An analysis of the multiscale structure and evolution of a meso-scale occluding convective system

The composite Doppler radar data at the Beijing, Tianjin and Qinhuangdao stations, the satellite data, the automatic meteorological observing stations data and the NCEP 1°×1° reanalysis data were used to analyze the multi-scale structure and evolution of a meso-scale occluding convective system in the east part of the Huabei plain, which caused severe heavy rainfall on 18 July 2007. The methods of the meso-scale filtering, the 4DVAR single Doppler radar retrieval, and the horizontal and vertical section analyses were employed in this paper. The results show that firstly, this MαCS experienced 3 phases including the intialization and developing, the maturation and the dissipation. The cold cloud top of the MαCS changed from an elongated top to a circle one with only single center again to a polygon top with several centers as shown in the satellite images. It is seen from the radar data that a meso-scale convective system under the cold top was a occluding squallline mesoscale convective system, whose two cross meso-β-scale line squall convective systems were composed of several MγCSs arranged in the form of a line with independent strong radar reflectivity center and life-cycle. In the occluding phase of the MαCS the active MγCSs caused local severe rainfall. In the occluded phase, those MγCSs were closely organized together so as to make their edges become obscured with the occluded point corresponding to the cold cloud top center of the MαCS. In the dissipation phase, with the pattern of crossing disappearing and cloud top dropping, the radar echo-top and the reflectivity both showed an eddy feature. Secondly, severe ascending motion existed in the MαCS whose center appears from 600 hPa to 500 hPa.At the height of 200 hPa, an anticyclonic circulation had an effect on the shape of cold cloud top. At the height of 700 hPa, the coldair flow brought by the cyclonic circulation weakened physically the intensity of MαCS. Thirdly, the structure and intensity in the MαCS closely related to the air flow at the lower troposphere. In the intialization and developing phase of the MαCS, the direction of the southwesterly currents entrancing that is consistant with the background one changed to form a convergence lines. After the mature phase, the dry-cold air flow from the west-north areas entranced into the MαCS instead of the wet-warm air flow, causing the convergence to disappear, the source of high energy to decrease, and the M-α-CS to be dissipated. Finally, the multi-scale convective systems were triggered and organized by the ground-convergence line in the areas of dense potential pseudo equivalent temperature lines. The energy fields concern directly the maintaining or weakening of the multi-scale convective systems. Comparing between the meso-scale occluding convective system and the occluding front weather system, it is found that they had difference in scale but they were resemble in pattern and genesis mechanism.