共查询到20条相似文献，搜索用时 46 毫秒
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《气象科技》1974,(4):25-31
一、概述本文对上海地区1964、1966、1968—1972年4—7月(梅雨季结束前)出现的22次大—暴雨(日雨量≥30毫米)过程进行了一些分析研究,并尝试用逐次回归统计方法,制作未来5—6天内大—暴雨的预报。毛主席指出:“离开具体的分析,就不能认识任何矛盾的特性。”经过调查,上海地区4—7月出现的大—暴雨过程主要是冷暖空气在长江中下游一带交绥,即冷暖空气这对矛盾斗争的结果。在本文中,我们就着重分析冷暖空气的活动,同时也注意本站要素的变化。通过对大—暴雨与本站要素之间、大—暴雨与天气系统之间以及反映这些天气系统的某些特征区(站)的要素变化,寻求其相互影响和相互制约的关系,最后挑选出一些特征区、特征站的 相似文献
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本文以下游低槽的加深,可能与上游低槽的发展有一定关系作为思路,统计了南欧经向环流与广西春季大范围大雨暴雨、大风天气的关系。经过用能反映下游暴雨环流特点的要素进行了两次消空,使广西大范围大雨暴雨大风天气的中期趋势和短期预报准确率有一提高。 我们将1956—1983年1—3月17个大范围大、暴雨过程进行了分析总结,得出了一些初步结果。 相似文献
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大—暴雨是三度空间各种要素场相互作用的结果,我们从分析每日08~s 500mb 温压场关键区指数着手,配合单站指标,运用概率统计方法,建立预报单站未来24小时大—暴雨的多元回归方程。由于大—暴雨出现前24小时气象要素呈现“量变到质变”的明显转折,把起报日建立在这一关键时刻,在汛期短期预报中,获得了较好的效果。一、预报思路:环流背景,不稳定能量贮备及源源不断地水汽供给是产生大—暴雨的重要条件。我 相似文献
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为了提高县站暴雨预报水平,我们对暴雨的预报因子进行了统计分析,采用了高空与地面、指标站与本站要素相结合,天气形势背景与本地特征变化相结合的办法。 预报实践表明,我区暴雨大多产生在高空槽前,河套有降温,地面为东高西低形势,本站湿度大的条件下。本文用1958—1977年7—8月的资料,对上述 相似文献
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提出一个综合评估多元判据的大—暴雨客观相似预报方案。方案应用细网格资料和数值天气预报产品，构造预报时刻前后的不同尺度天气系统和多层次各种要素的动态变化的客观相似判据。通过定义非线性的相似指数综合评估历史样本在多元判据下的相似程度，从而找到相似样本。应用相似样本对应的天气过程预报大—暴雨的发生与否，并通过相似指数的权重综合，进行大—暴雨的定量预测。预报试验表明该方案对大—暴雨发生具有一定的预报能力。 相似文献
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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
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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
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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
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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
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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
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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
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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
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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
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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