Richardson Number in a Moist Atmosphere and Its Application in the Analysis of Heavy Rainfall Events

To better describe the stability of a moist atmosphere, a new Richardson number, the Richardson number in a non-uniformly saturated moist atmosphere (Ri﹡), is defined in this paper. Ri﹡ is the same as the Richardson number (Ri) except that the generalized potential temperature introduced by Gao et al. in 2004 is used to calculate the Brunt-Vaisala frequency. Then, with outputs from simulations of two heavy rainfall events, Ri﹡ is applied to diagnosing instabilities of rainfall areas. The results show that convective instability is concentrated in the lower troposphere while instability determined by Ri﹡< 1 is mainly located in the middle and upper troposphere above the rainfall areas. The Ri﹡ is more appropriate than the Richardson number in a dry or uniformly saturated moist atmosphere for describing instability of rainfall areas. Moreover, in the two cases here, instability represented by Ri﹡ emerged a few hours prior to the rainfall occurrence, suggesting that Ri﹡ may be used to indicate the occurrence of rainfall. Thus, Ri﹡ could be a helpful index in estimating rainfall occurrence and development.