A Numerical Simulation of the Mei-Yu Front and the Associated Low Level Jet

A two-dimensional frontal model was used to study the structure and behavior of the Mei-Yu front over East Asia. The Mei-Yu front is characterized by mixed midlatitude-baroclinic and tropical-convective properties, with frequent occurrence of a low-level jet (LLJ) that is highly correlated with heavy convective rainfall. The quasi-steady state responses to a large-scale stretching deformation forcing were obtained by integrating the perturbation equations from an initial state of seasonal-mean zonal flow. Two major sets of experiments were conducted to simulate different midlatitude and subtropical conditions. The midlatitude front extends deeply into the upper troposphere with a strong poleward tilt, whereas the subtropical front is confined to the lower troposphere with less tilt, in good agreement with observations. Along the sloping front, slantwise updrafts develop with a multiband structure. This updraft is more evident in the subtropical cases and in the more moist midlatitude cases. For the subtropical cases, concurrent development of upper-level easterlies and low-level westerlies equatorward of the front is observed. The low-level westerly maximum at z=3–4 km resembles a LLJ, whose intensity increases when more moisture is included. The concurrent development suggests that the LIJ may be the result of a thermally direct secondary circulation that resembles a “reversed Hadley” cell. This circulation is revealed by a meridional–vertical streamfunction, with a strong lower branch return flow coinciding with the development of a LLJ in the more moist, subtropical cases. The Coriolis torque of the meridional circulation can develop and maintain the upper easterlies and the LLJ. Importance of cumulus convection and especially a slant-wise structure in developing the reversed Hadley cell and the LLJ is suggested. These conclusions are consistent with the observed intense convection and heavy rainfall in the Mei-Yu front, and a sinking region south of the Baiu front as revealed by Matsumoto's moisture analysis.