Local Initiation of Deep Convection on the Canadian Prairie Provinces     

John M. Hanesiak  Richard L. Raddatz  Scott Lobban 《Boundary-Layer Meteorology》2004,110(3):455-470

Our study found that local mesoscale circulations associated with highlandareas, and transient evapotranspiration discontinuities, are likely to influencethe timing and location of the initiation of deep convection across the Canadianprairie provinces when synoptic-scale forcing is weak (e.g., weak low level windsand no frontal boundaries). The cumulus congestus and cumulonimbus clouds thatformed over the highland areas were initiated by anabatic wind induced mesoscalecirculations. These deep convective clouds generally formed relatively early in theday (about 1030 Central Standard Time (CST)). In the relatively flat cropped grasslandregion, transient evapotranspiration gradients influenced the location of deep convectioninitiation, and the average soil moisture in the root zone had a direct impact on theirtiming. As root zone soil moisture declined from greater than 70% to less than 30%,convection initiation was delayed from about 0930 to 1630 CST. Cumulus congestusand cumulonimbus clouds that formed over the ephemeral evapotranspiration gradientswere initiated by land-land circulations. The study has improved the understanding ofthe influence of local surface forcing on the development of deep convective cloud onthe Canadian prairie provinces. The identification of areas where deep convection islikely to be initiated with weak synoptic forcing will also aid in the forecasting ofthunderstorms in this region.  相似文献   

Chemical, biological and hydrological controls on the C content of cold seep carbonate crusts: numerical modeling and implications for convection at cold seeps     

G. Aloisi  K. Wallmann  J.-F. Saliège 《Chemical Geology》2004,213(4):359-383

Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of ¹⁴C (D¹⁴C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on ¹⁴C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH₄ in the expelled fluids result in elevated crust D¹⁴C, while high Ca²⁺ and HCO₃⁻ concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D¹⁴C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D¹⁴C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the ¹⁴C-decay associated to the time needed to form the crust contribute in defining the D¹⁴C of carbonate crusts. We then use the model to reproduce the ¹⁴C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of ¹⁴C (−945.0<D¹⁴C ‰<−930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high ¹⁴C-content (−899.0<D¹⁴C ‰<−838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year⁻¹) of low salinity (20‰) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important.  相似文献   

On the onset of the south china sea summer monsoon in 1998   总被引：21，自引：0，他引：21  

Li Chongyin  Wu Jingbo 《大气科学进展》2000,17(2):193-204

1.IntroductionChinesescientistshavepointedoutsincethe1980sthattheAsiansummermonsooniscomposedoftheSouthAsian(Indian)monsoonsystemandtheEastAsianmonsoonsystem,whichhavetheirparticularcharacteristicsrespectivelybutalsointeractoneachother;andtheAsiansummermonsoonbreaksoutintheSouthChinaSea(SCS)regionatfirst,thenspreadsnorthwestwardandnorthwardrespectively,finallytheSouthAsiansummermonsoonandtheEastAsiansummermonsoonaresetup(TaoandChen,1987;JinandChen,1985;Zhuetal.,1986).Muchattentionhasbe…  相似文献   

对流电场、场向电流和极光区电集流变化的地磁响应   总被引：5，自引：5，他引：0       下载免费PDF全文

沈长寿  资民筠  高玉芬  索玉成  吴健 《地球物理学报》1999,42(6):725-731

对流电场、场向电流和极光区电集流是磁层一电离层耦合的主要物理过程．它们的演化发展时间分别为几分钟至半小时的量级．本文用１００°Ｅ和３００°Ｅ的两个地磁经度链附近各１１个台站的１ｍｉｎ均值地磁Ｈ和Ｚ分量资料,分析了１９９４年４月１６-１７日磁暴期间磁层耦合过程对极光区和中低纬区电离层扰动的地磁特征．强磁暴开始时,台站所处的地方时位置不同,则观测到的电离层和地磁响应也完全不同．这是磁层对流和一、二区场向电流共同作用的结果．一般说,扰时极光区的西向电集流变化更为强烈．随着耦合的发展,极光区范围会向南北扩展,电集流中心带则向低纬侧移动．在中低纬区,二区场向电流的建立能屏蔽一区场向电流所产生的扰动,并引起反向的电流及地磁变化．由此,中低纬区夜间有可能出现短时间的东向电场,又可通过ＥＸＢ的垂直向上漂移作用抬升Ｆ层等离子体,并发生同一经度链附近的多站电离层ｈ＇Ｆ同时突增现象．另一方面,磁赤道附近的台站则更多地受内磁层赤道环电流和电离层赤道电集流的影响．  相似文献   

Unstable steam-water convection as possible trigger to earthquakes     

V. S. Panfilov  G. A. Sobolev 《Pure and Applied Geophysics》1995,145(1):139-147

Convection occurs when two water reservoirs, the overlying and the underlying, are connected by a narrow channel and the fluid in the lower reservoir is heated to the stage of phase transition into steam. The laboratory study of the properties of unstable steam-water convection showed that under favourabieP-T conditions the convection can be the triggering mechanism of seismicity. This type of convection causes a sudden fall of pressure in the lower reservoir and in the connecting channel, the impulsive mechanical disturbances, and cyclicity. The point of initiation of this phenomenon can be located at a depth of 5–7 km from the earth's surface with subsequent propagation of the process of instability to larger and smaller depths. This model of the natural terrestrial conditions can account for the earthquake cyclicity in the same focal zones, the rise of temperature and of the level of ground waters during earthquakes, the enhancement of seismicity while filling the water storage basins, the effect of floating up of hypocenters of aftershocks and the greater intervals between them.  相似文献   

对计算土壤和岩石中氡浓度及其表面析出率表面边界条件的讨论     

何斌  过惠平 《铀矿地质》1998,14(5):302-307

本文对求解土壤或岩石中氡浓度及其表面析出率的边界条件进行了探讨。以空气中无穷远处氡浓度为零（空气中无氡源）的自然边界条件代替空气与土壤或岩石交界面氡浓度为零的边界条件,给出了土壤和岩石中氡浓度及其表面析出率的具体表达式。与前人研究结果比较,该式中增加了一项,表面氡析出率缩小了一个倍数。通过对土壤、岩石的具体计算．表明两种边界条件的计算结果有相当的偏差。同时,文中还讨论了在两类边界条件下对流对土壤或岩石中氡浓度及其表面析出率计算的影响。  相似文献   

One-dimensional modelling of mantle flow     

Wolfgang R. Jacoby 《Pure and Applied Geophysics》1978,116(6):1231-1249

A one-dimensional model of flow between a fixed boundary at the bottom and a moving one on top with no net flow through vertical sections is tested for geophysically interesting mantle viscosity-depth functions. Such a model, although simplistic, may help in answering the question to what depth the return flow extends, at least in the case of moving plates measuring many thousand kilometers across, such as the Pacific plate.It the viscosity in the asthenosphere is less than three orders of magnitude smaller than that of the mantle below, the return flow extends to great depth and the asthenosphere is a zone of concentrated shear. If the viscosity contrast is greater, the return flow is concentrated in the asthenosphere. For a wide range of model parameters typical flow velocities below the asthenosphere are about one-tenth of the plate velocity. The pressure gradient required by the mantle flow may be manifest in gravity trends across moving plates, but no excessive gravity anomalies are required by the model if the absolute viscosity values conform to those inferred from post-glacial rebound data. A thinner and lower-viscosity layer is favored over a thicker and more viscous layer if both fit glacial rebound evidence. The present model may not be applicable if down to the core the viscosity is as low as about 10²¹ N s m^–2 with a free-slip bottom boundary.  相似文献   

The alpha/beta ocean distinction: A perspective on freshwater fluxes, convection, nutrients and productivity in high-latitude seas   总被引：3，自引：0，他引：3  

Eddy C. Carmack   《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2578

Stratification is perhaps the most important attribute of oceans with regards to climate and biology. Two simple aspects of the ocean's climate system appear to have a surprisingly important role in transforming waters that feed the global thermohaline circulation, dominating patterns of biogeochemical flux and establishing macroecological domains. First, largely because of meridional distillation (mainly due to the atmospheric transport of freshwater across the Isthmus of Panama) the North Pacific is fresher than the North Atlantic. Second, largely because of zonal distillation (e.g., warming and evaporation at low latitudes and poleward transport of latent heat and moisture by the atmosphere) the upper layers of subtropical seas are permanently stratified by temperature (N_T²=gαdT/dz>0; here called alpha oceans), while the upper layers of high-latitude seas are permanently stratified by salinity (N_S²=gβdS/dz>0; here called beta oceans). The physical basis for the boundary separating alpha and beta oceans is unclear, but may lie in the thermodynamical equations published by Fofonoff [1961. Energy transformations in the sea. Fisheries Research Board of Canada, Report Series 109, 82pp]. Nevertheless, it is clear that the resulting thermohaline distributions establish a ‘downhill journey’ of low-salinity (and nutrient-rich) waters from the North Pacific to the Arctic and then into the North Atlantic. The Arctic Ocean—itself—acts a double estuary, whereby waters entering from the North Atlantic become either denser through cooling (negative estuary) or lighter by freshening (positive estuary) as they circulate within the basin and then return to the North Atlantic as a variety of components of the ocean's conveyor. Intermediate and deep waters generally form within cyclonic beta oceans in close proximity to alphas systems. Similar patterns of stratification, nutrients and biogeographical boundaries persist in the Southern Hemisphere. It is thus argued that this simple distinction—alpha versus beta oceans—provides a broad, conceptual framework for simple interpretation of key physical and biological processes and rates, including the impacts of climate variability.  相似文献   

Analysis of wake behind a rotating propeller using PIV technique in a cavitation tunnel     

Bu-Geun Paik  Jin Kim  Young-Ha Park  Ki-Sup Kim  Kwon-Kyu Yu 《Ocean Engineering》2007,34(3-4):594-604

A two-frame particle image velocimetry (PIV) technique is used to investigate the wake characteristics behind a marine propeller with 4 blades at high Reynolds number. For each of 9 different blade phases from 0° to 80°, 150 instantaneous velocity fields are measured. They are ensemble averaged to study the spatial evolution of the propeller wake in the region ranging from the trailing edge to one propeller diameter (D) downstream location. The phase-averaged mean velocity shows that the trailing vorticity is related to radial velocity jump, and the viscous wake is affected by boundary layers developed on the blade surfaces and centrifugal force. Both Galilean decomposition method and vortex identification method using swirling strength calculation are very useful for the study of vortex behaviors in the propeller wake region. The slipstream contraction occurs in the near-wake region up to about X/D=0.53 downstream. Thereafter, unstable oscillation occurs because of the reduction of interaction between the tip vortex and the wake sheet behind the maximum contraction point.  相似文献   

SENSITIVITY OF MESOSCALE CONVECTIVE SYSTEMS AND ASSOCIATED HEAVY RAINFALL TO SOIL MOISTURE OVER SOUTH CHINA     

蒙伟光  张艳霞  李江南  戴光丰  李昊睿  黄燕燕 《热带气象学报(英文版)》2017,23(1):91-102

The impacts of soil moisture(SM) on heavy rainfall and the development of Mesoscale Convection Systems(MCSs) are investigated through 24-h numerical simulations of two heavy rainfall events that occurred respectively on28 March 2009(Case 1) and 6 May 2010(Case 2) over southern China. The numerical simulations were carried out with WRF and its coupled Noah LSM(Land Surface Model). First, comparative experiments were driven by two different SM data sources from NCEP-FNL and NASA-GLDAS. Secondary, with the run driven by NASA-GLDAS data as a control one, a series of sensitivity tests with different degree of(20%, 60%) increase or decrease in the initial SM were performed to examine the impact of SM on the simulations. Comparative experiment results show that the 24-h simulated cumulative rainfall distributions are not substantially affected by the application of the two different SM data,while the precipitation intensity is changed to some extent. Forecast skill scores show that simulation with NASA-GLDAS SM data can lead to some improvement, especially in the heavy rain(芏50 mm) forecast, where there is up to 5% increase in the TS score. Sensitivity test analysis found that a predominantly positive feedback of SM on precipitation existed in these two heavy rain events but not with completely the same features. Organization of the heavy rainfall-producing MCS seems to have an impact on the feedback process between SM and precipitation. For Case 1, the MCS was poorly organized and occurred locally in late afternoon, and the increase of SM only caused a slight enhancement of precipitation. Drier soil was found to result in an apparent decrease of rainfall intensity,indicating that precipitation is more sensitive to SM reduction. For Case 2, as the heavy rain was caused by a well-organized MCS with sustained precipitation, the rainfall is more sensitive to SM increase, which brings more rainfall. Additionally, distinctive feedback effects were identified from different stages and different organization of MCS, with strong feedback between SM and precipitation mainly appearing in the early stages of the poorly organized MCS and during the late period of the well-organized MCS.  相似文献