Characteristics for the sources and sinks of gravity waves in an orographic heavy snowfall event

The characteristics of the mesoscale gravity waves during a snowfall event on November 30, 2018 over the Ili Valley and the northern slope of the Tianshan Mountains are analyzed based on the Weather Research and Forecasting model simulation. The vertical distribution of Ro is similar to that of the residual of the nonlinear balance equation (Delta NBE), with their high-value areas located over the leeward slope and the fluctuations extending upwardly with time, indicating the characteristics of strong ageostrophy and non-equilibrium of atmospheric motions. In addition, the Ro and Delta NBE are first developed in the lower layers over the leeward slope, revealing that the generation of the gravity waves is closely related to the topography. Thus, the topographic uplifting greatly affects this snowfall, and the ageostrophic motion in the whole troposphere and the lower stratosphere, as well as the unbalanced motions between convergence and divergence over the peak and the leeward slope are conductive to the development of the inertia-gravity waves. In terms of the horizontal scale of the gravity waves, the Barnes' bandpass filter is applied to separate the mesoscale waves and the synoptic-scale basic flow. The vertical distributions of the vorticity and divergence perturbations have a phase difference of pi/2, indicating the polarization state of gravity waves. The analyses on the sources and sinks of gravity waves by the non-hydrostatic wave equation show that the main forcing term for orographic gravity waves is the second-order nonlinear term, whose magnitude mainly depends on the nonlinear thermal forcing. This term is mainly related to the vertical transport of potential temperature perturbations. During the snowfall, the potential temperature perturbations are mainly caused by the topographic relief and the release of condensation latent heat. Therefore, the gravity waves in this snowfall are caused by the topographic forcing and condensation latent heating.