The Organization of Convection in a Mei-yu Frontal Rainband


Observations of a convective frontal rainband observed by TAMEX Doppler network were studied in this paper. The results showed that two different types of convection organization were observed in the Mei-yu front. Type A was the shallow convection triggered by cold air surface colliding with the prevailing southwesterly flow. This type of convection was observed on the cold-air leading edge of the surface cold front. The convection was shallow and had the maximum vertical velocity at low levels. The occurrence of maximum rainfall would follow the passage of surface cold front and dissipate rapidly. The appearance of strong vertical wind shear along the interface of the surface cold air and the lifted prevailing southwesterly monsoon flow provided the possibility of forming wave-type disturbances in the post-frontal region through Kelvin-Helmholtz instability. These wave-type disturbances would modulate the convective activity in the post-frontal region. This mechanism resembled a feature very similar to that frequently observed in mid-latitudes: colliding gust fronts associated with thunderstorm outflows.

Type B was the long-lived deep convection systems embedded in the prefrontal rainband region. This type of convection possessed complicated three-dimensional storm-scale features and was produced by a close interaction between the storm-scale features and their immediate mesoscale environmental flows. The tilted updraft, the intense downdraft, the formation of prefrontal mesoscale high pressure systems, and the deflected lowlevel jet between the frontal zone and the prefrontal mesohigh, all played important roles in triggering, organizing, and maintaining these long-lived convection systems.

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