Using wind run to predict sand drift

Conventional aeolian sand transport models relate mass transport rate to wind speed or shear velocity, usually expressed and empirically tested on a 1-s time scale. Projections of total sand delivery over long time scales based on these models are highly sensitive to any small bias arising from statistical fitting on empirical data. We analysed time series of wind speed and sand transport rate collected at 14 independent measurement stations on a beach during a prior field experiment. The results show that relating total sand drift to cumulative above-threshold wind run yields models which are more statistically robust when fitted on empirical data, generating smaller prediction errors when projected to longer time scales. Testing of different power exponents indicates that a linear relationship between sand drift and above-threshold wind run yields the best results. These findings inspire a speculative novel phenomenological model relating the mass flow of air in the boundary layer to the mass transport of sand over the surface. © 2020 John Wiley & Sons, Ltd.     

Elevation trends and shrink–swell response of wetland soils to flooding and drying

Given the potential for a projected acceleration in sea-level rise to impact wetland sustainability over the next century, a better understanding is needed of climate-related drivers that influence the processes controlling wetland elevation. Changes in local hydrology and groundwater conditions can cause short-term perturbations to marsh elevation trends through shrink–swell of marsh soils. To better understand the magnitude of these perturbations and their impacts on marsh elevation trends, we measured vertical accretion and elevation dynamics in microtidal marshes in Texas and Louisiana during and after the extreme drought conditions that existed there from 1998 to 2000. In a Louisiana marsh, elevation was controlled by subsurface hydrologic fluxes occurring below the root zone but above the 4 m depth (i.e., the base of the surface elevation table benchmark) that were related to regional drought and local meteorological conditions, with marsh elevation tracking water level variations closely. In Texas, a rapid decline in marsh elevation was related to severe drought conditions, which lowered local groundwater levels. Unfragmented marshes experienced smaller water level drawdowns and more rapid marsh elevation recovery than fragmented marshes. It appears that extended drawdowns lead to increased substrate consolidation making it less resilient to respond to future favorable conditions. Overall, changes in water storage lead to rapid and large short-term impacts on marsh elevation that are as much as five times greater than the long-term elevation trend, indicating the importance of long-term, high-resolution elevation data sets to understand the prolonged effects of water deficits on marsh elevation change.     

An investigation of the sensitivity of a land surface model to climate change using a reduced form model

 In an illustration of a model evaluation methodology, a multivariate reduced form model is developed to evaluate the sensitivity of a land surface model to changes in atmospheric forcing. The reduced form model is constructed in terms of a set of ten integrative response metrics, including the timing of spring snow melt, sensible and latent heat fluxes in summer, and soil temperature. The responses are evaluated as a function of a selected set of six atmospheric forcing perturbations which are varied simultaneously, and hence each may be thought of as a six-dimensional response surface. The sensitivities of the land surface model are interdependent and in some cases illustrate a physically plausible feedback process. The important predictors of land surface response in a changing climate are the atmospheric temperature and downwelling longwave radiation. Scenarios characterized by warming and drying produce a large relative response compared to warm, moist scenarios. The insensitivity of the model to increases in precipitation and atmospheric humidity is expected to change in applications to coupled models, since these parameters are also strongly implicated, through the representation of clouds, in the simulation of both longwave and shortwave radiation. Received: 27 March 2000 / Accepted: 11 September 2000     

Seasonal variations of the clear-sky greenhouse effect: the role of changes in atmospheric temperatures and humidities

We present an analysis of the factors which control the seasonal variations of the clear-sky greenhouse effect, based on satellite observations and radiative transfer simulations. The satellite observations include the radiation budget at the top of the atmosphere from the Earth Radiation Budget Experiment and the total column moisture content derived from the Special Sensor Microwave/Imager. The simulations were performed with the SAMSON system described in an earlier paper, using atmospheric temperatures and humidities from operational analyses produced by the European Centre for Medium Range Weather Forecasts. At low latitudes, the magnitude of the clear-sky greenhouse effect is dominated by the strong thermodynamic link between the total column moisture content of the atmosphere and sea surface temperatures, with minimal seasonal variations. In contrast, at middle to high latitudes there are strong seasonal variations, the clear-sky greenhouse effect being largest in winter and smallest in summer. These variations cannot be explained by the seasonal cycle in the total column moisture content, as this is largest in summer and smallest in winter. The variations are controlled instead by the seasonal changes in atmospheric temperatures. The colder atmosphere in winter enhances the temperature differential between the atmosphere and the sea surface, leading to a larger greenhouse effect despite the lower moisture contents. The magnitude of the clear-sky greenhouse effect is thus controlled by atmospheric humidity at low latitudes, but by atmospheric temperature at middle and high latitudes. These controls are illustrated by results from sensitivity experiments with SAMSON and are interpreted in terms of a simple model.     

Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern Mexico

Six volcanic rocks, reconnaissance samples representing most of the temporal and compositional variation in the Pinacate volcanic field of Sonora and Arizona, are characterized for major element and Nd---Sr isotopic compositions. The samples consist of basanite through trachyte of an early shield volcano, and alkali basalts and a tholeiite from later craters and cinder cones. With the exception of the trachyte sample, which has increased ⁸⁷Sr/⁸⁶Sr due to crustal effects, all ⁸⁷Sr/⁸⁶Sr values fall between 0.70312 and 0.70342, while ε_Nd values are all between + 5.0 and + 5.7. Clinopyroxene in a rare spinel-lherzolite nodule derived from the uppermost mantle beneath the field has ⁸⁷Sr/⁸⁶Sr of 0.70320 but ε_Nd of + 8.8, three ε_Nd units higher than the volcanic rocks. Both the volcanic rocks and the nodule record the presence of asthenospheric, rather than enriched lithospheric mantle beneath Pinacate. This is consistent with one or both of (a) proximity of Pinacate to the Gulf of California spreading center and (b) presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern USA. We consider the comparison to other southwestern USA magma sources as the more relevant alternative, although a definite conclusion is not possible at this stage.     

Consideration of fine-scale coastal oceanography and 3-D acoustics effects for the ESME sound exposure model

Results and recommendations for evaluating the effects of fine-scale oceanographic scattering and three-dimensional (3-D) acoustic propagation variability on the Effects of Sound on the Marine Environment (ESME) acoustic exposure model are presented. Pertinent acoustic scattering theory is briefly reviewed and ocean sound-speed fluctuation models are discussed. Particular attention is given to the nonlinear and linear components of the ocean internal wave field as a source of sound-speed inhomogeneities. Sound scattering through the mainly isotropic linear internal wave field is presented and new results relating to acoustic scattering by the nonlinear internal wave field in both along and across internal wave wavefront orientations are examined. In many cases, there are noteworthy fine-scale induced intensity biases and fluctuations of order 5-20 dB.     

Effects of East China Sea shallow-water environment on acoustic propagation

Operational environmental acoustics experiments were conducted over the frequency range of 25 to 800 Hz in September 1997 in the East China Sea, where the water depth was about 100 m. Objectives of the data analysis reported here are to characterize this environment and to assess its complexities as they may impact acoustic propagation as measured by its transmission loss (TL). Conductivity-temperature-depths and expendable bathy-thermographs sampled the ocean, such that its spatial and temporal variability could be approximately separated. The sound-speed profiles are downward refracting, involve two water masses associated with the Kuroshio Current and Taiwan Warm Current, and have thermocline variations caused by internal tides. The bottom geoacoustic characteristics, presumed to be approximately horizontally isotropic, were based on data atlases and were estimated from the measured TL, for some interpretations. The TL data were obtained in octave bands from explosive signal underwater sound sources and sonobuoy receivers, both deployed at a depth of about 18 m. Tests were conducted in directions approximately normal and parallel to the bathymetric contours and the measured TL was, to zero order, independent of the direction of propagation. To higher order, directional differences in the TL were observed and ascribed to anisotropies in bottom properties. A state-of-the-art TL model was adopted, based on environmental idealizations typical of operational forecasting and compared with the measured TL. The comparison yields a probability density function that quantifies the uncertainty of such a TL model, caused by the stochastic variability of the environment, typically unknown a priori. For the model used, the pdf has a standard deviation of about 2 dB from 50 to 800 Hz and larger below 50 Hz.     

Measurement of sound transmission and signal gain in the complex Strait of Korea

The experiment, The Acoustic Characterization Test III, was conducted in the oceanographically complex Strait of Korea to accurately measure the sound transmission under known environmental conditions. Geoacoustic profiles derived from geophysical measurements, measured bathymetry, and sound-speed profiles were the basis for range dependent parabolic equation (PE) calculations. Agreement between measured and calculated transmission loss was obtained with an attenuation profile in the near water-sediment interface layer with a dependence on frequency to the 1.8 power consistent with measurements in other sand-silt areas. Since the environment was oceanographically complex and the shipping noise levels were high, the coherency of the sound transmission was estimated using relative signal gain (RSG). RSG was taken as the difference between the gain calculated with PE and measured with the array and at longer ranges and higher frequencies was found to be approximately -2 dB with a signal gain coefficient of variation of 5%. This RSG degradation, attributed to the random signal phase fluctuations resulting from scattering from the surfaces and volume of the waveguide, yielded using a Gaussian coherence function a spatial coherence length of 30/spl lambda/ @ 400 Hz-40 km. In addition, high resolution imaging of five targets with two bottom mounted arrays illustrate the achievable performance of low-to-mid frequency active sonar in this environment.