Moist and Dry Dynamics in Shallow Water and General Circulation Models

Abstract

Various aspects of dry and moist atmospheric dynamics are explored through systems of varying com- newlineplexity, ranging from a 2-D shallow water model to a more complex 3-D general circulation model. newlineFirst, we examine the response of the nonlinear spherical shallow water equations to tropical vorticity newlineforcing. For a wide range of scenarios we show the emergence of a robust superrotating state. Unlike newlineprevious examples, this state of superrotation does not rely on any particular form of dissipation. In newlinefact, even in the absence of any damping we find propagating solutions that have superrotating zonal newlinemean winds. Further, our prescription allows for the construction of arbitrary equatorial zonal wind newlineprofiles. In all the cases, rotational eddy fluxes in the equatorial region are responsible for the required newlineeastward acceleration. Arguments based on the nature of potential vorticity (PV) and enstrophy are newlineput forth to shed some light on these results. newlineNext, we introduce moisture and examine the transient response of a spherical moist shallow water newlinesystem to tropical imbalances. In particular, our focus is on studying the response in the presence of newlineinhomogeneous moist saturation fields. While the initial moist response is similar to the dry reference newlinerun, albeit with a reduced equivalent depth, the long-time solution depends quite strikingly on the newlinenature of the saturation field. With a latitudinally varying background saturation, height imbalances newlineadjust to large-scale, low-frequency westward propagating modes. When the background saturation newlineenvironment is also allowed to vary with longitude, in addition to a westward quadrupole, a distinct newlinemoist PV conserving eastward propagating mode ( moist Rossby wave) emerges at long times. Many newlineof these basic features carry over to the response in the presence of realistic saturation fields, and we newlinecatalogue the changing nature of the nonlinear atmospheric responses in the presence of saturation newlinefields that characterize the summer and winter seasons... newline