Analysis of the quasi-stationary meso-scale convective systemsin the Northeast of Hubei on July 12,2012

By using the data of upper-air chart, NCEP (National Centers for Environmental Prediction) reanalysis field, weather radar andground observations, the characteristics and causes of the heavy rain-producing quasi-stationary MCS (mesoscale convective system) occurredin the northeast of Hubei on July 12, 2012 was analyzed. The main large scale weather systems were southern branch trough, northeastcold vortex and subtropical high. The MCS was formed between southwest airflow near the subtropical high and cold trough bottom. The factthat subtropical high kept stationary and cold trough moved southward slowly was favorable for the MCS to keep quasi-stationary. The southwestjet with deepening south branch trough has transported enough moisture, providing strong unstable energy to MCS. The MCS developedthe following features, i.e., an α-scale convective cloud with oval shape and smooth edges, low brightness temperature area located on thewest side of MCS, and finger-like, β-scale convective echo band corresponding to low brightness temperatures, composite reflectivity ofheavy rain ranging 45～55 dBz, echo top reaching 18 km, each center height being below 6 km. Heavy rain which corresponds to low brightnesstemperature area and strong echo band had high rainfall efficiency and long duration. Mesoscale analysis showed that convergence linewas the main cause for keeping the MCS quasi-stationary. Surface convergence line produced by terrain blocking triggered the initial convection;heavy rain produces cold pool, thunderstorm high and arc shape of outflow boundary; new convection was triggered again near outflowboundary areas where wind convergence and the temperature gradient were large. The MCS propagated southeast under pressure gradientforce which canceled the environment airflow advection. There were southwest and northwest airflows in MCS; northwest airflow passed intoMCS from its back, and formed convergence line with southwest airflow, then southwest airflow climbed along the northwest air and triggeredconvective, formed the front tilting from southeast to northwest. The outflow boundary on the ground and convergence line at high level wereshown as a frontal in wind field, whose position played an important role on echo strengthening and maintenance. The backward propagationproduced by convergent line on high level also acted to cancel the environment airflow advection.