中文摘要：

1 汷? 楷桴栠楥桧 ? 愠摮琠敨敲攠楸瑳摥愠 ? 癥摩湥 ? 牡慥漠 ? 浳污 ? 捩 ? 慰瑲捩敬 ? 湩琠敨挠潬摵琠灯 ? ?杧獥楴杮琠敨攠楸瑳湥散漠 ?? 瑳潲杮甠摰慲瑦椠 ? 桴 ? 汣畯獤

英文摘要：

The 1°×1° National Center for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） data and mesoscale numerical simulation data are analyzed to reveal a mechanism for the formation of heavy rainfall in Northern China; this mechanism is the non-uniformly saturated instability induced by a dry intrusion. The dry intrusion and the accompanying downward transport of air with a high value of potential vorticity （PV） are maintained during the precipitation event. As the dry air intrudes down into the warm and moist sector in the lower troposphere, the cold, dry air and the warm, moist air mix with each other, and, as a result, the atmosphere becomes non-uniformly saturated. On the basis of this non-uniform saturation, a new Brunt-Vaisaila frequency （BVF） formula is derived and applied to the precipitation event. It is shown that, compared to the conditions of either a dry or a saturated atmosphere, the BVF in a non-uniformly saturated, moist atmosphere （BVF） may be more appropriate for depicting the atmospheric instability in rainy regions.