R. John Wilson

The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL global coupled model CM3

AbstractThe Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emiss...

Comprehensive Model Simulation of Thermal Tides in the Martian Atmosphere

Abstract This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated w...

A general circulation model simulation of the Martian polar warming

This paper reports on a successful general circulation model simulation of a rapid warming phenomenon observed in the Martian winter polar atmosphere during global dust storm conditions. The model includes a self-consistent simulation of the dust distribution which is forced with a prescribed surface source. The warming is shown to be largely a response to the development of a pole-to-pole solstitial Hadley circulation resulting from the greatly increased dust loading. A crucial aspect of the simulation is a sufficiently deep computational domain that allows for the full development of this circulation. These simulations indicate that the thermal tides play a contributing role by providing zonal momentum flux divergence at the winter pole and by the advection of aerosol.

Forced waves in the martian atmosphere from MGS TES nadir data

Abstract We have analyzed the temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) nadir spectra to yield latitude-height resolved maps of various atmospheric forced wave modes as a function of season for a full Mars year. Among the isolated wave modes is the zonal mean, time mean temperature, which we used to derive zonal mean zonal winds and stationary wave quasi-geostrophic indices of refraction, diagnostic of their propagation. The diurnal Kelvin wave was isolated in the data, with results roughly consistent with models (Wilson and Hamilton, 1996 , J. Atmos. Sci. 33, 1290–1326). The s = 1 and s = 2 stationary waves were found to have significant amplitude in ducts extending up the winter polar jets, while the s = 3 stationary wave was found to be confined to near the surface. The s = 1 stationary wave was found to have little phase tilt with height during northern winter, but significant westward phase tilt with height in the southern winter. This indicates that the wave carries heat poleward, slightly more than that found in Barnes et al. (1996 ; J. Geophys. Res. 101, 12,753–12,776). The s = 1 stationary wave is likely the dominant mechanism for eddy meridional heat transport for the southern winter. We noted that the phase of the s = 2 stationary wave is nearly constant with time, but that the s = 1 stationary wave moved 90° of longitude from fall to winter and back in spring in the North. While interannual variability is not yet addressed, overall, these results provide the first comprehensive benchmark for forced waves in Mars’s atmosphere against which future atmospheric models of Mars can be compared.

Cyclones, tides, and the origin of a cross-equatorial dust storm on Mars

We investigate the triggering mechanism of a cross-equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off-line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally-modified return branch of the Hadley circulation.

Influence of water ice clouds on Martian tropical atmospheric temperatures

The Reanalysis derived from the UK Mars general circulation model assimilation of Thermal Emission Spectrometer temperature and dust opacity retrievals at present provides the best estimate of the evolving state of the Martian atmosphere over the course of the Mars Global Surveyor mapping mission. A Control simulation has also been carried out using the same evolving dust distribution as the Reanalysis, but without the temperature assimilation. Differences in zonal mean temperatures between these two simulations reflect possible biases in the representation of dynamical and radiative forcing in the assimilating model. We have identified a cold bias in the Control simulation of tropical temperature which develops in the northern hemisphere summer solstice season. We attribute this bias to the absence of radiatively active water ice clouds in the model and show that clouds likely play a prominent role in shaping the vertical thermal structure of the tropical atmosphere during this season.

Evidence for diurnal period Kelvin waves in the Martian atmosphere from Mars Global Surveyor TES data

Midlevel (∼25 km) atmospheric temperatures derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectra indicate the presence of stationary waves and thermal tides. Stationary waves are prominent at middle to high latitudes where westerly zonal flow is indicated by the meridional temperature gradient. Longitudinal variability within 30°S to 30°N is dominated by topographically-forced nonmigrating thermal tides that have westward and eastward propagating components. The MGS mapping data are available at two fixed local times so that it is not possible to distinguish between these components or isolate the sun-synchronous tide. A comparison with Mars general circulation model (MGCM) simulations suggests that the observed wave patterns are consistent with the presence of eastward propagating, diurnal period Kelvin waves with zonal wavenumbers one and two. These waves can propagate to great heights and may account for observed zonal variations in thermospheric density.

The stability and genesis of Rossby vortices

Abstract The stability and genesis of the vortices associated with long solitary divergent Rossby waves-the Rossby vortices–are studied numerically using the single-layer (SL) model with Jovian parameters. Vortex behavior depends on location and on balances among the translation, twisting, steepening, dispersion and advection processes. Advection is the main preserver of vortices. The solutions provide an explanation for the origin, uniqueness and longevity of the Great Red Spot (GRS). In midlatitudes, stable anticyclones exist in a variety of sizes and balances: from the large planetary-geostrophic (PG) and medium intermediate-geostrophic (IG) vortices that propagate westward, to the small quasi-geostrophic (QG) vortices that migrate equatorward. These vortices all merge during encounters. Geostrophic vortices in the f0-plane system adjust toward symmetry by rotating; those on the sphere adjust by rotating and propagating. Stable cyclones exist mainly at the QG scale or on the f0-plane. In low latitudes ...

A first look at dust lifting and dust storms near the south pole of Mars with a mesoscale model

Surface wind stresses and dust lifting in the south polar region of Mars are examined with a three-dimensional numerical model. The focus of this study is the middle to late southern spring period when cap-edge dust lifting events are observed. Mesoscale model simulations of high southern latitudes are conducted at three dates within this season (L_s = 225°, 255°, and 310°). Assuming that dust injection is related to the saltation of sand-sized grains or aggregates, the Mars MM5 mesoscale model predicts surface wind stresses of sufficient strength to initiate movement of sand-sized particles (∼100 μm), and hence dust lifting, during all three periods. The availability of dust and/or sand-sized particles is not addressed within this study. Instead, the degree to which the existence of sufficiently strong winds limit dust injection is examined. By eliminating forcing elements from the model, the important dynamical modes generating high wind stresses are isolated. The direct cap-edge thermal contrast (and topographic slopes in some locations) provides the primary drive for high surface wind stresses at the cap edge, while sublimation flow is not found to be particularly important, at these three dates. Simulations in which dust is injected into the lowest model layer when wind stresses exceed a threshold show similar patterns of atmospheric dust to those seen in recent observations. Comparison between these simulations and those without active dust injection shows no signs of consistent positive or negative feedback due to dust clouds on the surface wind stress fields during the late spring season examined here.

Stratospheric Ozone and Temperature Simulated from the Preindustrial Era to the Present Day

AbstractResults from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the...