PV‐based diagnostic quantities of heavy precipitation: Solenoidal vorticity and potential solenoidal vorticity

Abstract

Two new diagnostic quantities called solenoidal vorticity (SV) and potential solenoidal vorticity (PSV) are defined in this paper for a moist atmosphere based on the generalized moist potential vorticity (GMPV). SV and PSV are introduced to incorporate vorticity information along the isentropic surfaces that GMPV (or PV) has lost. SV is the scalar product of vorticity and the moist solenoidal term ∇ p × ∇ α*, while PSV is the scalar product of vorticity and (∇ p × ∇ α*) × ∇ θ* (a new vector). Application of GMPV, SV, and PSV to a diagnostic study of a heavy rainfall case shows that SV reflects the coupling of vertical wind shear and the baroclinity of the moist atmosphere weighted by the vertical gradient of pressure, while PSV reflects the coupling of vertical wind shear and baroclinity, the static stability, and the vertical gradient of pressure. Although all three quantities exhibit strong signals over the observed precipitation region, SV and PSV are more capable of distinguishing between precipitation and non‐precipitation areas. The following three main factors are concluded to contribute to the very different distribution patterns of GMPV, SV, and PSV: static stability,the vertical gradient of pressure, and different ways of coupling vertical wind shear and baroclinity. These factors are responsible for the good performance of SV and PSV in tracing and predicting heavy rainfall, as demonstrated by the correlation coefficient between SV (PSV) and the rainfall amount obtained from long time series analyses. The results of this study suggest that SV and PSV are good detectors of heavy rainfall and may be used as precipitation prediction parameters.