Geophysical and Astrophysical Fluid Dynamics

Abstract:

A meteorological data assimilation scheme has been developed for Mars, based on techniques used for the current operational scheme at the UK Meteorological Office (UKMO). The scheme has been designed to interface with a range of models of varying complexity, from a simple primitive equations model to a full martian general circulation model with detailed parameterizations of the most important physical processes. The scheme was originally intended to be used primarily for analysis of temperature profiles from the Pressure Modulator Infrared Radiometer (PMIRR) onboard the Mars Observer (MO) spacecraft. It is, however, capable of analysing any asynoptic, randomly-distributed dataset, which may include surface pressure, temperature, velocity or constituent data originating either from other past or future spacecraft missions or from models. It is proposed to use the scheme not only for interpretation of forthcoming data from future spacecraft but also for model inter-comparisons, for hind-casting earlier martian data for model validation and for data impact studies in the planning of new missions. Initial results have demonstrated successful assimilations of trial orbiter and lander data into models with different initial conditions and physics, although a large number of landers might be required if orbiter data were not available. An optimum approach would combine orbiter data with simultaneous measurements from a small number of landers.

Abstract:

The development and evolution of baroclinic transient waves are simulated in a high resolution, general circulation model (GCM) of the Martian atmosphere, including simulations both with and without a representation of the diurnal cycle of solar heating. Simulations with diurnally-averaged insolation are found to develop highly regular baroclinic transients during northern hemisphere autumn, winter and spring seasons, characterized by zonal wavenumber 1 or 2 and periods of 2-6 sols (a sol being a Martian day). With a diurnal cycle, however, transients are found to be significantly more irregular throughout the winter season, sporadically changing their dominant wavenumber and frequency, and resemble more closely data from the Viking Lander time series. These results are discussed with reference to some simple conceptual models in nonlinear dynamics, in terms either of a periodically-perturbed, self-exciting oscillator, or of attractor-merging near a crisis in a periodically perturbed bistable system.

Abstract:

In laboratory studies and associated theoretical and numerical work covering a very wide range of conditions (as specified by the key dimensionless parameters of the systems used) the phenomenon of sloping convection in rotating fluids can manifest itself in one of several spatial forms (waves, closed eddies, and combinations thereof), but all with strong local gradients (fronts, jet streams) and exhibiting various types of temporal behavior [steady, periodic vacillation, aperiodic (geostrophic) turbulence]. These general properties were first discovered in cylindrical (annular) systems, but they do not depend critically on geometry; differences between spherical and cylindrical systems are largely to be found in quantitative details. In all cases, the raison d'e tre of sloping convection is horizontal advective transfer, a process accompanied by upward advective heat transfer, which affects and may control vertical potential density gradients. It has been argued that sloping convection is the basic dynamical process underlying a wide variety of large-scale flow phenomena seen in planetary atmospheres (e.g., irregular waves in the Earth's atmosphere, regular waves in the Martian atmosphere, the Jovian Great Red Spot and other long-lived eddies seen in the atmospheres of the giant planets). In this review the extent to which this paradigm is upheld in the atmospheres of the major planets by recent work is discussed.