| dc.description.abstract | Various aspects of dry and moist atmospheric dynamics are explored through systems of varying complexity, ranging from a 2-D shallow water model to a more complex 3-D general circulation model. First, we examine the response of the nonlinear spherical shallow water equations to tropical vorticity forcing. For a wide range of scenarios we show the emergence of a robust superrotating state. Unlike previous examples, this state of superrotation does not rely on any particular form of dissipation. In fact, even in the absence of any damping we find propagating solutions that have superrotating zonal mean winds. Further, our prescription allows for the construction of arbitrary equatorial zonal wind profiles. In all the cases, rotational eddy fluxes in the equatorial region are responsible for the required eastward acceleration. Arguments based on the nature of potential vorticity (PV) and enstrophy are put forth to shed some light on these results.
Next, we introduce moisture and examine the transient response of a spherical moist shallow water system to tropical imbalances. In particular, our focus is on studying the response in the presence of inhomogeneous moist saturation fields. While the initial moist response is similar to the dry reference run, albeit with a reduced equivalent depth, the long-time solution depends quite strikingly on the nature of the saturation field. With a latitudinally varying background saturation, height imbalances adjust to large-scale, low-frequency westward propagating modes. When the background saturation environment is also allowed to vary with longitude, in addition to a westward quadrupole, a distinct moist PV conserving eastward propagating mode (“moist Rossby” wave) emerges at long times.
Many of these basic features carry over to the response in the presence of realistic saturation fields, and we catalogue the changing nature of the nonlinear atmospheric responses in the presence of saturation fields that characterize the summer and winter seasons.
After this, we proceed to moist steady and statistically stationary solutions that result from continuous forcing and dissipation. The moist analogue of the “Matsuno-Gill” problem yields a response which is comparatively localized in both latitude and longitude. The sensitivity of the moist solutions to various model parameters like strength of latent heating, condensation and evaporation timescales, momentum and radiative damping as well as the background saturation field and mean flows is examined. In the presence of a random large or small-scale forcing, i.e., moist turbulence, the system is seen to support significantly slower large-scale waves along with a smaller scale turbulent field. Also, in contrast to the dry shallow water turbulence, interscale kinetic energy transfer is inhibited and smaller scales are also dominated by rotational modes. Interestingly, unlike the dry momentum forced case, moist solution yields superrotation, which is again tied to tropical eddy fluxes. Further, the effects of varying forcing protocols on kinetic energy spectra, the relative roles of divergent and rotational components as well as the spontaneous aggregation of moisture fields is systematically documented.
Finally, we extend our theme of moist versus dry atmospheres to a more complex general circulation model. Specifically, dynamically dry and moist responses to uniform sea-surface temperature are studied in an aquaplanet setting by varying the latent heat of condensation. Despite having no meridional thermal gradients, Hadley and Ferrel cells with magnitude comparable to present-day Earth are observed for relatively moist cases. This tropical circulation, and the secondary Ferrel cell reverses as water vapor becomes dynamically inactive. In all cases, the Hadley cell is thermally indirect and is strongly influenced by eddy fluxes. Further, there is a systematic poleward transport of energy that remains almost invariant for relatively strong coupling of water substance. The emergence of tropical storm-like warm core vortices, associated extreme rainfall events and their sensitivity to moist coupling is examined. Finally, the changing nature of precipitation and tropical intraseasonal variability is documented for varying latent heats. Here, we see a systematic decrease in the dominant time period of the tropical disturbances with weaker dynamical coupling of water vapor. In fact, low-frequency modes such as the Madden Julian Oscillation (MJO) disappear when water vapor becomes dynamically passive in nature. | en_US |
