Physical basis of generalized potential temperature and its application to cyclone tracks in nonuniformly saturated atmosphere

This paper presents the physical basis and the conservation property of the generalized potential temperature theta* from the first law of thermodynamics. Here q* is defined when the commonly used step function is replaced by a smooth function of relative humidity to some power. This new variable, which includes the weighting function acting as a condensation probability factor in the expression of specific humidity, may be more appropriate to describe the property of nonuniform saturation of the real atmosphere. Generalized moist potential vorticity ( GMPV), defined as alpha zeta(a) .del theta* as expressed by Gao et al. ( 2004) ( a is the specific volume and zeta a is the absolute vorticity), is studied theoretically on its application to trace cyclones. It is shown through a case study that cyclone tracks can be well represented by the low negative GMPV centers. Thus the GMPV anomaly may be used as an alternative tool to the traditional use of surface pressure in cyclone tracks and to make a short-term prediction. What is more, other potential usages of GMPV are also pointed out in this study.