| Mesoscale Convective Systems (MCSs) cause heavy rain and other severe weather events during the warm season. Geostationary satellite infrared imagery with high spatial and temporal resolution can provide much available information for MCS surveillance. Hence, MCS census by means of satellite infrared imagery is one of the important aspects of MCS study. After orientation, interpolation and calibration, the raw satellite data can be translated into the grid data, as could be used in scientific computing.In previous studies on MCS census, because of the limitations of MCS identifying method, the census were confined to either a relatively small space and time frame or a particular type of MCS, which was bad for the research on MCS climatic characteristics. In this article, a new method of identifying MCS from infrared imagery by computer is developed, which contains three main steps:forming MCS profile, tracking the MCS, and judging the system according to MCS definitions. Results of the MCS census over China and its vicinity of the year1999show that, the wrong rate of the new method in identifying MaCS was lower than20%, which was about10%while identifying M(3CS. Based on the new method, the author developed a software, Satellite image and MCS plot.The automatic MCS identification method was used to capture four categories of MCS with different size and shape from satellite infrared numerical data. Forty-seven thousand four hundred and sixty-eight MCSs were found over Asia and western Pacific region during the warm season (May to October) from1995to2008. From this database, MCS characteristics such as shape, size, duration, velocity, geographical distribution, intermonthly variation, diurnal variation, and lifecycle were studied. The results indicated that linear MCSs were2.5times the number of circular MCSs. The former was of a larger size while the latter was of a longer duration. The500hPa steering flow played an important role in MCS movement. MCSs tended to move faster after they reached the maximum extent. Four categories of MCS had similar characteristics of geographical distribution, intermonthly variation. Basically, MCSs were in zonal distribution, with three zonations weakening from south to north. The intermonthly variation of MCS in one place was related to the seasonal adjustment of its large-scale circulation. MCS could be divided into four categories according to their diurnal variation characteristics:low latitude all-day MCS, high latitude all-day MCS, nocturnal MCS, and afternoon MCS. As far as the MCSs over China were concerned, they had different lifecycle characteristics over different areas. MCSs over plateau and hill areas, which had only one peak in their lifecycle curves, tended to form in the afternoon, mature at nightfall, and dissipate at night. On the other hand, MCSs over plain areas, which had several peaks in their lifecycle curves, might form either in the afternoon or at night, whereas MCSs over the oceans tended to form at midnight. Affected by sea-land breeze circulation, MCSs over coastal areas of Guangdong and Guangxi Province always came into being at about three or four pm local time, while MCSs over the Sichuan Basin which were affected by mountain-valley breeze circulation generally initiated nocturnally.By means of the LAPS data, MT1R satellite infrared imagery, and Doppler radar data, the process of heavy rain on June12,2008, was analyzed. The background environment, MCS evolution, as well as the formation and evolution of the storm in the MCS were researched in details. Results showed that:the MCS was formed in a favorable environment, and the southwest vortex in low levels played an important role in MCS evolution. The MCS was a MβECS from the satellite infrared imagery and a strong storm of squall line from radar imagery. The reflectivity gradient in low levels of inflow area was large, with a weak-echo region. The overhanging of echo in mid levels was quite clear. The radial velocity image showed that a convergence area existed in the mid levels of inflow area. The strong storm was formed in the environment with moderate CAPE and strong vertical wind shear.The rainstorm during August16th～18th,2009, was simulated by WRF3.2. The simulation succeeded in simulating the MCS and MCV activities during the rainstorm process. Results showed that:MCV formed beneath800hPa, and the rotation was most evident on850hPa. MCV formed in the convective precipitation area. When MCS formed, a vorticity variation couplet existed in the vortex area, and MCV moved along the direction of positive vorticity variation. In low levels of troposphere, the stretching term and tilting term were main source of positive vorticity when MCV formed, and the horizontal advection term and convection term restrained the growth of local vorticity. About this MCV case, one of the MCV formation mechanism was the atmospheric balance response to convective heating, and the other was the tilting of the horizontal vortex tube. After calculating the Rossby deformation radius at the time of MCV formation, the horizontal scale of the circulation was found to be larger than the Rossby deformation radius. It means that, the perturbation was "dynamically large", and the MCV was expected to be long lasting. |
