共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
Earlier GCM studies have expressed the concern that an enhancement of greenhouse warming might increase the occurrence of summer droughts in mid-latitudes, especially in southern Europe and central North America. This could represent a severe threat for agriculture in the regions concerned, where summer is the main growing season. These predictions must however be considered as uncertain, since most studies featuring enhanced summer dryness in mid-latitudes use very simple representations of the land-surface processes ("bucket" models), despite their key importance for the issue considered. The current study uses a regional climate model including a land-surface scheme of intermediate complexity to investigate the sensitivity of the summer climate to enhanced greenhouse warming over the American Midwest. A surrogate climate change scenario is used for the simulation of a warmer climate. The control runs are driven at the lateral boundaries and the sea surface by reanalysis data and observations, respectively. The warmer climate experiments are forced by a modified set of initial and lateral boundary conditions. The modifications consist of a uniform 3 K temperature increase and an attendant increase of specific humidity (unchanged relative humidity). This strategy maintains a similar dynamical forcing in the warmer climate experiments, thus allowing to investigate thermodynamical impacts of climate change in comparative isolation. The atmospheric CO 2 concentration of the sensitivity experiments is set to four times its pre-industrial value. The simulations are conducted from March 15 to October 1st, for 4 years corresponding to drought (1988), normal (1986, 1990) and flood (1993) conditions. The numerical experiments do not present any great enhancement of summer drying under warmer climatic conditions. First, the overall changes in the hydrological cycle (especially evapotranspiration) are of small magnitude despite the strong forcing applied. Second, precipitation increases in spring lead to higher soil water recharge during this season, compensating for the enhanced soil moisture depletion occurring later in the year. Additional simulations replacing the plant control on transpiration with a bucket-type formulation presented increased soil drying in 1988, the drought year. This suggests that vegetation control on transpiration might play an important part in counteracting an enhancement of summer drying when soil water gets limited. Though further aspects of this issue would need investigating, our results underline the importance of land-surface processes in climate integrations and suggest that the risk of enhanced summer dryness in the region studied might be less acute than previously assumed, provided the North American general circulation does not change markedly with global warming. 相似文献
3.
A. Henderson-Sellers 《Climatic change》1993,23(4):337-377
A simplified vegetation distribution prediction scheme is used in combination with the Biosphere-Atmosphere Transfer Scheme (BATS) and coupled to a version of the NCAR Community Climate Model (CCM1) which includes a mixed-layer ocean. Employed in an off-line mode as a diagnostic tool, the scheme predicts a slightly darker and slightly rougher continental surface than when BATS' prescribed vegetation classes are used. The impact of tropical deforestation on regional climates, and hence on diagnosed vegetation, differs between South America and S.E. Asia. In the Amazon, the climatic effects of removing all the tropical forest are so marked that in only one of the 18 deforested grid elements could the new climate sustain tropical forest vegetation whereas in S.E. Asia in seven of the 9 deforested elements the climate could continue to support tropical forest. Following these off-line tests, the simple vegetation scheme has been coupled to the GCM as an interactive (or two-way) submodel for a test integration lasting 5.6 yr. It is found to be a stable component of the global climate system, producing only ~ 3% (absolute) interannual changes in the predicted percentages of continental vegetation, together with globally-averaged continental temperature increases of up to + 1.5 °C and evaporation increases of 0 to 5 W m–2 and no discernible trends over the 67 months of integration. On the other hand, this interactive land biosphere causes regional-scale temperature differences of ± 10 °C and commensurate disturbances in other climatic parameters. Tuning, similar to the q-flux schemes used for ocean models, could improve the simulation of the present-day surface climate but, in the longer term, it will be important to focus on predicting the characteristics of the continental surface rather than simple vegetation classes. The coupling scheme will also have to allow for vegetation responses occurring over longer timescales so that the coupled system is buffered from sudden shocks. 相似文献
4.
Summary The boundary-layer wind field during weak synoptic conditions is largely controlled by the nature of the landscape. Mesoscale
(sub-synoptic) circulations result from horizontal gradients of sensible heat flux due to variation in local topography, variation
in surface-cover, and discontinuities such as land-sea contrasts. Such flows are usually referred to as thermally-driven circulations,
and are diurnal in nature and often predictable. In this paper we use a state-of-the-art non-hydrostatic computer model to
shed light on the physical mechanisms that drive a persistent easterly wind that develops in the afternoon in the Mackenzie
Basin, New Zealand. The easterly – Canterbury Plains Breeze (CPB) – is observed early in the afternoon and is often intense,
with mean wind speeds reaching up to 12 m s−1. Although computer modelling in mountainous terrain is extremely challenging, the model is able to simulate this circulation
satisfactorily. To further investigate the mechanisms that generate the Canterbury Plains Breeze, two additional idealized
model experiments are performed. With each experiment, the effects of the synoptic scale wind and the ocean around the South
Island, New Zealand were successively removed. The results show that contrary to previous suggestions, the Canterbury Plains
Breeze is not an intrusion of the coastal sea breeze or the Canterbury north-easterly, but can be generated by heating of
the basin alone. This conclusion highlights the importance of mountain basins and saddles in controlling near-surface wind
regimes in complex terrain. 相似文献
5.
T. Vrhovec 《Meteorology and Atmospheric Physics》1991,46(1-2):91-99
Summary A formation of a cold air lake in a basin is studied with a mesometeorological model.A dynamic Boussinesq hydrostatic mesoscale numerical model is developed in a staggered orthogonal grid with a horizontal resolution of 1 km and with a varying vertical grid. The topography is presented in a block shape so that computation levels are horizontal.The mesometeorological model is tested in three idealized topography cases (a valley, a single mountain, a basin) and test results are discussed.In an alpine basin surrounded by mountains and plateaus the air is supposed to be stagnant at the beginning of the night. Due to differences in radiation cooling an inversion layer is formed in the basin and local wind circulation is studied by model simulations.With 14 Figures 相似文献
6.
H. E. Eisma J. M. Tomas M. J. B. M. Pourquie G. E. Elsinga H. J. J. Jonker J. Westerweel 《Boundary-Layer Meteorology》2018,169(2):185-208
Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL. 相似文献
7.
Jonathan S. Price 《Boundary-Layer Meteorology》1991,57(4):391-406
Frequent fog severely restricts evaporation from blanket bogs in Newfoundland because it more than halves the radiant energy input, and it eliminates the vapor pressure deficit, resulting in evaporation at the equilibrium rate (average = 0.99 during fog). During these periods, there is no surface resistance to evaporation because the bog has been wetted by fog drip, and although the latent heat flux dominates over sensible heat (average = 0.8), both are small. In contrast, the surface dries during clear periods, increasing the surface resistance to evaporation so that sensible heat becomes more important ( = 1.05). When the mosses are dry, evaporation is below the equilibrium rate ( = 0.87), although the higher available energy ensures that actual evaporation is higher. During clear periods, daily evaporation averaged 2.5 mm, compared to 1.1 and 0.7 mm for fog and rain, respectively. The suppressed evaporation at this site is important in maintaining appropriate hydrological conditions for blanket bog development. 相似文献
8.
W. König 《Meteorology and Atmospheric Physics》1993,52(3-4):153-167
Summary An eddy effect of tropical deep convection on the large-scale momentum, resp vorticity budget is investigated. The process is specified by a simple parameterization approach which is based on a concept of rotating clouds exerting a momentum on the large-scale flow. The cloud rotation is associated with the thermal properties of a cloud ensemble by the principle of conservation of potential vorticity. A decomposition of cloud classes is applied in consistency with the thermodynamical parameterization scheme of Arakawa and Schubert (1974).The parameterization is tested with observations of GATE74, Phase III. The data are processed on a B/C-scale grid (55km) in a region within 9N and 16N, and between 21W and 27W, and with a vertical resolution of 41 levels. The parameterization results correspond to the observed patterns, especially in situations with strong large-scale wind shear. The findings suggest that certain large-scalle flow regimes provoke convective scale momentum generation rather than redistributing large-scale momentum by convective circulations.With 10 Figures 相似文献
9.
Spatial structure of a jet flow at a river mouth 总被引:1,自引:0,他引:1
The present work concentrates on the latest data measured in the Jordan river flow in lake Kinneret. Spectral characteristics of fluctuating velocity components have been obtained and processed. The three-dimensional structure of turbulence along the flow has been described. The main features of the jet flow turbulence in the river mouth are: a) The supply of turbulent energy changes due to different mechanisms along the flow. b) The structure of turbulence formed in the river decays rapidly along the flow, and c) In the sand area and beyond it, a significant generation of turbulent energy occurs. Quantitative estimations of the above effects were carried out. 相似文献
10.
11.
Internal waves incident on a sheared ocean pycnocline are studied using analytic and numerical methods. Linear analysis of the unstable modes of a sheared ocean pycnocline is used to demonstrate interactions between internal waves and shear instabilities. A new analytic solution for an asymmetric shear layer over a stratified layer is presented, illustrating modes which couple to internal waves, in addition to the well-known Holmboe modes. The robustness of these solutions is demonstrated using numerical methods for realistic shear profiles. Fully nonlinear numerical simulations illustrate the growth of these modes and demonstrate the excitation of shear instabilities by incident internal waves. The results may have implications for internal wave interactions with the ocean pycnocline and the local generation of internal solitary waves. 相似文献
12.
Howard J. Freeland 《大气与海洋》2013,51(3):288-302
Abstract The effect of an abrupt headland on a barotropic oceanic boundary current with variable bottom topography is investigated. The objective is to explore with a very simple model some of the observed features of flow past Brooks Peninsula, an obstacle to boundary currents on the west coast of Vancouver Island. It is shown that the seasonal variation in the background current field causes a large change in the response to the headland. The difference is both quantitative and qualitative and results from the ability of southward alongshore flows to support topographic Rossby lee waves. As a result of the presence of the lee waves a strong offshore flow occurs just downstream of the Peninsula and this ejects water from the continental shelf into deep water producing features reminiscent of the so‐called “squirts” and “jets”. 相似文献
13.
Flow over a two-dimensional obstacle and dispersion of a heavier-than-air gas near the obstacle were studied. Two species, one representing air and the other representing the heavier-than-air gas were treated. Equations for mass and momentum were cast in mass-averaged form, with turbulent Reynolds stresses and mass fluxes modeled using eddy-viscosity and diffusivity hypotheses. A two-equation k- turbulence model was used to determine the effective turbulent viscosity. Streamline curvature and buoyancy corrections were added to the basic turbulence formulation. The model equations were solved using finite difference techniques. An alternating-direction-implicit (ADI) technique was used to solve the parabolic transport equations and a direct matrix solver was used to solve the elliptic pressure equation.Mesh sensitivities were investigated to determine the optimum mesh requirements for the final calculations. It was concluded that at least 10 grid spaces were required across the obstacle width and 15 across the obstacle height to obtain valid solutions. A non-uniform mesh was used to concentrate the grid points at the top of the obstacle.Experimental measurements were made with air flow over a 7.6 by 7.6 cm obstacle in a boundary-layer wind tunnel. Smoke visualization revealed a low-frequency oscillation of the bubble downstream of the obstacle. Hot-wire anemometer data are presented for the mean velocity and turbulent kinetic energy at the mid-plane of the obstacle and the mid-plane of the downstream recirculation bubble. A single hot-wire probe was found to be suitable for determining mean streamwise velocities with an accuracy of 11 %. The downstream recirculation bubble was unsteady and had a length range from 3 to 8 obstacle lengths.The experimental results for flow over the obstacle were compared with numerical calculations to validate the numerical solution procedure. A sensitivity study on the effect of curvature correction and variation of turbulence model constants on the numerical solution was conducted. Calculations that included the curvature correction model gave a downstream recirculation bubble length of 5.9 obstacle lengths while excluding the correction reduced this length to 4.4.In the second part of the study, numerical calculations were performed for the dispersion of a heavier-than-air gas in the vicinity of the two-dimensional obstacle. Characteristics of an adiabatic boundary layer were used in these calculations. The densities of the contaminant gases were 0, 25 and 50% greater than the air density. Calculations were performed with the contaminant injection source upstream and downstream of the obstacle.Use of the pressure gradient model reduced the size of the dense gas cloud by as much as 12%. The curvature correction model also affected the cloud expanse by reducing the effective turbulent viscosity in the downstream recirculation bubble. The location of the injection source had the largest impact on the cloud size. The area of the cloud within the 5 % contour was three times larger for downstream injection than for upstream injection. 相似文献
14.
Measurements made with instruments aboard a remotely-piloted multicopter flying across the Weser river in Germany provide information on the thermal structure of the boundary layer over the river and adjacent land, in this case in summer for late afternoon, the evening transition and early night on a clear calm day. The river has a characteristic width of 100 m. The stratification over the land and river are of opposite signs at the lower levels, except during part of the evening transition. The multicopter allows to qualitatively estimate the evolution of the thermal contrast between both surfaces, showing that the river banks experience very significant daytime cooling and nocturnal warming due to river-bank circulations, with the change of sign taking place well before sunset. 相似文献
15.
连续10a气象报表预审无错情的体会 总被引:1,自引:1,他引:0
该文以连续10 a审核无错情经验为基础,从原始记录的审查、自记记录的检查、备注栏和记录要检的填写、微机审核报表等几方面,分析了基层台站如何做好气象报表的预审工作。 相似文献
16.
通过分析2004年8月25日发生在浙江省宁波市的一次台风前部龙卷发生发展的环境特征, 发现该龙卷发生在台风前部风切变区里, 尽管当时涡度、散度等物理量对于深对流发展不是非常有利, 但下湿中干、强的垂直风切变及地形条件等还是有利于局地弱龙卷的产生; 在宁波新一代天气雷达产品上表现为强的钩状回波, 速度场上有相邻的正负速度中心及强的组合切变值等。通过多个反射率产品、剖面产品等综合分析了该风暴的三维结构, 初步了解此类弱龙卷的发生机理, 为以后的预报提供一些经验。 相似文献
17.
18.
利用常规地面气象观测资料以及NCEP/NCAR再分析资料,对2012年7月21日发生在北京地区的一次特大暴雨过程的干侵入效应进行了定量诊断分析。结果表明:干侵入效应在该过程发生前表现明显,干侵入指数对暴雨落区和降水增幅具有一定的指示意义;干侵入指数的偶极分布特征反映了温带气旋发生发展过程中冷暖气流之间的相互作用,强降水区位于干侵入指数等值线密集带靠近其负值区一侧,当干侵入指数正负中心的零线越靠近,降水越趋于增强。在整个暴雨过程中,中高层干侵入指数正值区不断向中层扩展,当垂直方向上干侵入指数由一致的负值变为正负值相间、特别是中层干侵入指数由负值变为正值时,也即形成中层干冷、低层暖湿的不稳定层结时,易激发对流性降水,使得降水增强、暴雨增幅。对该过程使用等熵坐标系表征的干侵入指数同样能全面刻画干侵入过程,中低层等熵面上干侵入指数较高层等熵面上干侵入指数更具指示意义。另外,整层平均干侵入指数对此次特大暴雨过程有提前6~12 h的响应时间。
相似文献19.