Rossby wave interactions with Mediterranean and subtropical latitudes

The downstream influence of a Rossby wave on weather conditions in the Mediterranean and North Africa is studied. The objective is to gain a better understanding of the atmospheric processes in these regions and to improve their quantification. The emphasis is placed on high-impact weather events to improve numerical forecasts and warnings about these hazardous weather phenomena. For this purpose, 4 days from 5 to 8 February 1997 are used to investigate both a Mediterranean low and a subtropical African convective situation. Sensitivity studies, using a potential vorticity inversion tool associated with the French atmospheric model ARPEGE, are presented. The Mediterranean surface low under study is shown to be associated with the mid-latitude upper level potential vorticity anomaly, itself associated with a Rossby wave. A subtropical convective cell is shown to be related to upward vertical motions associated with a cut-off low; this cut-off low coming from a mid-latitude Rossby wave.     

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate. Correspondence to: CJC Reason     

Atlantic Meridional Overturning Circulation response to idealized external forcing

The response of the Atlantic Meridional Overturning Circulation (AMOC) to idealized external (solar) forcing is studied in terms of the internal (unforced) AMOC modes with the Kiel Climate Model (KCM), a coupled atmosphere-ocean-sea ice general circulation model. The statistical investigation of KCM’s internal AMOC variability obtained from a multi-millennial control run yields three distinct modes: a multi-decadal mode with a period of about 60?years, a quasi-centennial mode with a period of about 100?years and a multi-centennial mode with a period of about 300–400?years. Most variance is explained by the multi-centennial mode, and the least by the quasi-centennial mode. The solar constant varies sinusoidally with two different periods (100 and 60?years) in forced runs with KCM. The AMOC response to the external forcing is rather complex and nonlinear. It involves strong changes in the frequency structure of the variability. While the control run depicts multi-timescale behavior, the AMOC variability in the experiment with 100?year forcing period is channeled into a relatively narrow band centered near the forcing period. It is the quasi-centennial AMOC mode with a period of just under 100?years which is excited, although it is heavily damped in the control run. Thus, the quasi-centennial mode retains its period which does not correspond exactly to the forcing period. Surprisingly, the quasi-centennial mode is also most strongly excited when the forcing period is set to 60?years, the period of the multi-decadal mode which is rather prominent in the control run. It is largely the spatial structure of the forcing rather than its period that determines which of the three internal AMOC modes is excited. The results suggest that we need to understand the full modal structure of the internal AMOC variability in order to understand the circulation’s response to external forcing. This could be a challenge for climate models: we cannot necessarily expect that the response to external forcing is realistically captured by a model, even if only strongly damped modes are not well represented that do not account for much variance under present-day conditions.     

大气环流指数低频振荡机制的流体物理实验研究

使用旋转的环形水槽,用热力驱动的斜压流体作模拟实验,研究地形对斜压波流动状态的影响。使用对称分布的双峰正弦波地形,测量流体中层的流场,计算流函数并作谐波分析,对比无地形及有地形强迫的大尺度流动状态。实验表明,大尺度地形的作用是,对斜压波在时间和空间进行调制,使流动在时间和空间变得不均匀,使斜压波产生127转台旋转周期的振荡。量纲分析表明,这一周期相当于地球自转的26d。这使我们认为,大气环流指数的低频振荡的原因,可能是由于近似对称分布的亚洲及美洲山系大尺度地形的作用,造成大气动量随时间的变化,即对大气流动在时间和空间进行调制造成的。     

A three-dimensional variational single-Doppler velocity retrieval method with simple conservation equation constraint

Summary In this paper, a new three-dimensional variational analysis scheme capable of retrieving three-dimensional winds from single Doppler observations of convective storms is developed. The method incorporates, in a single cost function, Doppler radar observations, a background field, smoothness and mass continuity constraints, and the residual of reflectivity or radial velocity conservation. By minimizing this cost function, an analysis with the desired fit to these constraints is obtained in a single procedure. In tests with both simulated and real thunderstorm cases, detailed structures of the storms are well retrieved in comparison with reference analysis. Unlike most kinematic retrieval methods, our scheme is capable of directly dealing with data voids. When an analysis background is available, say from a proximity sounding, a wind profiler, or a numerical model forecast, the method naturally blends Doppler radar observations with it. Thus, a smooth transition is obtained between data-rich and data-void areas. These features, among others, are important if the analysis is to be used to initialize storm-scale numerical models or for diagnostic studies of storm structures.     

北太平洋涛动区500 hPa高度场季节变化特征及其对中国东北区降水的影响

利用国家气候中心1960~2000年500 hPa高度场10°×5°经纬度月平均资料，采用EOF、SVD方法，分析了北太平洋涛动区（25°~70°N，140°E~150°W）500 hPa高度场季节变化特征和中国东北区80个测站降水场相关。结果表明:（1）北太平洋涛动区500 hPa高度场冬季EOF第1载荷向量场呈由北向南的“-，+”波列分布，这种北南分布相反型，占总体方差贡献的40%，可以表现为北低南高，类似地面气压场涛动 (NPO) 的正位相阶段，反映了40年北太平洋中高纬度上空以东亚大槽为定常波的大气环流基本模态，亦是NPO呈正位相阶段的主要成因，反之，类似地面气压场涛动（NPO）的负位相阶段，第2载荷向量场呈整体“+”值分布，占方差贡献28%，该模态则表现为东亚大槽被长波脊替代的与气候基本模态呈相反分布的异常环流型；夏季第1载荷向量场基本模态则为全区的正值分布，占总体方差贡献的30%，第3载荷向量呈北“+”南“-”的分布，占方差贡献的13%，表明夏季500 hPa高度场NPO不是主要模态；春秋两季均呈现出较为明显的NPO模态；（2）北太平洋涛动区500 hPa高度冬季平均场与东北区夏季降水场呈由北向南的“+，-”相关波列，存在显著的相关性（α＞0.01）, 第1对SVD奇异向量占总方差贡献的49%，当NPO区前冬500 hPa高度场呈负位相阶段时，东北区夏季降水偏多，反之，东北区夏季干旱少雨，其它季节亦有类似隔季相关关系。     

1980-2010年丽水市土壤温度变化特征

利用1980-2010年丽水市国家气象观测站0-20 cm 的5层土壤温度逐月观测资料,采用气候倾向率、Mann-Kendall检验和气候相关性等气候分析统计方法,分析了丽水市土壤温度的时间变化趋势、气候突变及对气候变化的响应。结果表明：丽水市平均土壤温度呈极显著的增温趋势,升温率为0.18 ℃/10 a,其中冬季增温最明显,月变化特征呈单峰分布。土壤温度随深度的增加变化幅度减小,且在时间变化上有一定的滞后性。Mann-Kendall检验表明,丽水市土壤温度处于稳定增长的趋势,其中2004-2009年各层土壤温度增温最显著,通过0.01的显著性检验。丽水市各层土壤温度的突变点,随着深度的增加有所延后,平均土壤温度在1994-1996年发生突变。丽水市土壤温度对气候变化的响应,表现为与气温呈现显著的正相关,与春夏季降水有明显的负相关。     

UV irradiance on arbitrarily oriented surfaces: variation with atmospheric and ground properties

Summary The dependency of erythemal weighted solar UV irradiance on tilted surfaces with different orientation is investigated with respect to solar zenith angle, variable atmospheric conditions and albedo of the location. For overcast conditions or a cloud in front of the sun, the irradiance on a horizontal surface in general is largest, with the consequence that it is reduced for surfaces with any tilted position. For cloud free conditions the irradiance on a tilted plane, in comparison to that on a horizontal flat surface, is increased for orientations towards the sun, but reduced for other orientations. The increase is strongest for low sun in combination with clear atmosphere and high ground albedo, as is typical for snow covered mountain conditions.     

Corrections of Systematic Errors, Data Homogenisation and Climatic Analysis of the Padova Pressure Series (1725-1999)

A short history of the series with daily observations of barometric pressure in Padova (since 1725) is made, with special reference to the types of barometers used, their locations, the types of corrections (which were only partial in the early period), the calibrations and the comparison with primary instruments. The paper also describes the homogenisation of the series and the procedures used to fill the small gaps. The Padova series was compared with previously well investigated series from the ADVICE and IMPROVE projects and especially with the nearby Milan series in order to check its reliability. Trend analysis shows an increase in pressure (some 1 hPa) during the last a hundred year. This trend is a common feature for Northern Italy. An effect of the increased air pressure is a local lowering of the Northern Mediterranean Sea level by 1 cm. The part of the year mostly affected by this increase is from late spring to August and corresponds to an extension of the hot season, characterised by an earlier start, and longer duration of the Azores Anticyclone. This reduces the penetration of the Atlantic disturbances in the Northern Mediterranean and the precipitation associated with the passage of fronts. This change is associated with an increase in thermoconvective activity with thunderstorms and heavy precipitation. This explains why, in the last decades, the annual total amount of precipitation is slightly decreased and at the same time the frequency of intense rainfall is increased. Moreover, a comparison of the day-to-day pressure variability with the Western Mediterranean Oscillation (WeMO) indicates a significant positive correlation during the late autumn-early winter period. An increase of the WeMO index means a strengthening of the baric dipole from Azores to Northern Italy, that could be explained by a deepening of the cyclonic circulation over northern Italy from November to January.     

北京地区正地闪时间分布及波形特征

为了给雷电防护及机理研究提供必要的基础数据,利用闪电定位资料和电场快慢变化资料,对北京地区正地闪的时间分布规律及波形特征进行了统计分析。结果表明:北京地区正地闪分布随月份而不同,较大频次出现在5—7月,较高比例出现在春、秋季(3—5月和10—11月);而一天内的正、负地闪分布具有反对应关系,正地闪在傍晚(15:00—21:00,北京时)具有较大的频次和较高的比例,且该比例随闪电频次增大而呈减少的趋势;正地闪多为单次回击,多回击正地闪数占正地闪总数的3.89%;正地闪波形上升沿时间分布范围为5～28μs,平均为11.55μs,慢前沿时间分布在2.8～23.6μs之间,平均值为9.41μs,慢前沿幅度与回击峰值比为53%,过零时间为43.97μs,负反冲深度均值为20.75%,辐射场归一化到100 km的回击场强峰值为13.66 V/m;正地闪连续电流持续时间平均值为113 ms,具有连续电流的正地闪比例高达69.2%,其中48.7%具有长连续电流过程。     

Rainfall field estimates for hydrological models

The rainfall input in hydrological models concerns the mean area precipitation (MAP) values at a certain time resolution, which is required to be higher and higher in the rainfall-runoff modelling, but using space sampled data there is a limited MAP accuracy with respect to the involved time scale. Improved estimates of the rainfall field at small time intervals combine more accurate MAP values performed at a larger scale with the point time variability of data. The usual observation time scale for rain events is 1 h, with acceptable MAP errors, and according to the counting-box method applied in the case of the rainfall contour length there is a scale invariant below such a value.This paper suggests that an improved rainfall input may be obtained by transferring the shape of the hourly rain intensities to each hour in order to generate a finer time distribution of the MAP values, this being accepted if it leads to a better fitting of the simulated and observed hydrographs. This assumption is investigated by numerical experiments with a rainfall-runoff model at different catchment areas.     

Development of a risk-hedging CO2-emission policy, part I: Risks of unrestrained emissions

A rational global strategy with respect to greenhouse-gas emissions would seek to minimize total risk, which is the sum of the risk of negative impacts due to climatic change associated with a given level of emissions, and the risks associated with the process of achieving that emission level. Given the existence of reducible uncertainties in estimating these risks, and the possibility that an emission target thought to minimize total risk is later found to be not strict enough, a risk-hedging strategy is a more realistic policy objective. This paper is Part I of a two-part series in which these risks are reviewed and an interim risk-hedging emission level is proposed. Here, the risks associated with unrestrained greenhouse-gas emissions are reviewed. In particular, the carbon-cycle response to continuing CO₂ emissions; the heat trapping of projected greenhouse gas increases in comparison to other anthropogenic and natural heating or cooling perturbations; the climatic response to heating perturbations; and the impacts of projected climatic change on global agriculture, forests, coastal regions, coral reefs, water resources, terrestrial species, stratospheric and tropospheric ozone, and human comfort and welfare are critically examined. It is concluded that unrestrained emissions of greenhouse gases pose real and substantial risks to human societies and to ecosystems, and that these risks are likely to grow substantially if the climate warms beyond that associated with a CO₂ doubling. These risks clearly justify some action to limit emissions. The magnitude of emission restraint that is justified depends not only on the risks reviewed here, but also on the risks associated with measures to limit greenhouse-gas emissions, which are reviewed in Part II.     

地形强迫对大气长波调整的可能影响

地形作为大气的外部强迫,其动力和热力作用对波动结构演变及极端天气出现都有不能忽视的作用。本文通过数值求解考虑地形强迫的β平面正压准地转位势涡度方程,探讨了地形强迫作用对大气长波调整的可能影响,结果表明:同非线性作用和纬向非均匀基流作用一样,无基流情形下具有纬向差异的地形分布影响了大气长波结构的演变,也能强迫出大气长波调整现象。大气长波调整依赖于地形的高度和地形分布,地形越高,长波越容易出现波数的调整;地形波数越大,即地形结构复杂,越不易出现波数变化。大气长波调整还与纬度有关,纬度越高,β越小,地形强迫作用越突出,长波调整容易出现;反之,低纬度以β效应为主的线性波动不易出现波数调整。大气长波调整对波动初始波动的振幅不敏感,但依赖于波动的初始结构。此外,有基流作用时,地形强迫还是诱发定常波的重要因素,且定常波流场结构依赖于地形高度分布,与波动初始结构无关。     

Interdecadal variability over the North Pacific in a multi-century climate simulation

Interdecadal variability in the North Pacific region is investigated in a 500-y control integration of the Hamburg ECHAM+LSG coupled ocean-atmosphere general circulation model. The spectrum is predominantly red, but a significant peak with a period of about 18 y is detected in the spectrum of sea surface temperature (SST). This peak is shown to be associated with an irregular oscillation that involves both the model ocean and atmosphere. The SST, sea-level pressure, and geopotential height at 500 hPa all undergo a primarily standing oscillation with an extensive monopole structure centered near the date line. The surface anticyclone is situated to the northeast of the warm SST anomaly, and there is a small westward tilt with height; temporal changes are approximately in phase. The anomalous surface heat flux accompanying the warm phase of SST is primarily out of the ocean, but is compensated by anomalous warm advection by surface currents, allowing the SST anomaly to persist. Oceanic thermocline anomalies propagate northward in the western Pacific, and lag the atmosphere indicating a disequilibrium with the atmosphere; sub-surface thermal advection appears to play an important role. A comparison is made between the model's 18-y oscillation and oscillatory components identified in an analysis of the GISST observational SST dataset, which have periods of approximately 6 and roughly 30 y.     

On the transient adjustment of a mid-latitude abyssal ocean basin with realistic geometry: the constant depth limit

The early stages in the adjustment of a mid-latitude abyssal basin with realistic geometry are studied using an inverted one and one-half layer model of the Eastern Mediterranean Sea as a natural test basin. The model is forced with a localized sidewall mass source and a compensating distributed mass sink. A flat bottom basin is investigated for comparison with existing theories on abyssal gyral spin-up, and as a precursor to a study with realistic topography. As in existing theories, the early adjustment is dominated by sub-inertial Kelvin and Rossby waves. Obstacles and the varying coastal geometry do not impede the passage of the Kelvin wave, though the circuit time of the main Kelvin wave signal is reduced by an aggregate 6% for the abyssal Eastern Mediterranean basin. The scattering of the Kelvin wave due to small-scale variations in the coastline is also shown not to be significant to the adjustment. The relatively short period of time needed to reach a statistical steady state is attributed to western boundary current formation in response to local Kelvin wave dynamics. Upon cessation of the sidewall forcing, sub-inertial motion controls the spin-down adjustment with basin-scale Rossby waves becoming the most pronounced feature of the flow. Two dynamical issues of particular interest emerge in these simulations: the retardation of Kelvin wave propagation around the abyssal basin and the roles of detrainment and sidewall forcing in the interior vorticity balance. An idealized simulation using an elliptical basin is used to illustrate that the mechanism for Kelvin wave retardation is a geometrically induced dispersion due to large-scale variations in the coastline. A dynamical analysis of the interior circulation shows that detrainment alone does not develop a Sverdrup response. Both the localized sidewall injection and the detrainment are needed to describe the interior dynamics, with both poleward and equatorward flows developing during the adjustment.     

华北地区雪密度不同的两次降雪过程对比分析

利用加密降雪观测资料、地面常规观测、FY-2E卫星观测及ERA5再分析资料对华北地区两次融化比存在显著差异的降雪过程其降雪特征、云内垂直热动力结构、降水粒子垂直分布、地面气温和地表温度等进行了对比分析,揭示了热动力垂直结构和水汽条件对降雪过程的雪密度影响。结果表明:融化比较大降雪过程(简称"0103"过程)整层温度偏低,位于对流层低层的-18～-12℃温度层较为深厚,与最大上升运动中心、水汽饱和区相重合,有利于树枝状雪花的形成进而产生较大融化比,其云中粒子以冰相粒子为主;融化比较小降雪过程(简称"1129"过程)整层温度偏高,前述温度层位于对流层高层,较为浅薄,且位于最大上升运动中心下方,其云层下部存在较多过冷水滴,有利于凇附作用进而产生较小融化比;"0103"过程短波槽较浅,导致最大动力抬升层次低,-18～-12℃温度层位于暖锋锋区附近,锋前暖平流有利于深厚温度层的建立和维持,水汽主要来自低层偏东气流输送,导致其水汽含量偏小;"1129"过程主要由高空槽前暖湿气团沿冷锋锋面爬升所引起,动力抬升位于中高层,-18～-12℃温度层位于冷锋锋区上部,温度直减率大,导致-18～-2℃温度层较...     

青藏高原边坡地形重力波拖曳方案应用的个例研究

本文针对我国西南地区的一次持续性暴雨过程,利用WRF模式引入地形重力波拖曳方案进行120h的模拟,对比结果认为引入地形重力波拖曳的模拟效果总体看来略有优势,并详细分析了不同模拟时段的位势高度场,风场,海平面气压场与降水,其结果认为,引入与未引入地形重力波模拟的差异主要出现在48~72h之后,位势高度场方面,72h之后,在青藏高原主体上引入拖曳方案后的模拟结果相对未引入拖曳方案的结果是正偏差,高原下游地区为负偏差,引入后的模拟相对准确;风场模拟方面同样72h以内的模拟差距较小,72h之后高原上出现一处气旋性涡旋,其中心位置的模拟,引入拖曳方案后的结果相对准确,本次个例中出现的一例西南低涡,两实验都能够较好的模拟出700h Pa的气旋性涡旋;降水模拟的结果表明,在72h之后,对于四川地区降水中心位置的模拟,引入拖曳方案的模拟有所提高,但中心依然过强。这方面主要考虑降水的模拟牵涉模式内部更为复杂的处理过程和其他因素,地形重力波拖曳只是其中一个影响因素而不是核心因素。另外此方案对温度场及海平面气压场模拟的影响不明显。理论上讲引入地形重力波拖曳效应的模拟是物理过程相对更为完善的,在本次个例中模拟效果也略优,但在高原下坡复杂地形处长期应用的模拟效果仍需要更多的测试,将在后续工作中加以完善。      

A cold air lake formation in a basin — a simulation with a mesoscale numerical model

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     

Variation in summer surface air temperature over Northeast Asia and its associated circulation anomalies

This study investigates the interannual variation of summer surface air temperature over Northeast Asia(NEA) and its associated circulation anomalies.Two leading modes for the temperature variability over NEA are obtained by EOF analysis.The first EOF mode is characterized by a homogeneous temperature anomaly over NEA and therefore is called the NEA mode.This anomaly extends from southeast of Lake Baikal to Japan,with a central area in Northeast China.The second EOF mode is characterized by a seesaw pattern,showing a contrasting distribution between East Asia(specifically including the Changbai Mountains in Northeast China,Korea,and Japan) and north of this region.This mode is named the East Asia(EA) mode.Both modes contribute equivalently to the temperature variability in EA.The two leading modes are associated with different circulation anomalies.A warm NEA mode is associated with a positive geopotential height anomaly over NEA and thus a weakened upper-tropospheric westerly jet.On the other hand,a warm EA mode is related to a positive height anomaly over EA and a northward displaced jet.In addition,the NEA mode tends to be related to the Eurasian teleconnection pattern,while the EA mode is associated with the East Asia-Pacific/PacificJapan pattern.     

Interaction between an eddy and a zonal jet : Part II. Two-and-a-half-layer model

In a two-and-a-half-layer quasi-geostrophic model, a process study is conducted on the interaction between a vortex and a zonal jet, both with constant potential vorticity. The vortex is a stable anticyclone, initially located north of the eastward jet. The potential vorticity of the jet is allowed to have various vertical structures, while the vortex is concentrated in only one layer. The flow parameters are set to values characteristic of the Azores region.First, the jet is stable. Weak vortices steadily drift north of the jet without crossing it while strong vortices can cross the jet and tear off a cyclone with which they pair as a heton (baroclinic dipole). This heton often breaks later in the shear exerted by the jet; the two vortices finally drift apart. When crossed by deep anticyclones, the jet develops meanders with 375 km wavelength. These results exhibit a noticeable similarity with the one-and-a-half-layer case studied in Part I.Secondly, the jet is allowed to be linearly unstable. In the absence of the vortex, it develops meanders with 175 km wavelength and 25-day e-folding time on the β-plane. For various vertical structures of the jet, baroclinic instability is shown to barely affect jet–vortex interaction if the linear growth rate of unstable waves is smaller than 1/(14 days). Further simulations with a linearly unstable, nonlinearly equilibrated jet evidence its strong temporal variability when crossed by a deep vortex on the β-plane. In particular, long waves can dominate the spectrum for a few months after jet crossing by the vortex. Again in this process, the deep vortex couples with a surface cyclone and both drift southwestward.