrafts and surface convergence, which in turn induce strong vertical stretching and intensification of MVa into an intense mesovortex. In contrast, the weaker and shorter-lived mesovortex, MVb, gradually moves away from the bow apex, accompanied by localized convective-scale downdrafts. Lagrangian circulation and vorticity budget analyses reveal that the vertical vorticity of MVs in general originate from the tilting of near-surface horizontal vorticity, which is mainly created via surface friction. The circulation of the material circuit that ends up to be a horizontal circuit at the foot of the MVs increases as the frictionally generated horizontal vortex tubes pass through the tilted material circuit (tilted following backward trajectories defining the material circuit) surface, especially in the final few minutes prior to mesovortex genesis. The tilted material circuit becomes horizontal at the MV foot, turning associated horizontal vorticity into vertical. The results show at least qualitatively that, in addition to baroclinicity, surface friction can also have significant contributions to the generation of low-level MVs, which was not considered in previous MV studies.

Using convection‐resolving Weather Research and Forecasting simulation, this work studies a heavy rainfall event that occurred in the Shanxi Plateau, North China on 21–22 May 2017. Of particular interest is the influence of multiscale orography on the initiation and maintenance of the precipitating convective system. Results showed that the initial convection was triggered over the small‐scale Taiyue Mountain in the Shanxi Plateau. In the nighttime, with a stable boundary layer, orographic gravity waves were generated by the Taiyue Mountain. The gravity wave‐induced lifting cooled and saturated the lower troposphere adiabatically, thereby producing a moist absolutely unstable layer favorable for subsequent convection initiation. In the daytime, a mountain–plains solenoid (MPS) was established due to the differential solar heating between the Taiyue Mountain and Changzhi Basin. The MPS upslope wind increased the low‐level convergence above the mountain, which eventually promoted the convection. This convection moved eastward and grew into a quasi‐stationary, quasi‐linear convective system in the eastern Shanxi Plateau, which was significantly impacted by the daytime MPS caused by the Shanxi Plateau and North China Plain. The low‐level easterlies associated with the plateau‐scale MPS hindered the eastward propagation of the precipitating system and enhanced the low‐level vertical wind shear ahead of the quasi‐linear convective system, resulting in an overall balance with the system cold pool circulation. These findings provide some new insights into the development of a precipitating convective system under the influence of multiscale orography in a relatively pure environment of weak synoptic forcing and convective available potential energy.

A derecho-producing bow-echo event over the central United States on 8 May 2009 is analyzed based on radar observations and a successful real-data WRF simulation at 0.8-km grid spacing. Emphasis is placed on documenting the existence, evolution, and characteristics of low-level mesovortices (MVs) that form along the leading edge of the bowing system. The genesis of near-surface high winds within the system is also investigated. Significant MVs are detected from the radar radial velocity using a linear least squares derivatives (LLSD) method, and from the model simulation based on calculated vorticity. Both the observed and simulated bow-echo MVs predominantly form north of the bow apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. Vortex mergers occur between MVs during their forward movement, which causes redevelopment of some MVs in the decaying stage of the bow echo. MVs located at (or near) the bow apex are found to persist for a notably longer lifetime than the other MVs. Moreover, the model results show that these bow-apex MVs are accompanied with damaging straight-line winds near the surface. These high winds are mainly caused by the descent of the rear-inflow jet at the bow apex, but the MV-induced vortical flow also has a considerable contribution. The locally enhanced descent of the rear-inflow jet near the mesovortex is forced primarily by the dynamically induced downward vertical pressure gradient force while the buoyancy force only plays a minor role there.

This study evaluates the precipitation over East Asia simulated by the variable resolution CAM‐MPAS featuring the Model for Prediction Across Scales (MPAS) atmospheric dynamical core coupled with the physics parameterizations of the Community Atmosphere Model (CAM) version 5.4. Two CAM‐MPAS experiments, one with a global quasi‐uniform resolution mesh of 120 km (MPAS‐UR) and the other with a variable resolution mesh of 30–120 km refined over East Asia (MPAS‐VR), are conducted from 1989 to 2005 following the Atmospheric Model Intercomparison Project protocol. Two regional climate model simulations at ∼25 km resolution from the Coordinated Regional Downscaling Experiment East Asia second plan are also analyzed for comparison. Results show that CAM‐MPAS performs better than the selected regional models in simulating the precipitation climatology over East Asia. Compared with MPAS‐UR, MPAS‐VR with refinement over East Asia better simulates the precipitation over and around the Tibetan Plateau (TP), the upper‐level circulation, the precipitation frequency and intensity in various regions of China, and the interannual precipitation pattern associated with the East Asian summer monsoon. Both decreased grid spacing and the use of finer‐scale terrain information contribute to the improvements in MPAS‐VR relative to MPAS‐UR. However, decreased grid spacing is more essential for simulating the precipitation magnitude over TP and the location of peak precipitation south of TP, likely due to the better resolved physical and dynamical processes associated with orographic precipitation. The 30–120 km CAM‐MPAS model is shown to be a promising tool for precipitation simulation over East Asia.