A Quasigeostrophic Model for Moist Storm Tracks

Abstract

he effect of moisture and latent heat release is investigated in the context of a three-level quasigeostrophic model on the sphere. The model is based on an existing dry model that was shown to be able to reproduce the midlatitude synoptic and low-frequency variability of the troposphere. In addition to potential vorticity equations, moisture evolution equations are included with a simple precipitation scheme. The model can be forced using reanalysis datasets to represent the observed climatology.After the description of the model, the Northern Hemisphere midlatitude climatic characteristics of the moist model are compared to its dry counterpart and to the 40-yr ECMWF Re-Analysis (ERA-40). The jet of the moist model is weakened in its central and northern part and enhanced in its southern part compared to the dry version, which generally decreases the model biases compared to reanalyses. Latent heating processes are mainly responsible for the global decrease in westerlies in the jet-core regions. Precipitation mainly occurs slightly poleward of the jet axes, thereby reducing the meridional temperature gradient and the wind through thermal wind balance. The mean synoptic activity is reduced according to the decrease in baroclinicity, as well as the mean low-frequency variability. A diagnosis of synoptic wave breaking is performed and the characteristics of the moist model are closer to the ones found in reanalyses, especially with more occurrence of cyclonic wave breaking. Weather regimes are slightly better represented in the moist model, although changes are weak compared to the intrinsic model biases. The behavior of the moist model around its climatology indicates that it could be used to run sensitivity experiments.