On the Mei-Yu Rainband and its Role in Southeast Asian Summer Monsoon

The Mei-Yu rainband is an integral component of the East Asian summer monsoon and is linked to many high impact weather events. The evolution and nature of the Mei-Yu rainband, including its role in the organization of convection over East Asia, is investigated using objective identification to form feature relative composites from reanalysis and gridded precipitation datasets. The objective identification methodology, based on mid tropospheric wetbulb potential temperature, produces a climatology that closely matches subjective equivalents and expectations based on synoptic experience. Using data for the period 1979-2013, this climatology shows the development of the rainband over southern China at approximately the same time as the observed onset of the monsoon, followed by intensification and poleward movement during the warm season. The feature relative composites reveal that the rainband is associated with strong moisture gradients that separate the tropical and extratropical airmasses, exhibiting a vertical tilt akin to extratropical warm fronts, and that the periods of peak rainfall correspond to rainband being close to the latitude of the core of the subtropical upper-tropospheric jet. Computations of vertical mass flux show that the overturning circulation orthogonal to the rainband is approximately an order of magnitude larger than that along it, with maximum ascent occurring in a relatively narrow band along the rainband coupled with maximum descent in the extratropical airmass immediately poleward. Along the composite rainband, the environmental conditions are most favorable for long-lived deep convection; exhibiting troposphere deep moisture, low level convergence and strong vertical wind shear. Either side of the rainband the environment is less favorable for long-lived convection due to the lack of vertical wind shear coupled with vertical thermodynamic profile that will produce strong downdrafts via mixing.