Upgrades to the Boundary-Layer Scheme in the Met Office Numerical Weather Prediction Model   总被引：1，自引：1，他引：0  

A. R. Brown  R. J. Beare  J. M. Edwards  A. P. Lock  S. J. Keogh  S. F. Milton  D. N. Walters 《Boundary-Layer Meteorology》2008,128(1):117-132

Recent upgrades to the boundary-layer scheme in the UK Met Office operational global Numerical Weather Prediction model are documented. These comprise a reduction in turbulent mixing in stable conditions over the sea, and the inclusion of non-local momentum mixing in convective conditions. The dependence of low-level winds on changing stability is shown to have been significantly improved. Crucially, it is also found that these improvements in local performance have been achieved without degrading the model skill in terms of synoptic evolution—something that has proved difficult to achieve in the past in many operational models. In fact some aspects of the large-scale flow (e.g. zonal mean winds) have been slightly improved.  相似文献   

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis   总被引：2，自引：0，他引：2  

Holly A. Michael  Clifford I. Voss 《Hydrogeology Journal》2009,17(7):1561-1577

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.  相似文献   

Permafrost Degradation and Ecological Changes Associated with a WarmingClimate in Central Alaska   总被引：4，自引：0，他引：4  

M. Torre Jorgenson  Charles H. Racine  James C. Walters  Thomas E. Osterkamp 《Climatic change》2001,48(4):551-579

Studies from 1994–1998 on the TananaFlats in central Alaska reveal that permafrostdegradation is widespread and rapid, causing largeshifts in ecosystems from birch forests to fens andbogs. Fine-grained soils under the birch forest areice-rich and thaw settlement typically is 1–2.5 mafter the permafrost thaws. The collapsed areas arerapidly colonized by aquatic herbaceous plants,leading to the development of a thick, floatingorganic mat. Based on field sampling of soils,permafrost and vegetation, and the construction of aGIS database, we estimate that 17% of the study area(263,964 ha) is unfrozen with no previous permafrost,48% has stable permafrost, 31% is partiallydegraded, and 4% has totally degraded. For thatportion that currently has, or recently had,permafrost (83% of area), 42% has been affected bythermokarst development. Based on airphoto analysis,birch forests have decreased 35% and fens haveincreased 29% from 1949 to 1995. Overall, the areawith totally degraded permafrost (collapse-scar fensand bogs) has increased from 39 to 47% in 46 y. Based on rates of change from airphoto analysis andradiocarbon dating, we estimate 83% of thedegradation occurred before 1949. Evidence indicatesthis permafrost degradation began in the mid-1700s andis associated with periods of relatively warm climateduring the mid-late 1700s and 1900s. If currentconditions persist, the remaining lowland birchforests will be eliminated by the end of the nextcentury.  相似文献   

The Geysers-Cobb Mountain Magma System, California (Part 2): timescales of pluton emplacement and implications for its thermal history     

Axel K Schmitt  T.Mark Harrison  Oscar Lovera  Mark Walters 《Geochimica et cosmochimica acta》2003,67(18):3443-3458

Over 400 ion microprobe U-Pb isotopic ages measured for zircons extracted from 24 geothermal wells that penetrate the Geysers Plutonic complex (GPC) allow us to conclude that the entire known extent of the GPC crystallized during the early Pleistocene. Nine samples of the microgranite porphyry that forms the shallow cupola (100-1,500 m below sea-level, mbsl) of the GPC yield the oldest model U-Pb age (1.75 ± 0.01 Ma after correction for initial U series disequilibrium; errors 1σ). Twelve samples from the main intrusive phase (orthopyroxene-biotite granite) present at depths >1,250 mbsl define a crystallization age of 1.27 ± 0.01 Ma. This coincides with the age determined for a structurally and compositionally distinct body of granodiorite (1.25 ± 0.01 Ma; N = 5 samples) that is intruded over a similar depth range. Two petrographically distinct varieties of orthopyroxene-biotite granite yield ages of 1.46 ± 0.03 (GPC21-6000) and 1.16 ± 0.02 Ma (CA5636 74F 21; three samples). U-Pb zircon ages for dikes intruded in metagraywacke country-rocks overlap with those obtained from the main body of the GPC and include the youngest material identified (dike sample NEGU2 ST1-7700: 1.11 ± 0.03 Ma). Overall, the U-Pb results demonstrate that the main body of the GPC (∼300 km³) was emplaced and crystallized within the upper crust over a short time interval (0.2 Ma) that overlaps with zircon crystallization ages of overlying silicic volcanic units.  相似文献   

Partition coefficients for olivine-melt and orthopyroxene-melt systems   总被引：3，自引：0，他引：3  

Paul Beattie  Clifford Ford  Douglas Russell 《Contributions to Mineralogy and Petrology》1991,109(2):212-224

Thermodynamic analysis shows that olivinemelt and orthopyroxene-melt partition coefficients for many elements should be approximately linear functions of D_Mg. These simple relationships can be combined with the constraint of mineral stoichiometry to allow the direct calculation of partition coefficients for these elements if the major element chemistry of the melt phase is known. A large dataset of published and unpublished experimental mineral-melt pairs for compositions in the range komatiite to andesite has allowed the determination of the empirical constants required for this calculation. The precision of these parameterisations is demonstrated by comparing the values calculated with those observed. Comparison of phenocryst-matrix partition coefficients with those measured from experimental mineral-melt pairs demonstrates that experimentally determined partition coefficients are equivalent to those in magmatic processes. There are therefore no significant kinetic factors precluding magmatic partitioning being reproduced on an experimental timescale. The model provides a set of simple tests for equilibrium and enables the chemical evolution of a magma fractionating olivine or orthopyroxene to be modelled. An empirical equation for distinguishing orthopyroxene from other low-Ca pyroxenes in chemical analyses of experimental runs is also presented.  相似文献   

Periglacial processes,ed. C. A. M. King,Benchmark Papers in Geology No. 27. Dawden,Hutchinson & Ross Inc., Stroudsburg,Pa., U.S.A., (1976), No. of Pages: 459. Price: $42.00, £24.70     

Clifford Embleton 《地球表面变化过程与地形》1977,2(1):92-92

  相似文献   

Mineral-chemical and isotopic studies of Namaqualand granulites,South Africa: A grenville analogue     

Tom N. Clifford  Eugen F. Stumpfl  Alwyn J. Burger  Terence S. McCarthy  David C. Rex 《Contributions to Mineralogy and Petrology》1981,77(3):225-250

The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area. U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°–650° C and ca. 7–8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.  相似文献   

Internet-based open source software for learning about GPS   总被引：1，自引：0，他引：1  

Clifford Kelley 《GPS Solutions》2002,6(3):201-205

  相似文献   

SWICA—2 M³: Second Conference on Salt Water Intrusion in Coastal Aquifers: Monitoring, Modeling, and Management     

Luis E. Marin  Clifford I. Voss 《Ground water》2004,42(3):318-318

  相似文献   

The future of hydrogeology     

Clifford I. Voss 《Hydrogeology Journal》2005,13(1):1-6

  相似文献