Mud consolidation during a short time interval

Flume observations of the consolidation of mud beds deposited from an unsteady flow show that 10-mm layer undergoes considerable consolidation during the time of deposition and within the first hour after deposition ceases, but little additional consolidation during the next 24 hr. A certain minimum thickness, between 3 and 10 mm, is required for any consolidation to occur. The results support the hypothesis that thick mud layers may form in nearshore subtidal areas as the result of accumulation of thinner layers deposited during successive tidal cycles, but only if each individual layer is able to withstand erosion during the subsequent cycle.     

Evolution of Baffin Bay Water Masses and Transports in a Numerical Sensitivity Experiment under Enhanced Greenland Melt

We used a numerical model forced with three different scenarios to analyze Baffin Bay circulation sensitivity to runoff around Baffin Bay, especially the Greenland runoff, for the past (1970–2010) and future (2010–2099). We observed an overall decrease in transport from the Arctic to the North Atlantic for the volume, heat, and freshwater over the time period as well as an augmentation of the freshwater and heat in Baffin Bay. In the early 1990s, the increase in heat in Baffin Bay was consistent with an increase in the West Greenland Irminger Water (WGIW) inflow at Davis Strait while later West Greenland Shelf Water played an important role in the heat import, sustaining the idea that the West Greenland Current might have an impact on the melt of West Greenland tidewater glaciers. The increase in freshwater and later in heat in Baffin Bay leads to changes in the steric height inside Baffin Bay, which leads to changes in the circulation. After 1978, the WGIW reaches the North Water polynya and recirculates into the Baffin Bay gyre where it accumulates over time. In the future experiment, the dynamic changes in Baffin Bay are mainly related to the accumulation of heat inside the gyre.     

Equity in the distribution of urban environmental amenities: the case of Washington,D.C.

This study examines equity issues in access to urban environmental amenities using Washington, D.C. as a case study. The amenities that are the focus of this study, urban trees and urban parks, have a range of social and health benefits for city dwellers, and Washington, D.C. is a major city with a large, diverse population and long-standing geographic divisions by race and class. Using spatial regression techniques, the study examines disparities in access to these amenities by race, ethnicity, and socioeconomic status at citywide and sub-city scales. Mixed evidence is found for disparities in access across amenities and across scales. The results emphasize the need for studies of access to urban environmental amenities to pay closer attention to the differences between amenities and to issues of scale.     

A four-dimensional variational system for skillful operational prediction of convective storms

正Forecasting convective storms using NWP models is an important goal and a highly active area of ongoing research.Skillful and reliable NWP of convective storms could allow for severe weather warnings with longer lead times,as operational forecasters begin to incorporate convective-scale forecasts into severe weather forecast operations(Stensrud et al.,2009,2013).This would then provide vulnerable individuals and industries with more time to seek shelter and/or mitigate     

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier,Alaska, USA

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.     

Nonlinear waves on a coupled density front

Abstract

It is shown that the inclusion of the nonlinear terms in the equations of motion of a coupled density front of zero potential vorticity results in wave solutions which merely propagate with time. The linear theory, on the other hand, predicts an exponential temporal growth. The nonlinear equation admits steady solutions representing standing waves whereas if the nonlinear terms are omitted no steady solutions exist. The general initial value problem is difficult to solve numerically since the linear problem is ill posed.

In addition we prove that the general similarity solution of the nonlinear equation tends to zero for large times, at any point in space, regardless of the initial condition.     

Vegetation changes in the Neotropical Gran Sabana (Venezuela) around the Younger Dryas chron

The occurrence of the Younger Dryas cold reversal in northern South America midlands and lowlands remains controversial. We present a palaeoecological analysis of a Late Glacial lacustrine section from a midland lake (Lake Chonita, 4.6501 °N, 61.0157 °W, 884 m elevation) located in the Venezuelan Gran Sabana, based on physical and biological proxies. The sediments were mostly barren from ～15.3 to 12.7 k cal a BP, probably due to poor preservation. A ligneous community with no clear modern analogues was dominant from 12.7 to 11.7 k cal a BP (Younger Dryas chronozone). At present, similar shrublands are situated around 200 m elevation above the lake, suggesting a cooling‐driven downward shift in vegetation during that period. The interval from 11.7 to 10.6 k cal a BP is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence. The intensification of local and regional fires at this interval could have played a role in the vegetation shift. A change to wetter, and probably warmer, conditions is deduced after 11.7 k cal a BP, coinciding with the early Holocene warming. These results support the hypothesis of a mixed origin (climate and fire) of the Gran Sabana savannas, and highlight the climatic instability of the Neotropics during the Late Glacial. Copyright © 2011 John Wiley & Sons, Ltd.     

An invariant theory of the linearized shallow water equations with rotation and its application to a sphere and a plane

The system of linearized shallow water equations is formulated in this paper on any rotating and smooth surface M in terms of differential geometry. The system decouples into two separate equations: a scalar one for the height deviation and a vector one for the velocity field. For low and high frequencies these equations yield asymptotic equations whose solutions are the generalizations of the Poincare and Rossby waves to smooth surface. The application of these equations to the β-plane yields both new and previously known equations for the height deviation and for the velocity components. The application of the equations to the rotating spherical Earth shows that the meridional amplitudes of Poincare and Rossby waves are both described by the prolate angular spheroidal wave functions. The asymptotic and the power series expansions of the eigenvalues of these functions yield new approximations for the dispersion relations of these waves on a sphere. The new dispersion relations are very accurate in the physically relevant range of the single nondimensional model parameter – the square of the nondimensional gravity waves’ phase speed. The invariant formulation can also be applied to other surfaces that are of geophysical interest such as an oblate ellipsoid of revolution.