A modified moist potential vorticity, its properties, and application

Abstract

AbstractA modified moist potential vorticity (MMPV) and its tendency equation are derived in terms of modified potential temperature and the specific volume of dry air. MMPV holds not only the simplicity of virtual potential vorticity but also the conservation, impermeability, and invertibility principles of potential vorticity under certain conditions. More importantly, the effects of various nonconservative processes are explicitly included in its tendency equation. It can be seen from the MMPV tendency equation that contributions of moist species (water vapor and hydrometeors) to moist potential vorticity anomalies are twofolds: (i) the latent heating and the mass forcing of water vapor associated with moist microphysical processes and (ii) the solenoidal terms associated with the nonuniform distribution of moist species. Thus, we believe that it will be convenient to use MMPV in investigating the impact of diabatic processes and moist species gradients on the evolution of weather systems. A case study indicates that the MMPV does show strong signals aligned with the horizontal distribution of precipitation, suggesting its potential use as an accurate diagnostic variable. Since the MMPV tendency equation is derived from fully compressible and nonhydrostatic moist atmosphere system, the MMPV will also be a valid tool for evaluating and improving mesoscale numerical models such as the Advanced Research Weather Research and Forecasting model.