20世纪后30年中国西北西部降水年代际变化机理分析

在全球气候变暖背景下1980年代中期中国西北西部气候发生了年代际突变,降水明显增加且主要是极端降水的贡献. 本文用欧洲中期预报中心再分析资料ERA-40计算了瞬变涡动、环流、水汽输送及其散度等的年代际变化并进行了分解分析. 结果表明, 1987年后大气环流年代际变化形成"东高西低"环流背景场,西风带向该区的水汽输送增加, 水汽辐合增强,其中主要是定常波水汽输送的贡献, 其结果是造成该区及周边空气水汽含量上升, 有利于降水或极端降水的增加. 夏季该区瞬变涡动增强且造成水汽辐合, 增加了降水天气出现的频率; 冬季瞬变涡动减弱, 天气过程减少, 说明降雪增加的原因应该是空气水汽含量增加, 导致降雪强度增加. 因此该区降水的增加即有中高纬度大气变暖变湿的贡献,也有瞬变涡动的动力作用. 未来西北西部降水的趋势主要取决于瞬变涡动与大气水汽含量变化的竞争．     

混沌系统中可预报性的研究

针对简化的气候模式、Rossler吸引子和超混沌系统,进一步阐明了不确定原理,在数值求解时由于计算机固有精度而引起的舍入误差,造成对解的不确定性,存在最优步长和最大有效计算时间.运用自忆性原理,导出了各混沌系统的自忆性方程,取最优步长时,其预报性能有明显的改善 关键词： 自忆性 不确定原理 最优步长     

二维小波变换及在青藏高原地形重构中的应用

计算和检验了4种不同小波基底和不同阶小波基底对单脉冲和尖峰两个信号的分解及重构的性质，结果表明双正交小波的精度最高，Coifman小波的精度最低．通过设阈值对小波和快速Fourier变换进行截断后再重构，说明小波具有重构精度高、所用非零系数少以及误差被局域化等优点，这就削弱了“Gibbs现象”，限制了它的影内范围．在此基础上，用二维快速Fourier和二维离散小波变换对实际青藏高原及其附近地形做了分解、重构及截断后重构等，说明二维小波变换能以较之FFT更少的非零系数和误差局域化等重构大地形，表明了二维小波变换在未来大气环流数值模式中的应用潜力。     

近60年新疆降水趋势与波动机制分析

用小波多尺度分析和回归等方法研究了近60年新疆降水记录的波动与趋势特征, 结果发现21世纪头十年降水亦然保持增加趋势, 但春、夏降水已呈弱的非线性下降趋势, 秋、冬季节降水增加趋势明显. 这可能与气候暖化的季节不均匀性有关. 年降水量的振荡与北大西洋涛动(NAO)关系密切, 二者约80%的方差都集中在年际尺度和多年代尺度成分两个带上, 年际变化之间呈显著负相关关系. NAO的多年代尺度成分位相超前降水10年左右, 而大西洋海温的多年代尺度涛动与之相差5年, 后者与降水的关联更为密切. "极差"分析表明, 对应于NAO的极端正或负位相事件, 下游的降水和水汽环境明显不同. 在NAO极端负位相年, 中亚和新疆降水都明显增加, 来自欧洲南部的水汽输送和中纬度瞬变涡动水汽输送均偏强, 中亚大气可降水量上升, 冷空气路径偏西, 瞬变天气系统易得到西南水汽输送带的配合, 水汽辐合增强, 有利于新疆和中亚的降水. 反之, 西风带水汽输送偏弱, 天气过程偏少, 降水量偏小, 易发生干旱. 因此, NAO的异常是新疆和中亚降水年际波动的主要原因之一. 关键词： 新疆降水 北大西洋涛动 水汽输送 瞬变涡动     

Warm–dry collocation of recent drought in southwestern China tied to moisture transport and climate warming

This study aims to investigate the recent drought in southwestern China and its association with environmental changes in moisture transport(MT) and atmospheric circulation. Climatic Research Unit grid data, in situ observations in China,and ERA-interim reanalysis are used to study the characteristics of the drought and the associated mechanism. Recent precipitation trends show a pattern of "Northern wetting and Southern drying", similar to the anti-phase of the climate pattern prevailing during 1980–2000 in mainland China; southwestern China incurred a severe drought during 2009–2013.Wavelet analysis reveals that the drought coincides with a warm–dry phase of temperature and precipitation on a period of about 20 years and beyond 100 years, where contributions account for 43% and 57% of the deficiency of the precipitation,averaged for 2003–2012, respectively. A further investigation reveals that the drought results chiefly from the decline of the southwestern monsoon MT toward southwestern China, in addition to mid-latitude circulation changes, which leads to more blockings near the Ural Mountains and the Sea of Okhotsk in the rainy season and negative anomalies around Lake Baikal and northeast China in the dry season. These anomalies are likely to be correlated with global sea surface temperature changes and need to be studied further.     

克立格法在时间域上做外延预报的可行性

利用Lorenz系统混沌解序列和全国160个站的气象资料研究了线性地质统计学在时间域上做外延预报的可行性并与自回归过程AR模型进行比较.结果表明：时间域上普通克立格法（Ordinary Kriging,简称OK）的外延计算虽与矩估计法建模的AR模型法相似,但由于前者使用了无偏性条件使其在平稳或非平稳序列上都具有更好的外延能力;泛克立格法（Universal Kriging,简称UK）由于引入内插中漂移概念反而限制其外延计算,使在平稳序列上的外延解也呈现出类似漂移多项式的变化.同时还提出克立格法有关外延计算的3个试验方案.对实际气象资料的计算表明试验方案存在对OK法外延精度改善的可能性,但这还有赖于外延序列的性质.     

A decomposition study of moisture transport divergence for inter-decadal change in East Asia summer rainfall during 1958-2001

In this paper,we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958-1979.The aim is to explore the mechanism of summer rainfall change in the region after abrupt changes.The relevant changes are calculated using ERA-40 daily reanalysis datasets.The results show that both stationary and transient eddy moisture transports to the Chinese mainland have declined since the abrupt change in atmospheric general circulation in the late 1970s,leading to more rainfall in South China and less in the North.The anomalous rainfall pattern coincides well with anomalous large-scale moisture divergence in the troposphere,of which stationary-wave or monsoon transport is dominant,in comparison with the contribution of the transient eddies.Furthermore,their divergences are found to be in opposite phases.In addition,meridional divergence is more important than its zonal counterpart,with an opposite phase in East Asia.Abnormal zonal moisture convergences appear in northwestern and northeastern parts of China,and are related to the excess rainfalls in these regions.An increase in transient eddy activity is one of the major mechanisms for excess rainfall in northern Xinjiang.Consequently,the anomalous rainfall pattern in East Asia results from a decline of the East Asian monsoon after the abrupt change,while the rainfall increase in northwestern China involves anomalies of both stationary waves and transient eddies on boreal westerly over the midand high latitudes.     

Numerical solution of Helmholtz equation of barotropic atmosphere using wavelets

The numerical solution of the Helmholtz equation for barotropic atmosphere is estimated by use of the waveletGalerkin method. The solution involves the decomposition of a circulant matrix consisting up of 2-term connection coefficients of wavelet scaling functions. Three matrix decompositions, i.e. fast Fourier transformation (FFT), Jacobian and QR decomposition methods, are tested numerically. The Jacobian method has the smallest matrix-reconstruction error with the best orthogonality while the FFT method causes the biggest errors. Numerical result reveals that the numerical solution of the equation is very sensitive to the decomposition methods, and the QR and Jacobian decomposition methods, whose errors are of the order of 10-3, much smaller than that with the FFT method, are more suitable to the numerical solution of the equation. With the two methods the solutions are also proved to have much higher accuracy than the iteration solution with the finite difference approximation. In addition, the wavelet numerical method is very useful for the solution of a climate model in low resolution. The solution accuracy of the equation may significantly increase with the order of Daubechies wavelet.     

外强迫作用下正压大气非线性特征数值模拟

在两种边界条件下用正压大气非线性位势涡度方程模拟了强迫、耗散和非线性共同作用下大气运动的若干特征.在南北两端为齐次边界条件时，单纯的热力强迫下只出现了波状定常解；在只有经向加热和波状地形共同强迫下出现了在两种基本流型之间振荡的准周期解，十分类似于旋转地球上大气运动的纬向和经向流型之间的准周期循环，前者对应于强的西风环流，后者对应于弱西风流型.在周期边界条件下，弱的下垫面加热导致定常解，较强的加热强迫可以得到周期解.尽管下垫面的波状地形也能强迫一个类似的准周期振荡，但弱西风流型是对称的，不像大气中的弱西风流型.因此，在周期边界条件下用低阶截谱模式求出的解析或数值解，不一定适合描述中高纬度大气的非线性动力特征. 关键词： 正压大气 热力强迫 地形起伏 周期解 准周期振荡     

小波抽样与气象要素场插值

首先用6种插值方法对一个带限与非带限一维信号分别进行等距插值计算检验，6种方法是基于Shannon抽样定理的插值法、两种基于小波抽样的插值法、三次样条插值法、三次卷积插值法和快速Fourier变换插值法．结果显示小波和三次样条插值法最好，其中小波插值误差的局域性最好．最差的是快速Fourier变换法．其次，剔除快速：Fourier变换插值法，将其它5种插值公式推广到二维的情形，对大气层中层500 hPa高度场、海平面气压场和降水量场也进行了插值计算，误差分析表明对于降水量场三次样条小波插值最好，对高度场和海平面气压场三次样条插值法仍然是最好的．在此基础上，提出了既保证降水量场插值精度又能避免插出“负降水”的插值处理方法并进行了计算检验，最后指出了小波插值法有待改进的问题。