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

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

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

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

Assessment of Mars Atmospheric Temperature Retrievals from the Thermal Emission Spectrometer Radiances

Motivated by the needs of Mars data assimilation. particularly quantification of measurement errors and generation of averaging kernels. we have evaluated atmospheric temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) radiances. Multiple sets of retrievals have been considered in this study; (1) retrievals available from the Planetary Data System (PDS), (2) retrievals based on variants of the retrieval algorithm used to generate the PDS retrievals, and (3) retrievals produced using the Mars 1-Dimensional Retrieval (M1R) algorithm based on the Optimal Spectral Sampling (OSS ) forward model. The retrieved temperature profiles are compared to the MGS Radio Science (RS) temperature profiles. For the samples tested, the M1R temperature profiles can be made to agree within 2 K with the RS temperature profiles, but only after tuning the prior and error statistics. Use of a global prior that does not take into account the seasonal dependence leads errors of up 6 K. In polar samples. errors relative to the RS temperature profiles are even larger. In these samples, the PDS temperature profiles also exhibit a poor fit with RS temperatures. This fit is worse than reported in previous studies, indicating that the lack of fit is due to a bias correction to TES radiances implemented after 2004. To explain the differences between the PDS and Ml R temperatures, the algorithms are compared directly, with the OSS forward model inserted into the PDS algorithm. Factors such as the filtering parameter, the use of linear versus nonlinear constrained inversion, and the choice of the forward model, are found to contribute heavily to the differences in the temperature profiles retrieved in the polar regions, resulting in uncertainties of up to 6 K. Even outside the poles, changes in the a priori statistics result in different profile shapes which all fit the radiances within the specified error. The importance of the a priori statistics prevents reliable global retrievals based a single a priori and strongly implies that a robust science analysis must instead rely on retrievals employing localized a priori information, for example from an ensemble based data assimilation system such as the Local Ensemble Transform Kalman Filter (LETKF)

Isentropic formation of the tropopause

Large-scale horizontal rolls can have a significant influence on turbulent transport across the atmospheric boundary layer. The formation and maintenance of such rolls is dependent on the thermal and dynamic stability of the boundary layer (BL). The authors present aircraft observations of boundary layers, both with and without roll circulations, off the coast of California. The contribution of the rolls to the turbulent fluxes of heat, moisture, and momentum, and the variances of the three velocity components are determined for four cases. The fractional roll contributions to the u and w variances, and the sensible heat and along-wind momentum fluxes, show a near linear increase with altitude, from less than 10% at 30 m to more than 70% at the top of the BL. The variance in v and crosswind momentum flux are more scattered, although the variance shows a slight increase with altitude from about 40% to 60%. The latent heat flux also shows a great deal of scatter, especially in the lower third of the BL where the total flux is small; above this, values range between about 40% and 85% but show no clear trends. A stability parameter in the form of a bulk Richardson Ri number is calculated for each of 13 profiles through the boundary layer; it is found that the Richardson number successfully identifies those cases where rolls are present, and its value corresponds to some extent with the strength of the rolls. Values close to zero correspond to cases with well-defined rolls; for 0.1 < Ri < 0.25 rolls are found to exist, but they tend to be weak and patchy; and no rolls are found where Ri is greater than the critical value of approximately 0.25. Reynolds numbers are calculated from a number of different definitions and indicate the dynamic instability of the shear dominated boundary layers

A regional CO₂ observing system simulation experiment for the ASCENDS satellite mission

Top–down estimates of the spatiotemporal variations in emissions and uptake of CO₂ will benefit from the increasing measurement density brought by recent and future additions to the suite of in situ and remote CO₂ measurement platforms. In particular, the planned NASA Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) satellite mission will provide greater coverage in cloudy regions, at high latitudes, and at night than passive satellite systems, as well as high precision and accuracy. In a novel approach to quantifying the ability of satellite column measurements to constrain CO₂ fluxes, we use a portable library of footprints (surface influence functions) generated by the Stochastic Time-Inverted Lagrangian Transport (STILT) model in combination with the Weather Research and Forecasting (WRF) model in a regional Bayesian synthesis inversion. The regional Lagrangian particle dispersion model framework is well suited to make use of ASCENDS observations to constrain weekly fluxes in North America at a high resolution, in this case at 1° latitude × 1° longitude. We consider random measurement errors only, modeled as a function of the mission and instrument design specifications along with realistic atmospheric and surface conditions. We find that the ASCENDS observations could potentially reduce flux uncertainties substantially at biome and finer scales. At the grid scale and weekly resolution, the largest uncertainty reductions, on the order of 50%, occur where and when there is good coverage by observations with low measurement errors and the a priori uncertainties are large. Uncertainty reductions are smaller for a 1.57 μm candidate wavelength than for a 2.05 μm wavelength, and are smaller for the higher of the two measurement error levels that we consider (1.0 ppm vs. 0.5 ppm clear-sky error at Railroad Valley, Nevada). Uncertainty reductions at the annual biome scale range from ~40% to ~75% across our four instrument design cases and from ~65% to ~85% for the continent as a whole. Tests suggest that the quantitative results are moderately sensitive to assumptions regarding a priori uncertainties and boundary conditions. The a posteriori flux uncertainties we obtain, ranging from 0.01 to 0.06 Pg C yr⁻¹ across the biomes, would meet requirements for improved understanding of long-term carbon sinks suggested by a previous study