Rheology of solid methane and nitrogen: applications to Triton

The existing information about the rheological properties of solid methane and nitrogen is reviewed. The main limitation is the absence of creep data for solid nitrogen, but estimated properties are derived on the basis of simple scaling arguments for van der Waals bonded materials. On Triton solid methane and most likely nitrogen are too soft to support any topography over billions of years. Topographic support by water ice-rich materials seems almost certain. Nitrogen and methane do not intermix by solid-state diffusion. The weakness (absence?) of the opposition effect on Triton can be explained by annealing of nitrogen grains, provided the latter have sizes characteristic of freshly deposited frost

Enhancement of Atmospheric Radiation by an Aerosol Layer

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of “photon trapping.” We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogeneous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20°N latitude, summer, by as much as 45% at 2900 Å. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column

Interpretation of the near-IR spectra of the Kuiper Belt Object (136472) 2005 FY_9

Visible and near-IR observations of the Kuiper Belt Object (136472) 2005 FY_9 have indicated the presence of unusually long (1 cm or more) optical path lengths in a layer of methane ice. Using microphysical and radiative transfer modeling, we show that even at the frigid temperatures in the outer reaches of the solar system, a slab of low-porosity methane ice can indeed form by pressureless sintering of micron-sized grains, and it can qualitatively reproduce the salient features of the measured spectra. A good semiquantitative match with the near-IR spectra can be obtained with a realistic slab model, provided the spectra are scaled to a visible albedo of 0.6, at the low end of the values currently estimated from Spitzer thermal measurements. Consistent with previous modeling studies, matching spectra scaled to higher albedos requires the incorporation of strong backscattering effects. The albedo may become better constrained through an iterative application of the slab model to the analysis of the thermal measurements from Spitzer and the visible/near-IR reflectance spectra. The slab interpretation offers two falsifiable predictions: (1) Absence of an opposition surge, which is commonly attributed to the fluffiness of the optical surface. This prediction is best testable with a spacecraft, as Earth-based observations at true opposition will not be possible until early next century. (2) Unlikelihood of the simultaneous occurrence of very long spectroscopic path lengths in both methane and nitrogen ice on the surface of any Kuiper Belt Object, as the more volatile nitrogen would hinder densification in methane ice

A Regional CO2 Observing System Simulation Experiment Using ASCENDS Observations and WRF-STILT Footprints

Knowledge of the spatiotemporal variations in emissions and uptake of CO2 is hampered by sparse measurements. The recent advent of satellite measurements of CO2 concentrations is increasing the density of measurements, and the future mission ASCENDS (Active Sensing of CO2 Emissions over Nights, Days and Seasons) will provide even greater coverage and precision. Lagrangian atmospheric transport models run backward in time can quantify surface influences ("footprints") of diverse measurement platforms and are particularly well suited for inverse estimation of regional surface CO2 fluxes at high resolution based on satellite observations. We utilize the STILT Lagrangian particle dispersion model, driven by WRF meteorological fields at 40-km resolution, in a Bayesian synthesis inversion approach to quantify the ability of ASCENDS column CO2 observations to constrain fluxes at high resolution. This study focuses on land-based biospheric fluxes, whose uncertainties are especially large, in a domain encompassing North America. We present results based on realistic input fields for 2007. Pseudo-observation random errors are estimated from backscatter and optical depth measured by the CALIPSO satellite. We estimate a priori flux uncertainties based on output from the CASA-GFED (v.3) biosphere model and make simple assumptions about spatial and temporal error correlations. WRF-STILT footprints are convolved with candidate vertical weighting functions for ASCENDS. We find that at a horizontal flux resolution of 1 degree x 1 degree, ASCENDS observations are potentially able to reduce average weekly flux uncertainties by 0-8% in July, and 0-0.5% in January (assuming an error of 0.5 ppm at the Railroad Valley reference site). Aggregated to coarser resolutions, e.g. 5 degrees x 5 degrees, the uncertainty reductions are larger and more similar to those estimated in previous satellite data observing system simulation experiments

Coupled weather research and forecasting–stochastic time-inverted lagrangian transport (WRF–STILT) model

This paper describes the coupling between a mesoscale numerical weather prediction model, the Weather Research and Forecasting (WRF) model, and a Lagrangian Particle Dispersion Model, the Stochastic Time-Inverted Lagrangian Transport (STILT) model. The primary motivation for developing this coupled model has been to reduce transport errors in continental-scale top–down estimates of terrestrial greenhouse gas fluxes. Examples of the model’s application are shown here for backward trajectory computations originating at CO2 measurement sites in North America. Owing to its unique features, including meteorological realism and large support base, good mass conservation properties, and a realistic treatment of convection within STILT, the WRF–STILT model offers an attractive tool for a wide range of applications, including inverse flux estimates, flight planning, satellite validation, emergency response and source attribution, air quality, and planetary exploration.Engineering and Applied Science

Atmospheric Inverse Estimates of Methane Emissions from Central California

Methane mixing ratios measured at a tall-tower are compared to model predictions to estimate surface emissions of CH{sub 4} in Central California for October-December 2007 using an inverse technique. Predicted CH{sub 4} mixing ratios are calculated based on spatially resolved a priori CH{sub 4} emissions and simulated atmospheric trajectories. The atmospheric trajectories, along with surface footprints, are computed using the Weather Research and Forecast (WRF) coupled to the Stochastic Time-Inverted Lagrangian Transport (STILT) model. An uncertainty analysis is performed to provide quantitative uncertainties in estimated CH{sub 4} emissions. Three inverse model estimates of CH{sub 4} emissions are reported. First, linear regressions of modeled and measured CH{sub 4} mixing ratios obtain slopes of 0.73 {+-} 0.11 and 1.09 {+-} 0.14 using California specific and Edgar 3.2 emission maps respectively, suggesting that actual CH{sub 4} emissions were about 37 {+-} 21% higher than California specific inventory estimates. Second, a Bayesian 'source' analysis suggests that livestock emissions are 63 {+-} 22% higher than the a priori estimates. Third, a Bayesian 'region' analysis is carried out for CH{sub 4} emissions from 13 sub-regions, which shows that inventory CH{sub 4} emissions from the Central Valley are underestimated and uncertainties in CH{sub 4} emissions are reduced for sub-regions near the tower site, yielding best estimates of flux from those regions consistent with 'source' analysis results. The uncertainty reductions for regions near the tower indicate that a regional network of measurements will be necessary to provide accurate estimates of surface CH{sub 4} emissions for multiple regions

I. Microphysics of frost metamorphism : applications to Triton and Mars. II. A global analysis of the ozone deficit in the upper stratosphere and lower mesosphere. III. The diabatic circulation in the stratosphere as diagnosed from microwave limb sounder data

The present thesis reflects work I have done as a graduate student at Caltech. It is devoted to two broad subjects, planetary frost metamorphism and the terrestrial middle atmosphere, and consists of three papers. Paper I considers frost metamorphism on the surfaces of Triton and Mars. Based on an analysis of the microphysical processes involved in pressureless sintering, it is concluded that fine-grained nitrogen and carbon dioxide frosts can undergo seasonal metamorphism into semitransparent layers on the surface of Triton and in the martian seasonal polar caps, respectively. The presence of such layers explains a host of facts about Triton's surface and about the martian seasonal caps. The Triton portion of the paper has been published in the Journal of Geophysical Research, while the Mars portion has been submitted to Icarus. Paper II is devoted to elucidating a long-standing issue in the terrestrial middle atmosphere chemistry, the so-called "ozone deficit problem." Based on an analysis of data acquired by the Limb Infrared Monitor of the Stratosphere (LIMS) instrument between October 1978 and May 1979, it is concluded that current photochemical models systematically underestimate observed ozone abundances in the upper stratosphere and lower mesosphere. Three modifications to the accepted photochemical scheme, capable of providing a global solution to this problem, are proposed and discussed. This paper is in press by the Journal of Geophysical Research. Paper III differs from the other two in that it reports on results from an ongoing research effort. It considers the diabatic circulation in the stratosphere and lower mesosphere, using ozone and temperature measurements acquired by the Microwave Limb Sounder (MLS) instrument onboard the Upper Atmosphere Research Satellite (UARS). The present study extends past analyses of the diabatic circulation by considering a full annual cycle November 1991 - November 1992 and by taking advantage of the high vertical resolution of MLS data. In the tropical upper stratosphere and lower mesosphere, a semiannual oscillation (SAO) is observed in the computed circulation, with the region of downwelling reaching maximum spatial extent ~1 month before the equinox. The projected lifetime of UARS should enable the present analysis to be extended to several SAO cycles

Angular momentum transfer in low velocity oblique impacts: Implications for asteroids

The efficiency of angular momentum transfer ζ in low velocity oblique impacts was studied experimentally. ζ is defined as the fraction of incident angular momentum transferred to the rotation of the target. Plaster, mortar, cement, and one granite target were studied. Lead and aluminum projectiles were used. Only cratering impacts were considered. ζ was found to decrease with increasing incidence angle φ (relative to the surface normal). For example, for impacts into cylindrical mortar targets a least-squares fit of the form ζ = A(cos φ)β with A = 0.9 and β = 1.7 was found to match the data points reasonably well. In addition, β decreased from 1.9 to 1.4 as the kinetic energy density ϵ (= kinetic energy of the projectile/projectile volume) increased from 0.5 to 1.8 × 10^9 J m^(−3). This suggests that more energetic impacts transfer angular momentum more efficiently. ζ decreased as the indentation hardness H of the target increased: at φ ∼ 35°, ζ = 0.07 for granite (H = 850 kg mm^(−2) and ζ = 0.7 for plaster (H = 7.5 kg mm^(−2)). Cement and mortar (H = 76 kg mm^(−2)) yielded intermediate values, although the values for cement (ζ ∼ 0.3) were appreciably lower than for mortar (ζ ∼ 0.6). In all cases where the velocity of the ricochetted projectile was determined, the fraction of angular momentum carried away by ejecta was found to be less than 30%. Finally, the results were only weakly dependent on the material of the projectile