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This paper investigates the relationship between mei-yu and North Atlantic sea surface temperature anomalies (SSTA). Results show that they are significantly associated with each other on the decadal timescale. Both mei-yu precipitation and mei-yu duration are characterized by significant decadal variability. Their decadal components are closely correlated with a triple mode of North Atlantic SSTA in the preceding winter. Regression analysis demonstrates that the wintertime North Atlantic SSTA may impose a delayed impact on East Asia Summer Monsoon (EASM) circulation and mei-yu on the decadal timescale. The persistency of SSTA plays an important role during this course. The triple SSTA mode can persist from winter until late spring. It is suggested that the springtime SSTA may excite a stationary wave-train propagating from west Eurasia to East Asia and exert an impact on mei-yu.  相似文献
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Four-dimensional variational data assimilation (4DVar) is one of the most promising methods to provide optimal analysis for numerical weather prediction (NWP). Five national NWP centers in the world have successfully applied 4DVar methods in their global NWPs, thanks to the increment method and adjoint technique. However, the application of 4DVar is still limited by the computer resources available at many NWP centers and research institutes. It is essential, therefore, to further reduce the computational cost of 4DVar. Here, an economical approach to implement 4DVar is proposed, using the technique of dimension-reduced projection (DRP), which is called ``DRP-4DVar." The proposed approach is based on dimension reduction using an ensemble of historical samples to define a subspace. It directly obtains an optimal solution in the reduced space by fitting observations with historical time series generated by the model to form consistent forecast states, and therefore does not require implementation of the adjoint of tangent linear approximation. To evaluate the performance of the DRP-4DVar on assimilating different types of mesoscale observations, some observing system simulation experiments are conducted using MM5 and a comparison is made between adjoint-based 4DVar and DRP-4DVar using a 6-hour assimilation window.  相似文献
4.
各种非绝热物理过程在中尺度模式中的作用   总被引:6,自引:6,他引:39  
张大林 《大气科学》1998,22(4):548-561
随着中尺度模式水平分辨率的不断提高,考虑加入尽可能合理的各种非绝热物理过程极为重要。如积云参数化需包括湿下沉气流、中上层的云卷出和非降水性浅对流。显式云物理方案需同时加入含有水相和冰相的预报方程,以计入水负荷、凝结蒸发、冻结融化和凝华升华的影响。本文首先从实测角度介绍上述物理过程在产生中β尺度环流结构的作用,然后通过一些敏感性数值试验来阐述它们如何帮助成功模拟不同的中尺度对流系统。这些物理过程的相对作用取决于网格距大小或可分辨尺度垂直运动的强度。当网格距在20~50 km之间,本文特别强调积云参数化和显式云物理方案的同时使用。最后对各种非绝热物理过程的耦合以及中尺度模式的局限性作了适当讨论。  相似文献
5.
A Review of Recent Advances in Research on Asian Monsoon in China   总被引:6,自引:0,他引:6       下载免费PDF全文
This paper reviews briefly advances in recent research on monsoon by Chinese scholars, including primarily: (1) the establishment of various monsoon indices. In particular, the standardized dynamic seasonal variability index of the monsoon can delimit the geographical distribution of global monsoon systems and determine quantitatively the date of abrupt change in circulation. (2) The provision of three driving forces for the generation of monsoon. (3) The revelation of the heating-pump action of the Tibetan Plateau, which strengthens southerlies in the southern and southeastern periphery of the Plateau and results in a strong rainfall center from the northern Bay of Bengal (BOB) to the Plateau itself. (4) Clarification of the initial onset of the Asian Summer Monsoon (ASM) in the BOB east of 90°E, Indochina Peninsula (ICP) and the South China Sea, of which the rapid northward progression of tropical convection in the Sumatra and the rapid westward movement of the South Asia High to the Indochina Peninsula are the earliest signs. (5) The provision of an integrated mechanism for the onset of the East Asian Summer Monsoon (EASM), which emphasizes the integrated impact of sensible heat over Indian Peninsula, the warm advection of the Tibetan Plateau and the sensible heat and latent heat over the Indochina Peninsula on the one hand, and the seasonal phase-lock effect of the northward propagation of low frequency oscillation on the other. (6) The revelation of the "planetary-scale moisture transport large-value band" from the Southern Hemisphere through to the Asian monsoon region and into the North Pacific, which is converged by several large-scale moisture transport belts in the Asian-Australian monsoon regions and whose variation influences directly the temporal and spatial distribution of summer rainfall in China. (7) Presenting the features of the seasonal advance of the EASM, the propagation of intraseasonal oscillation, and their relationship with rainfall in Ch  相似文献
6.
The study investigated the effects of global direct radiative forcing due to carbonaceous aerosol on the climate in East Asia, using the CAM3 developed by NCAR. The results showed that carbonaceous aerosols cause negative forcing at the top of the atmosphere (TOA) and surface under clear sky conditions, but positive forcing at the TOA and weak negative forcing at the surface under all sky conditions. Hence, clouds could change the sign of the direct radiative forcing at the TOA, and weaken the forcing at the surface. Carbonaceous aerosols have distinct effects on the summer climate in East Asia. In southern China and India, it caused the surface temperature to increase, but the total cloud cover and precipitation to decrease. However, the opposite effects are caused for most of northern China and Bangladesh. Given the changes in temperature, vertical velocity, and surface streamflow caused by carbonaceous aerosol in this simulation, carbonaceous aerosol could also induce summer precipitation to decrease in southern China but increase in northern China.  相似文献
7.
The Impact of the Storm-Induced SST Cooling on Hurricane Intensity   总被引:5,自引:0,他引:5       下载免费PDF全文
The effects of storm-induced sea surface temperature (SST) cooling on hurricane intensity are investigated using a 5-day cloud-resolving simulation of Hurricane Bonnie (1998). Two sensitivity simulations are performed in which the storm-induced cooling is either ignored or shifted close to the modeled storm track. Results show marked sensitivity of the model-simulated storm intensity to the magnitude and relative position with respect to the hurricane track. It is shown that incorporation of the storm-induced cooling, with an average value of 1.3℃, causes a 25-hPa weakening of the hurricane, which is about 20 hPa per 1℃ change in SST. Shifting the SST cooling close to the storm track generates the weakest storm, accounting for about 47% reduction in the storm intensity. It is found that the storm intensity changes are well correlated with the air-sea temperature difference. The results have important implications for the use of coupled hurricane-ocean models for numerical prediction of tropical cyclones.  相似文献
8.
参数化次表层上卷海温改进ENSO模拟   总被引:4,自引:4,他引:0       下载免费PDF全文
通过参数化次表层上卷海温改进了一个热带太平海洋模式的SSTA模拟。这种参数化方案通过经验方法将海洋上混合层底部海温变化与海表面起伏联系起来,从而可以方便地利用模式模拟的海表起伏描述温跃层的变化情况及其对混合层海温变化的影响。三组数值试验表明通过上述方法显著改善了SST年际变化的模拟,与观测相比,在赤道东太平洋及南美沿岸,距平相关系数由原来的0.7左右提高到0.8以上,均方根误差在赤道东太平洋由原来0.8℃降到0.6℃,在南美沿岸由1.3℃以上降为0.9℃。这表明在赤道东太平洋及南美沿岸,温跃层的变化通过夹卷过程及垂直扩散过程可以显著影响混合层的温度,OGCM对这些过程描述不足是导致SST年际变化模拟偏弱的一个重要原因,通过强调这些过程可以改善模拟效果。同时在热带西太平洋的改进也是显著的。  相似文献
9.
The Madden-Julian oscillation (MJO) is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The effect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher- frequency eddies are delineated.  相似文献
10.
Background error covariance plays an important role in any variational data assimilation system, because it determines how information from observations is spread in model space and between different model variables. In this paper, the use of orthogonal wavelets in representation of background error covariance over a limited area is studied. Based on the WRF model and its 3D-VAR system, an algorithm using orthogonal wavelets to model background error covariance is developed. Because each wavelet function contains information on both position and scale, using a diagonal correlation matrix in wavelet space gives the possibility to represent some anisotropic and inhomogeneous characteristics of background error covariance. The experiments show that local correlation functions are better modeled than spectral methods. The formulation of wavelet background error covariance is tested with the typhoon Kaemi (2006). The results of experiments indicate that the subsequent forecasts of typhoon Kaemi’s track and intensity are significantly improved by the new method.  相似文献
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