Studies On Formation Mechanism And Multi-scale Features Of A Rainstorm Process In Qinling Area

A rainstorm process which occurred in Qinling Region from 28 to 29 July 2011 is studied with multiple observed data, NCEP final analysis data and high resolution simulation by mesoscale model WRF. We focus on the circulation of mesoscale system, kinetic energy conversion between rotational and divergent components of wind, moisture transport features and interactions between synoptic scale and mesoscale system with the result of scale separation by two-dimensional Discrete Cosine Transform (2D-DCT) to meteorological fields. The mainly results as following:(1)Synoptic analyses indicate that upper level jet, stable subtropical high, short-wave trough and low-votex shear provide the favorable large-scale circulation background. And low level shear moving with the shear are the main weather systems for this rainstorm.(2)Diagnoses of the physics fields suggest that stronger upper divergence and week lower convergence associated with positive vorticity and strong vertical motions in the strong precipitation region during the developing phase of the rainstorm. At the initial phase of the rainstorm, CAPE (Convective Available Potential Energy) was accumulated and which was triggered and released rapidly (decreased 2200J·kg-1 in 12 hours) by strong vertical motions, which caused the produce of strong rainstorm. The strong precipitation in 3h occurred around the shear line at 850hPa which was associated with high CAPE value. The analysis on the residual of Non-linear Balance Equation (NBE) features shows that the high residual of NBE appeared around the heavy precipitation area.(3)The peak of divergent energy is ahead of the maximum of precipitation about 1-2h. The moisture transportation at low levels of troposphere is mainly due to the convergence of divergent wind component. Advections of rotational and divergent wind are relatively small. The obvious kinetic energy conversion from divergent wind component to the rotational part when the rainstorm was enhanced, which means the conversion term of divergent energy and rotational energy can be viewed as an indicator of precipitation.(4)Separated circulation shows that synoptic wind field could represent the basic features of the original field. The southwest warm-type shear in low levels of troposphere and southwest flow ahead of short wave trough at middle levels appeared in synoptic scale. The center of synoptic scale moisture convergence is consistent with the distribution of Qinling mountain, which means Qinling mountain has obvious impacts on the transportation of moisture and formation of precipitation. The convergent center of mesoscale wind and moisture is consistent with center of rainfall. The distribution of mesoscale vorticity is larger about an order in magnitude but smaller about an order in horizontal size than that in synoptic scale and accompanied with clearly updraft and downdraft.(5)Interactions between synoptic scale and mesoscale system indicate that kinetic energy converts from mesoscale to synoptic scale at the upper level of troposphere (200-300hPa), while it converts inversely under 300hPa, the energy converts from synoptic scale to mesoscale by the integral of whole layers.