Mixing in the ocean produced by tropical cyclones     

JOHAN NILSSON 《地球，A辑:动力气象学与海洋学》1996,48(2):342-355

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States of maximum entropy production in a one-dimensional vertical model with convective adjustment   总被引：2，自引：0，他引：2  

By TONI PUJOL  JOAQUIM FORT 《地球，A辑:动力气象学与海洋学》2002,54(4):363-369

We investigate the hypothesis that the atmosphere is constrained to maximize its entropy production by using a one-dimensional (1-D) vertical model. We prescribe the lapse rate in the convective layer as that of the standard troposphere. The assumption that convection sustains a critical lapse rate was absent in previous studies, which focused on the vertical distribution of climatic variables, since such a convective adjustment reduces the degrees of freedom of the system and may prevent the application of the maximum entropy production (MEP) principle. This is not the case in the radiative-convective model (RCM) developed here, since we accept a discontinuity of temperatures at the surface similar to that adopted in many RCMs.
For current conditions, the MEP state gives a difference between the ground temperature and the air temperature at the surface ≈10 K. In comparison, conventional RCMs obtain a discontinuity ≈2K only. However, the surface boundary layer velocity in the MEP state appears reasonable (≈m s⁻¹). Moreover, although the convective flux at the surface in MEP states is almost uniform in optically thick atmospheres, it reaches a maximum value for an optical thickness similar to current conditions. This additional result may support the maximum convection hypothesis suggested by Paltridge (1978 ).  相似文献   

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet   总被引：9，自引：0，他引：9  

TEGNER  CHRISTIAN; CAWTHORN  R. GRANT; KRUGER  F. JOHAN 《Journal of Petrology》2006,47(11):2257-2279

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

Deglaciation sequences in the Permo-Carboniferous Karoo and Kalahari basins of southern Africa: a tool in the analysis of cyclic glaciomarine basin fills   总被引：2，自引：0，他引：2  

JOHAN N. J. VISSER 《Sedimentology》1997,44(3):507-521

The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice-rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles. The massive diamictites formed as aprons and fans in front of the ice-grounding line, whereas the stratified diamictites represent more distal debris-flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes. Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter-term fluctuation of the ice margin is present within the major advance-retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea-level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono-eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer-term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.  相似文献   

Major Addition of Magma at the Pyroxenite Marker in the Western Bushveld Complex, South Africa   总被引：3，自引：3，他引：3  

CAWTHORN  R. GRANT; MEYER  PETER S.; KRUGER  F. JOHAN 《Journal of Petrology》1991,32(4):739-763

A major, but gradual, reversal in the cryptic variation patternof the plagioclase and pyroxenes, of 13 mol% anorthite and 10mol% Mg/ (Mg + Fe) respectively, is documented in the Main Zoneof the western Bush veld Complex. These changes are accompaniedby a decrease in initial ⁸⁷Sr/⁸⁶Sr ratio from > 0.708 to< 0.707. The Pyroxenite Marker, a distinctive orthopyroxenitelayer, occurs close to the top of this reversed differentiationsequence. This is attributed to addition of less differentiatedmagma. On the basis of a mass balance calculation of the initial⁸⁷Sr/ ⁸⁶Sr ratios, it is estimated that the volume of magmaadded was comparable to that of the resident magma. Increases in the Fe₂O₃, TiO₂, Al₂O₃, and Na₂O contents of thepyroxenes above the level of magma addition indicate that thenew magma had a lower silica activity and higher fO₂ than theresident magma. Quantification of the trace element and REEcontent of the two magmas is hampered by the very low proportionof trapped intercumulus component in these adcumulate rocks.However, semi-quantitative modelling indicates that the traceand REE signatures of the two magmas were similar, with moderateLREE enrichment and flat HREE profiles. The new magma had aslightly higher La/ Sm ratio than the resident magma, consistentwith its more alkaline nature. The new magma was probably added gradually, while 100–150m of cumulates formed. It probably intruded at an intermediatelevel within an existing stratified magma chamber, where itcooled and crystallized, and composite packets of liquid pluscrystals plunged to the base of the chamber. The cores of plagioclasegrains formed during this mixing interval show a wider rangeof compositions than in other sections, and plagioclase primocrystsfrom both magmas may be preserved within single samples. Therefore,although intimate physical mixing of packets of unknown sizeof the two magmas occurred, re-equilibration of the major oxidecomposition of the plagioclase primocrysts was not achieved.However, the data and calculations based on diffusion ratesindicate that partial Sr isotopic resetting of plagioclase mayhave occurred.  相似文献   

Suspension and calibration of a sediment trap   总被引：5，自引：0，他引：5  

By Lars Håkanson 《Aquatic Sciences - Research Across Boundaries》1984,46(1):171-175

The aim of this technical note is to introduce a new, simple construction of a buoy-carried, suspended sediment trap that meets given demands concerning design (geometry of vessels) and stability during the registration period. The device is based on a ball-and-socket joint in the center and two Plexiglas cylinders (D=5cm, H=30cm) on PVC plastic arms. The suspension consists of only three parts. To determine the optimal construction, the apparatus has been calibrated in a circulating flume at various water velocities.  相似文献   

运用上月各气象要素的特征来预告下月雨量——(沈阳夏季各月雨量作例)     

杨鉴初  刘钟玲 《气象学报》1953,24(1):159-172

一个地方下月的雨量,能不能从当地上月天气演变的结果所记录下来的各种气象要素的数值来推测呢?现在这篇报告的目的就想解答这个问题。关於这方面的研究,以前早有人注意,例如竺可桢教授研究过冬天温度与夏季雨量的关系,  相似文献   

Deglaciation of western Central Norway   总被引：4，自引：0，他引：4  

JOHAN LUDVIG SOLLID  LEIF SØRBEL 《Boreas: An International Journal of Quaternary Research》1979,8(2):233-239

The glacier movements and corresponding ice margins in Central Norway during Younger Dryas and Preboreal are reconstructed. Scattered, older marginal deposits are difficult to correlate. Raised beach features indicate that the deep fjords became ice-free at an early stage due to calving. In Møre og Romsdal county the glacier front lay at the fjord heads during Younger Dryas, with extensive local glaciation in the intervening mountain areas, and a limit of glaciation 500–600 m lower than the present. In certain places local moraines older than Younger Dryas have been preserved. Autochthonous block fields are widespread in the mountains of Møre og Romsdal. The lower limit of block fields lies at c. 500 m above sea level on the outermost coast and rises to c. 1500 m above sea level in the interior fjord country. No erratics, striation or lateral moraines from the inland ice have been found above this limit. Its gradient, which in outer fjord districts is about 1%, seems to indicate the ice surface at the last maximum of Weichsel glaciation.  相似文献   

Intermediate time error growth and predictability: tropics versus mid-latitudes     

By DAVID M. STRAUS  DAN PAOLINO 《地球，A辑:动力气象学与海洋学》2009,61(5):579-586

The evolution of identical twin errors from an atmospheric general circulation model is studied in the linear range (small errors) through intermediate times and the approach to saturation. Between forecast day 1 and 7, the normalized error variance in the tropics is similar to that at higher latitudes. After that, tropical errors grow more slowly. The predictability time τ taken for tropical errors to reach half their saturation values is larger than that for mid-latitudes, especially for the planetary waves, thus implying greater potential predictability in the tropics.
The discrepancy between mid-latitude and tropical τ is more pronounced at 850 hPa than at 200 hPa, is largest for the planetary waves, and is more pronounced for errors arising from wave phase differences (than from wave amplitude differences).
The spectra of the error in 200 hPa zonal wind show that for forecast times up to about 5 d, the tropical error peaks at much shorter scales than the mid-latitude errors, but that subsequently tropical and mid-latitude error spectra look increasingly similar.
The difference between upper and lower level tropical τ may be due to the greater influence of mid-latitudes at the upper levels.  相似文献   

On the selection of prognostic moments in parametrization schemes for drop sedimentation     

By ULRIKE WACKER  CHRISTOF LÜPKES 《地球，A辑:动力气象学与海洋学》2009,61(4):498-511

Common parametrization models for cloud microphysical processes use condensate mass density and/or particle number density as prognostic properties. However, other moments of the particle size distribution can likewise be chosen for prediction. This study deals with parametrization models with one and two, respectively, prognostic moments for the sedimentation of drop ensembles. The spectral resolving model defines the reference solution.
The evolution of the vertical profiles of liquid water content, drop number density and rain rate strongly depend on the choice of the prognostic moments in the parametrization models. In models with a single prognostic moment, its vertical profile is copied by all other moments. The moment of most physical pertinence is recommended for prediction. In models with two prognostic moments, the vertical profiles of all moments differ. The orders of the prognostic moments should be chosen close to the order of moments of highest relevance. Otherwise large errors occur. For example, comparison of modelled versus observed radar reflectivity (6th moment with respect to diameter) does not tell much about the quality of other properties if reflectivity is diagnosed from for example, number density and mass density. Furthermore, mass conservation is fulfilled only if mass density is forecasted.  相似文献