Moist Baroclinic Instability along the Subtropical Mei-Yu Front

Abstract

Abstract Differing from classic midlatitude dry baroclinic instability theory, atmospheric motions over the subtropical mei-yu front in boreal summer are dominated by synoptic-scale disturbances coupled with precipitation and moisture under a weaker background vertical shear. This moisture–precipitation–circulation interactive feature, along with a preferred zonal wavelength of about 3400 km and eastward phase propagation, is explained by a moist baroclinic instability theoretical framework. The new framework is an extension of a traditional two-level baroclinic model by considering a prognostic moisture equation, the moisture–precipitation–circulation feedback, and an interactive planetary boundary layer. An eigenvalue analysis of the model shows that the most unstable mode has a preferred zonal wavelength of 3400 km, a westward-tilted vertical structure, and a phase leading of maximum moisture and precipitation anomalies relative to a lower-tropospheric trough, all of which are in good agreement with observations. Both anomalous horizontal and vertical advection processes contribute to the moisture increase. Further sensitivity tests show that the instability and the zonal-scale selection primarily arise from the moisture–convection–circulation feedback, while the vertical shear provides an additional energy source for the perturbation growth. This moist baroclinic instability theory explains well the observed characteristics of the development of synoptic-scale disturbances along the mei-yu front.