Evaluation of two WAM white capping parameterizations using parallel unstructured SWAN with application to the Northern Gulf of Mexico, USA

The performance of two well accepted formulations for white capping and wind input of third generation wave models, viz., WAM-3 and WAM-4, were investigated using parallel unstructured SWAN (PunSWAN). Several alternative formulations were also considered to evaluate the effects of higher order steepness and wave number terms in white capping formulations. Distinct model configurations were calibrated and validated against available in situ measurements from the Gulf of Mexico. The results showed that some of the in situ calibrated models outperform the saturation level calibrated models in reproducing the idealized wave growth curves. The simulation results also revealed that increasing the power of the steepness term can enhance the accuracy of significant wave height (H_s), at the expense of a higher bias for large waves. It also has negative effects on mean wave period (T_a) and peak wave period (T_p). It is also demonstrated that the use of the quadratic wave number term in the WAM-3 formulation, instead of the existing linear term, ameliorates the T_a underestimation; however, it results in the model being unable to reach any saturation level. In addition, unlike H_s and T_p, it has been shown that T_a is sensitive to the use of the higher order WAM-4 formulation, and the bias is decreased over a wide range of wave periods. However, it also increases the scatter index (SI) of simulated T_a. It is concluded that the use of the WAM-4 wind input formulation in conjunction with the WAM-3 dissipation form, is the most successful case in reproducing idealized wave growth curves while avoiding T_a underestimation of WAM-3 and a potential spurious bimodal spectrum of WAM-4; consequently, this designates another perspective to improve the overall performance of third generation wave models.     

Constraining climate model properties using optimal fingerprint detection methods

 We present a method for constraining key properties of the climate system that are important for climate prediction (climate sensitivity and rate of heat penetration into the deep ocean) by comparing a model's response to known forcings over the twentieth century against climate observations for that period. We use the MIT 2D climate model in conjunction with results from the Hadley Centre's coupled atmosphere–ocean general circulation model (AOGCM) to determine these constraints. The MIT 2D model, which is a zonally averaged version of a 3D GCM, can accurately reproduce the global-mean transient response of coupled AOGCMs through appropriate choices of the climate sensitivity and the effective rate of diffusion of heat anomalies into the deep ocean. Vertical patterns of zonal mean temperature change through the troposphere and lower stratosphere also compare favorably with those generated by 3-D GCMs. We compare the height–latitude pattern of temperature changes as simulated by the MIT 2D model with observed changes, using optimal fingerprint detection statistics. Using a linear regression model as in Allen and Tett this approach yields an objective measure of model-observation goodness-of-fit (via the residual sum of squares weighted by differences expected due to internal variability). The MIT model permits one to systematically vary the model's climate sensitivity (by varying the strength of the cloud feedback) and rate of mixing of heat into the deep ocean and determine how the goodness-of-fit with observations depends on these factors. This provides an efficient framework for interpreting detection and attribution results in physical terms. With aerosol forcing set in the middle of the IPCC range, two sets of model parameters are rejected as being implausible when the model response is compared with observations. The first set corresponds to high climate sensitivity and slow heat uptake by the deep ocean. The second set corresponds to low sensitivities for all magnitudes of heat uptake. These results demonstrate that fingerprint patterns must be carefully chosen, if their detection is to reduce the uncertainty of physically important model parameters which affect projections of climate change. Received: 19 April 2000 / Accepted: 13 April 2001     

Modelling the effects of permafrost loss on discharge from a wetland‐dominated,discontinuous permafrost basin

Permafrost degradation in the peat‐rich southern fringe of the discontinuous permafrost zone is catalysing substantial changes to land cover with expansion of permafrost‐free wetlands (bogs and fens) and shrinkage of forest‐dominated permafrost peat plateaux. Predicting discharge from headwater basins in this region depends upon understanding and numerically representing the interactions between storage and discharge within and between the major land cover types and how these interactions are changing. To better understand the implications of advanced permafrost thaw‐induced land cover change on wetland discharge, with all landscape features capable of contributing to drainage networks, the hydrological behaviour of a channel fen sub‐basin in the headwaters of Scotty Creek, Northwest Territories, Canada, dominated by peat plateau–bog complexes, was modelled using the Cold Regions Hydrological Modelling platform for the period of 2009 to 2015. The model construction was based on field water balance observations, and performance was deemed adequate when evaluated against measured water balance components. A sensitivity analysis was conducted to assess the impact of progressive permafrost loss on discharge from the sub‐basin, in which all units of the sub‐basin have the potential to contribute to the drainage network, by incrementally reducing the ratio of wetland to plateau in the modelled sub‐basin. Simulated reductions in permafrost extent decreased total annual discharge from the channel fen by 2.5% for every 10% decrease in permafrost area due to increased surface storage capacity, reduced run‐off efficiency, and increased landscape evapotranspiration. Runoff ratios for the fen hydrological response unit dropped from 0.54 to 0.48 after the simulated 50% permafrost area loss with a substantial reduction of 0.47 to 0.31 during the snowmelt season. The reduction in peat plateau area resulted in decreased seasonal variability in discharge due to changes in the flow path routing, with amplified low flows associated with small increases in subsurface discharge, and decreased peak discharge with large reductions in surface run‐off.     

The Solar Energetic Particle Event of 14 December 2006

Solar Physics - The solar energetic particle event on 14 December 2006 was observed by several near-Earth spacecraft including the Advanced Composition Explorer (ACE), STEREO A and B, SOHO and...     

Arsenic and uranium transport in sediments near abandoned uranium mines in Harding County,South Dakota

Sediment samples were analyzed as part of ongoing environmental investigations of historical U mining impacts within Custer National Forest in Harding County, South Dakota. Correlations between As and U content, grain size and soil mineralogy were determined to identify contaminant fate and transport mechanisms. Soil samples collected near the mining source zone and up to 61 km downgradient of the minesites were analyzed. Samples were homogenized and wet sieved through polymer screens, and metal(loid) concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Powder X-ray diffraction (XRD) analysis identified quartz as the primary mineral for all size fractions, with varying amounts of analcime, indicative of volcanic origin. Selected samples were examined for trace mineral composition using scanning electron microscopy (SEM). The presence of Fe sulfides and Fe (hydr)oxides indicate heterogeneity in redox potentials on a microscopic scale. Elevated metal(loid) concentrations were associated with trace concentrations of Fe sulfide, indicating an influence on metal transport during weathering. Sequential chemical extractions (SCE) performed on source sediment fractions demonstrated that most As and U was adsorbed to Fe- and Mn-oxides and carbonates with lesser amounts bound by ion exchange, organics and Fe sulfides. Large changes in U/Th and As/Th ratios were observed to coincide with geochemical changes in the watershed, suggesting that metal(loid)–Th ratios may be used in environmental investigations to identify geochemically-significant watershed conditions.     

Particle size characteristics of suspended sediment in Southern Ontario rivers tributary to the great lakes

In spite of the important relationship between sediment particle size and the transport/deposition of adsorbed pollutants in fluvial systems, little information regarding the size characteristics of suspended sediment transported by southern Ontario Great Lakes tributaries is currently available. This paper examines long-term sediment and hydrometric data collected by the Water Resources Branch of Environment Canada in order to provide information on (1) typical particle size distributions of suspended sediment, (2) relationships between source material and particle size characteristics of suspended sediment, and (3) temporal variation in the particle size characteristics of suspended sediment from six southern Ontario rivers. Results illustrate the complex behaviour and variability of sediment particle size transport in these rivers and demonstrate the need for a better understanding of seasonal effects on sediment availability and conveyance processes in fluvial systems.