Publications – Abstracts

We will present results from multi-annual simulations of methane condensation in the troposphere of Titan, using the global version of the TitanWRF atmospheric model. TitanWRF is the Titan version of the PlanetWRF model (Richardson et al., 2007, JGR, in press). PlanetWRF differs from the model upon which it is based, NCAR’s Weather Research and Forecasting model (www.wrf-model.org), in that it (a) can be run in global as well as limited area mode, and (b) is structured to be easily adapted to planets other than the Earth.

The methane condensation scheme includes for example (i) a surface source, depending on the sub-saturation of the lowest atmospheric layer and the strength of near-surface winds, as in Tokano et al. (2001, Icarus, 153, 130-147); (ii) determination of the type of condensate produced (methane ice or a binary liquid including dissolved nitrogen) depending on temperature; (iii) inclusion of a decrease in saturation vapor pressure for the binary liquid; (iv) different assumptions about the fate of any condensate produced.

Preliminary results show some interesting similarities to the observed location and timing of Titan clouds, and suggest further investigations which will also be discussed.

This work was funded by the Outer Planets Research Program and by the Applied Information Systems Research Program.

Since the work of Eden and Vonnegut (1973, Science, 180, 962-963), it has been hypothesized that the lower atmosphere of Mars might be highly electrically active during dusty conditions. This electrical activity supposedly results from the charging of dust and sand particles by collisions with other dust and sand particles of contrasting size and/or composition and the discharge of these particles into an atmosphere with a low breakdown potential. Farrell et al. (1999, JGR, 104, 3795-3801) and Renno et al. (2003, GRL, 30, 2140) have proposed that these electrical discharges might be a source of intense non-thermal radiation and therefore detectable.

In this work, we synthesize the work of Farrell et al. (1999) and Renno et al. (2003) into a model of non-thermal radiation (primarily UHF) produced by emission from arc discharges between sand and dust particles. Although this model is highly unconstrained, it can provide insight into the source characteristics of such radiation. We use this model to analyze the observations of Busch et al. (this meeting) and consider the implications of our analysis for surface spacecraft operations, enhancement of dust lifting by strong electric fields (Kok and Renno, 2006, GRL, 33, L19S10), meteorological monitoring, and lower atmosphere chemical synthesis on Mars (Atreya et al., 2006, Astrobiology, 6, 439).

Valles Marineris is the deepest canyon on Mars, and its topography affects the climate both globally and locally. The equatorial cloud belt appears from areocentric solar longitudes of 45 to 150 degrees, and the clouds associated with the valley are a part of it. Before the main belt developed in 2004, the High Resolution Stereo Camera (HRSC) and the spectrometer OMEGA on board Mars Express observed a dusty haze floating inside the valley. It became thinner after three days, and disappeared within ten days.

The Planetary Weather Research and Forecasting (WRF) model has been modified at Caltech from the terrestrial mesoscale WRF model developed mainly by the National Center for Atmospheric Research (NCAR). It can simulate the diurnal / seasonal change of wind direction and temperature profile in a limited area with high spatial resolution. Formations of clouds and dust hazes strongly depend on these phenomena. We will present the results of mesoscale simulations of Valles Marineris in Northern spring to summer.

PlanetWRF is a global, planetary version of the mesoscale, Earth-based WRF (Weather Research and Forecasting) model (www.wrf-model.org). With minimal changes, and using the same basic dynamical core and parameterizations of physical processes, it may be run as a global, mesoscale, LES (large eddy simulation), latitude-height or one-dimensional model. This makes it exceptionally flexible and applicable to a range of studies, including for example its use as a radiative-convective model to test a new radiative transfer scheme, or as a three-dimensional global model to examine wave-mean flow interactions. It is also easily configured for any planetary atmosphere, and has so far been applied to the atmospheres of Earth, Mars, Titan and most recently Venus. We will present an overview of results to date, including tests of the dynamical core using simplified forcing, Mars simulations that compare very well with observations, and Mars simulations using a `rotated’ grid in which the poles are dealt with particularly well. We will also outline future work using high resolution `nests’ placed within the global domain to enable multi-scale feedbacks.

This work is partially funded by NASA’s AISR and OPR research programs, and we would also like to acknowledge our use of Caltech’s new 1024 node Geological and Planetary Sciences Dell cluster, CITerra.

TitanWRF is the Titan version of the PlanetWRF model, which is a global, planetary version of the mesoscale, Earth-based WRF (Weather Research and Forecasting) model (www.wrf-model.org). It uses a full radiative transfer scheme (a more recent version of that described in McKay, Pollack and Courtin, “The Thermal Structure of Titan’s Atmosphere”, Icarus 1989) including diurnal and seasonal variations in solar forcing, and is fully three-dimensional allowing waves and their effect on the mean flow to be represented explicitly. This required us to use a computationally efficient model – Titan’s thick sluggish atmosphere has very long dynamical time-scales, and a Titan year is 30 Earth years, meaning that long simulations are needed to `spin up’ the model atmosphere. PlanetWRF was therefore an excellent choice, as its basis (the WRF model) was designed to run efficiently on parallel machines, such as the new 1024 node Geological and Planetary Sciences Dell cluster, CITerra, available to us at Caltech.

We will present results from the latter stages of model spin-up, and show that the model atmosphere (having been started from rest) takes many Titan years to reach an equilibrium state in which there is no net transfer of angular momentum from surface to atmosphere when averaged over one year. We will also show that our model begins to produce significant equatorial super-rotation after several years, and will identify the mechanism behind this in TitanWRF. Validation of the equilibrium model state is our next step once it is available, but we will also outline future plans after this has been accomplished. These include allowing advection of the radiatively active haze distribution by model winds, and the inclusion of simple methane microphysics to study cloud formation in Titan’s lower atmosphere.

This work is funded by NASA’s AISR and OPR research programs.

The NCAR terrestrial Weather Research and Forecast (WRF) atmospheric model has been converted into a global, planetary model. The model is fully compressible, has Coriolis and curvature treatment and has hydrostratic and non-hydrostatic options.

The model has been converted for use on Venus, initally using the linearized forcing and dissipation parameterizations of recent Venus GCMs (Lee et al.(2005), Yamamoto and Takahashi (2003)). Preliminary analysis of the momentum transports, atmospheric circulation, and super-rotation diagnostics from this model will be presented.

The next stage of the study will be to use more realistic thermal forcing data derived from radiative models, and to adapt a radiative transfer model for the Venus atmosphere in order to provide more realistic heating rates and allow radiative feedback processes to be included in the GCM.

Some big questions relating to Titan’s atmosphere are: How much equatorial superrotation occurs? What determines the way in which albedo patterns change over time at different wavelengths? and What determines the size, frequency and location of the recently observed southern hemisphere clouds?

To investigate such questions properly requires a three-dimensional model of Titan’s thick atmosphere, which can examine different scenarios in multi-year simulations. Such simulations are computationally demanding due to Titan’s long radiative and seasonal timescales, as well as its slow rotation rate, all of which mean a long time is required to spin up then conduct each run. This demands the use of an efficient, accurate, and mass- and momentum-conserving model.

One such model is the newly developed Planetary Weather Research and Forecasting model (PWRF), which is based on a pre-existing terrestrial mesoscale model (WRF). WRF is a highly parallelized and numerically efficient model, which is also able to place high resolution ‘nests’ (with information passing both inwards and outwards) over selected regions where resolving small-scale features is most necessary. This is very useful, as increasing the resolution everywhere would slow the model considerably. PWRF retains these features, but extends to using a global mother domain (no separate global model required), and has also been adapted to include a planetary timing system (using areocentric longitude) and other planetary options (such as radiation schemes).

We will present results from the Titan version of PWRF to demonstrate how well the model reproduces key aspects of Titan’s circulation (e.g. superrotation, temperature structure). We will describe our plans to include aerosol transport, and our eventual aim to model the recently observed (telescopically from Earth, e.g. Griffith et al. Nature 1998, Brown et al. Nature 2002, and from Cassini, e.g. Porco et al. Nature 2005) methane clouds.

This work is funded by NASA.

This presentation will describe the progress to-date on the development of an acoustic anemometer for the in-situ measurement of wind speeds on Mars, funded by NASA PIDDP. Improved measurements of Martian winds are needed for several reasons: better prediction and understanding of global and regional weather, direct measurement of fluxes between surface/atmosphere of momentum, heat, and trace atmospheric constituents, characterizing and monitoring boundary layer winds that influence the safe delivery of spacecraft to/from the Martian surface, and improved characterization of geologically important aeolian processes that can pose a hazard to future exploration via dust storms and dust devils.

Prior attempts to measure surface winds have been limited in capability and difficult to calibrate. Sonic anemometry, measuring wind speed via sound pulse travel-time differences, can overcome many of these issues. Sonic anemometry has several distinct advantages over other methods such as hot wire techniques: higher sensitivity ( <5 cm/s), higher time resolution (10-100 Hz), and fewer intrinsic biases for improved accuracy. Together, these open the possibility of resolving turbulent boundary layer eddies to directly capture surface-to-atmospheric fluxes for the first time.

We will describe the results of our development of an acoustic anemometer using capacitive micro-machined devices, optimized for acoustic coupling in a low-pressure medium like the Martian atmosphere. This development includes transducer characterization tests in a pressure chamber at Ball Aerospace with Mars-relevant CO2 pressures. We will also describe experimental results showing that the addition of water in a low-pressure CO2 atmosphere can significantly increase acoustic attenuation. Finally we will describe plans for further optimization of the instrument for future Mars payloads.

Atmospheric loss processes have played a major role in the evolution and habitability of the terrestrial planet atmospheres in our solar system. The hydrogen escape rate to space is the key parameter that controls the composition of the primitive terrestrial atmosphere, including its possible methane concentration. Most models of the early atmosphere assume that hydrogen escapes at the diffusion-limited rate (Walker, 1977), but this need not necessarily have been true. A CO2- or CH4-rich primitive atmosphere may have been relatively cool in its upper regions, and the escape may therefore have been limited by energy considerations rather than by diffusion. Resolving questions of hydrogen escape for these cases requires solving a set of hydrodynamic equations for conservation of mass, momentum, and energy. Hydrodynamic escape may also be important for Close-in Extrasolar Gas Giants (CEGPs) such as HD209458b, which has recently been observed to be losing hydrogen by the Hubble Space Telescope (Vidal-Madjar et al., 2003; 2004). Although planetary hydrodynamic escape models have been created in the past (Watson et al., 1981; Kasting and Pollack, 1983; Chassefiere, 1996), the problems were solved by integrating the coupled, time independent mass, momentum, and energy equations for the escaping gas from the homopause out to infinity. Solving the one-dimensional, steady state approximation becomes problematic at the distance where the outflow becomes supersonic. A new technique has been developed for the treatment of hydrodynamic loss processes from planetary atmospheres that overcomes the instabilities inherent in modelling transonic conditions by solving the coupled, time dependent mass, momentum, and energy equations, instead of integrating time independent equations.

We validate a preliminary model of hydrodynamic escape against simple, idealized cases (viz., steady state and isothermal conditions) showing that a robust solution obtains and then compare to existing cases in the literature as cited above. The general tools developed here are applied to the problems of hydrodynamic escape on the early Earth and close-in extrasolar gas giant planets and results from these analyses are shown.

The water cycle is one of the most important mechanisms shaping the current climate of Mars. Recent progress in climate monitoring with the Mars Global Surveyor’s TES instrument has revealed new features of the water cycle, including its strong coupling with the atmospheric circulation. We have implemented comprehensive simulation of hydrological processes in the GFDL Mars General Circulation Model, including at- mospheric transport and microphysics of water ice clouds and their coupling with dust and radiative balance. It is shown that the seasonal migration of water between polar regions and tropics is strongly influenced by the microphysical properties of clouds and the Hadley cell circulation. The intensity and zonal structure of this circulation is, in turn, sensitive to radiative forcing by aerosols. In order to determine how various mechanisms influence the water cycle, we present sensitivity studies against dust load- ing and microphysical processes in clouds. Multiannual simulations with a reduced self-consistent microphysical scheme have been carried out to address the stability of the current surface water inventory and the origin of its prominent interhemispherical asymmetry.

This work has been supported by NASA JURRISS program.