Mei-Yu frontogenesis

Mei-Yu fronts are different from usual midlatitude cold fronts in that the southwestern sector of Mei-Yu fronts in the late spring season, or the entire Mei-Yu fronts in the summer season appear to be without any appreciable temperature gradient. Several mechanisms which might either initiate or enhance Mei-Yu frontogeneses are discussed. First, the CISK as the mechanism responsible for Mei-Yu frontogenesis is re-examined by coupling a well mixed boundary layer model with a free atmosphere model. It is found that CISK as proposed in Cho and Chen (1995) is effective in Mei-Yu frontogenesis only if one of the following conditions is met: (1) atmosphere is moist so that the heating parameter is large, (2) the background vorticity of the atmosphere is a few times larger than the Coriolis parameter, or (3) a combination of both. A few other mechanisms are also discussed; these include the small temperature perturbations at the mesoscale induced either by surface inhomogeneities or by mesoscale potential vorticity anomalies at low-levels, and the change in inertial stability due the line of large vorticity values along the front Observations will be needed to determine which process or processes may be responsible for Mei-Yu frontogenesis.