Diagnosis of Thermal and Dynamic Mechanisms of Two Rainstorm Processes in Northern Shaanxi

Based on the conventional meteorological observations and 6 h 1°×1° NCEP FNL analysis data, two heavy rain processes occurred on 21-22 July 2013 and 8-9 July 2014 in northern Shaanxi are diagnosed with synoptic method and dynamic diagnosis method. It shows that both processes can be attributed to the intersection of the warm moist air flow along the edge of subtropical high at 500 hPa and the cold air brought from plateau troughs. Low-level jet plays an important role, as it provides adequate water vapor and water vapor convergence lifts on the left side of the shear line. The vertical secondary circulation produced by the coupling of upper-and low-level jet is an important triggering factor. Heavy rainfall in "0721" process occurs mainly at Yan'an where strong coupling of upper-level jet and low-level jet is located. There is strong convective instability in the atmosphere in the initial stage of precipitation during both rainstorm processes. Convergence lifting from the low-level shear line triggers convection energy, resulting in strong precipitation. The latent heat of condensation released by the precipitation extends downward to the middle atmosphere, and leads to thermal discontinuity of the middle and lower atmosphere. Atmospheric wet baroclinicity and frontogenesis significantly enhances, causing the uplift of whole layer saturated atmosphere and strong precipitation, finally producing heavy rainfall process. Because the convective precipitation is stronger and latent heat released is greater in "0721" rainstorm process, the feedback of the low-level jet and the middle atmosphere frontogenesis is stronger, and therefore the precipitation is heavier. The vertical component of generalized convective vorticity vector describes the enhancing of vertical wind shear very well, and describes the frontogenesis which is increased by condensation latent heat that released by water vapor phase transition in the middle and lower layers very well. Therefore, the changing trend of generalized convective vorticity vector can reflect the development and decrease of precipitation. The large value center and high gradient area on the south side of the vertically integrated moist thermodynamic advection parameters is consistent with rainstorm fall area, and it appears about 6 hours before the precipitation, indicating it can be used to effectively forecast regional precipitation.