Howard C. Houben

A Boundary-Layer Model for Mars: Comparison with Viking Lander and Entry Data

Abstract A one-dimensional boundary-layer model for Mars is described, and its results are compared with Viking data. The model equations are similar to Earth boundary-layer models in that they include contributions from Coriolis, pressure gradient, and frictional forces for momentum; and radiation, sensible heat flux convergence, and advection for heat. Turbulent fluxes are computed from the level-2 second-order closure theory of Mellor and Yamada with similarity relations employed for boundary conditions. The pressure gradient force can be specified or computed from a simple slope model. Radiative heating is due to the absorption of solar and infrared radiation by CO2 gas and suspended dust particles. Ground temperatures are computed by solving a surface heat budget using an accurate treatment of conduction into the Martian soil. The data used for comparison were obtained by the Viking 1 and 2 landers for early northern summer. At each site, these data include a single profile of wind and temperature be...

Radiatively forced dispersion of the Mt. Pinatubo volcanic cloud and induced temperature perturbations in the stratosphere during the first few months following the eruption

A combined 3-dimensional circulation model and aerosol microphysical/transport model is used to simulate the dispersion of the Mt. Pinatubo volcanic cloud in the stratosphere for the first few months following the eruption. Radiative heating of the cloud due to upwelling infrared radiation from the troposphere is shown to be an important factor affecting the transport. Without cloud heating, cloud material stays mostly north of the equator, whereas with cloud heating, the cloud is transported southward across the equator within the first two weeks following the eruption. Generally the simulations agree with TOMS, AVHRR, and SAGE satellite observations showing the latitude distribution of cloud material to be between about 20°S and 30°N within the first few months. Temperature perturbations in the stratosphere induced by the aerosol heating are generally 1–4 K, in the range of those observed.

Baroclinic Instability in the Venus Atmosphere

Abstract A three-dimensional, spherical, primitive equation eigenvalue model is used to investigate the baroclinic stability properties of the wind and temperature fields in the Venus atmosphere as measured by Pioneer Venus. It is found that baroclinic instability occurs in the region of the middle cloud deck. The most unstable modes have growth times less than eight days and are vertically confined to the region near the middle cloud layer. The most unstable baroclinic mode at zonal wavenumber 2 has characteristics similar to those observed for the high latitude rotating dipole thermal feature. Certain planetary scale baroclinic modes can penetrate to relatively high altitudes under the right circumstances, and may therefore explain some of the wave features observed between 60 and 90 km. For example, thermal oscillations with periods between four and seven days occurring at middle latitudes have characteristics which appear to be consistent with computed properties of planetary scale baroclinic modes.

Characteristics of finite amplitude stationary gravity waves in the atmosphere of Venus

Abstract This paper extends the study of stationary gravity waves generated near the surface of Venus reported previously by Young et al. to include finite amplitude effects associated with large amplitude waves. Waves are forced near the surface of Venus by periodic forcing. The height-dependent profiles of static stability and mean wind in the Venus atmosphere play a very important role in the evolution of the nonlinear behavior of the waves, just as they do in the linear wave solutions. Certain wave properties are qualitatively consistent with linear wave theory, such as wave trapping, resonance, and wave evanescence for short horizontal wavelengths. However, the finite amplitude solutions also exhibit many other interesting features. In particular, for forcing amplitudes representative of those that could be expected in mountainous regions such as Aphrodite Terra, waves generated near the surface can reach large amplitudes at and above cloud levels, with clear signatures in the circulation pattern. At...

A simplified three-dimensional model for Martian climate studies

We describe a simplified physics approach to generating time-varying global wind and temperature fields suitable for conducting transport studies on Mars. The goal is to develop a computationally fast algorithm that will enable long-term simulations of the planets climate system. Thus we use a three-dimensional log-pressure global spectral model forced with a simplified physics package based on Newtonian cooling and Rayleigh friction. We examine the models performance at northern winter solstice for clear and dusty conditions and find that the simulated mean meridional circulation and transient eddies compare well with a more sophisticated general circulation model by tuning a single parameter: the thermal damping time. Best results are obtained for a thermal damping time of 2 days for clear conditions and 1 day for dusty conditions.

Dynamics of Planetary-Scale Baroclinic Waves during Southern Hemisphere Winter

Abstract An investigation is conducted of the characteristics of linear and finite amplitude baroclinic waves during Southern Hemisphere winter. Emphasis is placed on the dynamics of planetary scale modes, which propagate into the stratosphere. Linear stability analysis of the June–August 1979, time mean geostrophic zonally averaged wind and temperature fields shows that both Charney and Green baroclinic modes exist at planetary scales, with the Green modes having smaller than, but comparable growth rates to, those of the Charney modes. Barotropic unstable modes of the June–August mean geostrophic zonal wind had growth rates too small to make them dynamically significant. Fully nonlinear computations showed that wave-wave coupling among intermediate scale baroclinic modes was important for determining the evolution of planetary scale modes. Computed maximum amplitudes of planetary modes are comparable to peak amplitudes observed for eastward traveling modes in Southern Hemisphere winter. Lack of coherence...

Microphysical Processes Affecting the Pinatubo Volcanic Plume

In this paper we consider microphysical processes which affect the formation of sulfate particles and their size distribution in a dispersing cloud. A model for the dispersiorl of the Mt. Pinatubo volcanic cloud is described. We then consider a single point in the dispersing cloud and study the effects of nucleation, condensation and coagulation on the time evolution of the particle size distribution at that point.