The Studies On The Formation Mechanism Of Heavy Rainfall Occurred In Huabei Areas In China

The 1o×1o data of the NCEP/NCAR data and the meso-scale numerical simulated results are analyzed to reveal two kinds of mechanism to form heavy rainfall occurred in Huabei areas in China, dry intrusion and frontogenesis. By using FleXPaRT Lagrangian particle model, the physical image illustrating dry intrusion's reaching down to warm and moist region in the middle and lower troposphere is anatomised. The instability induced after dry intrusion is dynamical analyzed. The research clue of"dry intrusion - nonuniformly saturated - instability"is brought forward. The generalized frontogenesis function denotative by the local change rate of the gradient of deformation, and the frontogenesis function in the non-uniformly saturated moist atmosphere are derived and used to diagnose frontogenesis. Therefore, the dynamical process forming the heavy rainfall in Huabei areas in China are systemically studied.1. The analysis of NCEP/NCAR data reveals the dry intrusion and frontogenesis characteristics during two heavy rainfall occurred in Huabei areas in China. It is found that, tropopause folds, high-level jet, and dry intrusion coexist in the higher troposphere. The high potential vorticity downwards spreads along with the dry intrusion. The warm and moist flow exists in the lower troposphere. The front stretches upwards to the higher troposphere from the ground. The sinking airflow comprised of the downward branches of the frontogenesis second circulation and the dry intrusion airflow, transports cold and dry air downwards from the ambient region of tropopause folds in the higher troposphere and the lower stratosphere.When the cold and dry airflow approaches the boundary of the warm and moist airflow, the gradients of temperature and/or moisture increases, and the frontogenesis is strengthened. When the cold and dry airflow reaches above the warm and moist airflow, instability induces convection and precipitation. Therefore, dry intrusion and frontogenesis are tow kinds of important mechanisms forming heavy rainfall in Huabei areas in China. The dry intrusion has very great influence on the development of front, the excitation of instability, and induced precipitation.The analysis of dry intrusion on isentropic surface shows that, the dry intrusion has been existed before the occurrence of precipitation and maintained during the precipitation process. The dry intrusion in the higher troposphere exceeds that in the lower troposphere. The precipitation just occurs within the regions with large gradient of relative humidity anterior the dry intrusion. On the basis of isobaric analysis, a new dry intrusion parameter on the isentropic is used to analyze the evolution of dry intrusion, to track the origin and trace of dry intrusion, so as to be convenience to study its three-dimensional structure. The analysis shows that, the new induced dry intrusion parameter can better quantity dry intrusion intensity and indicative of the evolution of dry region in satellite infrared and moisture images. The contrastive analysis on isentropic and corresponding isobaric surfaces shows that, the analyses of dry intrusion on two surfaces are identical. The precipitation increases along with the intensification of dry intrusion. The dry intrusion drives the precipitation to move southeastwards.2. Beginning from the studies of deformation, the generalized frontogenesis function denotative by the local change of the gradient of deformation is derived, and the local change rate of the gradient of total deformation is set up. Then according to the non-uniformly characteristics of real atmosphere, the frontogenesis function in non-uniformly saturated moist atmosphere is derived and the Lagrange change rate of the gradient of generalized potential temperature is built. By the study of heavy rainfall case in Huabei, it is found that the generalized frontogenesis function denotative by the local change of the gradient of deformation can diagnose the frontogenesis of the flow-pattern dominated by deformation in middle and lower troposphere, while the frontogenesis function in non-uniformly saturated moist atmosphere denotative by the Lagrange change rate of the gradient of generalized potential temperature is valid to the front with great moisture gradient.The analysis of the component terms in generalized frontogenesis function formula shows that, the coaction of strong large-scale horizontal convergence, deformation, horizontal advection, horizontal pressure gradient force and vertical velocity, maintains strongθe gradient, which has identical coverage and orientation with the total frontogenesis zone and can better reflect the intensity and orientation of frontogenesis. The vertical section analysis shows that, the strong frontogenesis zone stretches vertically upwards to 300 hPa within frontal zone, and slants southwards (i.e., slants towards warm region) in the lower troposphere, northwards (cold region) in the higher troposphere. Up to higher troposphere, typical deformation(saddle)flow pattern and confluent shear do not exist, therefore the frontogenesis function denotative by the local time change rate of horizontal gradient of deformation has striking effect in the front with deformation flow-pattern and large moisture gradient. While the large decrease of moisture in the higher troposphere and the lack of confluence of deformation on moisture, lead to the deformation frontogenesis is not distinct. Thus, the generalized frontogenesis function is effective in the middle and lower troposphere, especially in the flow pattern dominated by deformation and with large moisture gradient.The analyses of frontogenesis and dry intrusion shows that, the axis of strong frontogenesis locate at large gradient region between the warm moist airflow and cold dry airflow, slants towards cold side with height. Thus, cold and dry advection brought by the dry intrusion reaches the boundary of warm and moist airflow, the gradient contrast with the warm and moist airflow is strengthened, the frontogenesis is intensified. Furthermore, the dry intrusion drive the front to move towards the warm and moist region.The analysis of frontogenesis function in non-uniformly saturated moist atmosphere shows that, horizontal convergence, the deformation, and the diabatic heat dominates the frontogenesis in non-uniformly saturated moist atmosphere. The frontogenesis function is fit for depiction of the frontogenesis problem in real atmosphere in theory. Furthermore the case analysis proves its validity on reflecting the frontogenesis in non-uniformly saturated moist atmosphere.3. The studies on the dynamical mechanism of instability induced by the dry intrusionBy using the simple and convenient dynamical index to estimate insatbility, Richardson (Ri) number, the dry and moist Ri tendency equations are derived. For the selected heavy rainfall case in Huabei areas, the credible meso-scale data generated by the WRF model output are used to calculate and analyze comparatively the component terms in dry and moist Ri tendency equations. The analysis shows that, the vertical shear of horizontal velocity dominates the influence on Ri change and instability in dry process, whereas in the moist case, the influence of numerator of Ri (i.e., Brunt-Vaisala frequency, named BVF) on Ri change and instability can reach up to the same magnitude order as the denominator of Ri. The new BVF formula appropriate for the moist case is explored to modify BVF and Ri, so that the instability is better estimated in moist case. Thus in non-uniformly saturated moist atmosphere induced after dry intrusion reaches above warm and moist airflow, the new Ri formula is given.To the selected precipitation case, the BVF profiles and Ri images in dry, moist saturated, and non-uniformly saturated moist atmosphere are comparatively analyzed. It is shown that, the Ri in non-uniformly saturated moist atmosphere may be more appropriate to depict the instability in rainy region. It can more accurately diagnose the location of heavy rainfall.