Two large‐eddy simulations are carried out to investigate the vertical structure and daytime evolution of trapped lee waves (TLWs) triggered by mountains of two heights (500 and 1500 m, denoted as HM500 and HM1500, respectively) based on a typical subtropical winter troposphere, in which a steady upper‐level jet and a clear diurnal evolution of the atmospheric boundary layer (ABL) are present. Multimode TLWs co‐exist at three altitudes with dominant wavelengths increasing with altitude in HM500, while a single‐mode TLW dominates throughout most of the troposphere in HM1500. The wave amplitudes for both experiments increase from midday, reaching peaks in the afternoon, likely related to the reduction of the wave absorption by the ABL. Whereas the growth of the dominant wavelength of TLWs with time is mainly limited to layers near the ABL top for HM500; the dominant wavelength in HM1500 stays steady with time. The TLW pattern and evolution can be largely explained by linear theory. In HM500, the multimode pattern is due to the perturbation source at a low altitude where high wavenumbers are supported, and the wavelength lengthening near the ABL top can be explained by the decreasing Scorer parameter in the afternoon. In HM1500, the large‐amplitude single‐mode TLW is due to the enhanced and elevated perturbation source of the higher mountain where the Scorer parameter is smaller and affected less by the ABL. The continual amplification of the dominant TLW in HM1500 may be caused by the instability from the wave‐induced momentum deficit at the upper ABL, which further facilitates the wave propagation. Our findings are beneficial for improving our process‐scale understanding of the vertical structure and diurnal evolution of TLWs constrained by the upper‐level jet and ABL evolution, and have implications for improving orographic gravity‐wave parameterization especially when the model resolution approaches around 10 km.

This paper presents a turbulent orographic form drag (TOFD) parameterization scheme that takes into account the directional effects stemming from the angle between the low‐level wind and the principal axis of small‐scale orography. Suitable for models with both kilometer and subkilometer horizontal resolutions, this scheme builds upon prior theoretical and numerical studies to formulate surface TOFD based on the slope and direction of the sinusoidal hills. In this study, we proposed a straightforward function to calculate the surface TOFD for various orographic aspect ratios and directional parameters. The vertical decay of the drag is modeled with a scale twice the standard deviation of the sub‐grid orography. A comparison with numerous large‐eddy simulations featuring a single ellipsoidal hill demonstrates that our scheme effectively captures the dependence of drag on factors such as maximum slope, aspect ratio, the angle between low‐level wind and the principle axis, and hill height. We recommend calculating the sub‐grid orographic parameters using a well‐established method and digital terrain elevation data with a horizontal resolution less than a 100 m. This will allow for representation on orographic scales of both kilometers and subkilometers. Real‐case numerical weather prediction tests are conducted and verified with dense surface wind observations. The proposed scheme improves surface wind simulation compared to a renowned TOFD scheme, and also effectively exhibits the wind response to orographic anisotropy.

Devastating tornadoes in China have received growing attention in recent years, but little is known about their formation, structure, and evolution on the tornadic scale. Most of these tornadoes develop within the East Asian monsoon regime, in an environment quite different from tornadoes in the U.S. In this study, we used an idealized, highresolution (25-m grid spacing) numerical simulation to investigate the deadly EF4 (Enhanced Fujita scale category 4) tornado that occurred on 23 June 2016 and claimed 99 lives in Yancheng, Jiangsu Province. A tornadic supercell developed in the simulation that had striking similarities to radar observations. The violent tornado in Funing County was reproduced, exceeding EF4 (74 m s^–1), consistent with the on-site damage survey. It was accompanied by a funnel cloud that extended to the surface, and exhibited a double-helix vorticity structure. The signal of tornado genesis was found first at the cloud base in the pressure perturbation field, and then developed both upward and downward in terms of maximum vertical velocity overlapping with the intense vertical vorticity centers. The tornado’s demise was found to accompany strong downdrafts overlapping with the intense vorticity centers. One of the interesting findings of this work is that a violent surface vortex was able to be generated and maintained, even though the simulation employed a free-slip lower boundary condition. The success of this simulation, despite using an idealized numerical approach, provides a means to investigate more historical tornadoes in China.