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This paper demonstrates that, for a moist baroclinic frontal system, the large-value deformation belt in the low-level atmosphere overlaps with precipitation. To precisely describe the relationship between deformation and heavy precipitation, deformation is introduced into the non-geostrophic Q#-vector. Q# is then decomposed into three parts: the divergence-related term, the vorticity-related term, and the deformation-related term. By calculating the divergence of Q# and its components, it is found that in strong ascending areas within precipitation regions the non-geostrophic Q#-vector divergence shows strong negative values. Its deformational component can contribute about 68% to these negative values. This verifies that strong deformation in a precipitating atmosphere is favorable for the development of convection and precipitation. In addition, by calculating the correlation coefficients between the Q#-vector (including its components) divergence and vertical motions, it is also found that the Q#-vector divergence shows higher correlation with vertical motion within the precipitation belt and lower correlation in the non-precipitation areas, which indicates a larger contribution of Q# to vertical motion when precipitation occurs and implies an effect of Q# to the precipitation distribution or spatial variability. Among the three components of the Q#-vector, the correlation coefficients between the deformational component and vertical motion are the most similar in pattern to that of the correlation coefficients between the Q#-vector and vertical motion, which further reflects the important contribution of deformation to the large spatial variability of precipitation.

The moist atmosphere with occurring precipitation is considered to be a multiphase fluid composed of dry air, water vapor and hydrometeors. These compositions move with different velocities: they take a macroscopic motion with a reference velocity and a relative motion with a velocity deviated from the reference velocity. The reference velocity can be chosen as the velocities of dry air, a gas mixture and the total air mixture. The budget equations of continuity and momentum are formulated in the three reference-velocity frames. It is shown that the resulting equations are dependent on the chosen reference velocity. The diffusive flux due to compositions moving with velocities deviated from the reference velocity and the internal sources due to the phase transitions of water substances result in additional source terms in continuity and momentum equations. A continuity equation of the total mass is conserved and free of diffusive flux divergence if the reference velocity is referred to the velocity of the total air mixture. However, continuity equations in the dry-air and gas-mixture frames are not conserved due to the mass diffusive flux divergence. The diffusive flux introduces additional source terms in the momentum equation. In the dry-air frame, the diffusive flux of water substances and the phase transitions of water substances contribute to the change of the total momentum. The additional sources of total momentum in the frame of a gas mixture are associated with the diffusive flux of hydrometeors, the phase transitions of hydrometeors and the gas-mixture diffusive flux. In the frame of total air mixture, the contribution to the total momentum comes from the diffusive flux of all atmospheric compositions instead of the phase transitions. The continuity and momentum equations derived here are more complicated than the traditional model equations. With increasing computing power, it becomes possible to simulate atmospheric processes with these sophisticated equations. It is helpful to the improvement of precipitation forecast. [ABSTRACT FROM AUTHOR]

We used a weather research and forecasting model to simulate a torrential rainstorm that occurred in Xinjiang, China during June 16–17, 2016. The model successfully simulated the rainfall area, precipitation intensity, and changes in precipitation. We identified a clear wave signal using the two-dimensional fast Fourier transform method; the waves propagated westwards, with wavelengths of 45–20 km, periods of 50–120 min, and phase velocities mainly concentrated in the −25 m/s to −10 m/s range. The results of wavelet cross-spectral analysis further confirmed that the waves were gravity waves, peaking at 11:00 UTC, June 17, 2016. The gravity wave signal was identified along 79.17–79.93○E, 81.35–81.45○E and 81.5–81.83○E. The gravity waves detected along 81.5–81.83○E corresponded well with precipitation that accumulated in 1 h, indicating that gravity waves could be considered a rainstorm precursor in future precipitation forecasts. [ABSTRACT FROM AUTHOR]