塔克拉玛干沙漠春季大气气溶胶光学特性研究

本文利用多波段光度计在新疆塔克拉玛干沙漠探测的1986年4月11日至5月10日的太阳直接辐射强度和天空亮度资料,分析了塔克拉玛干沙漠晴空大气和沙暴期间大气气溶胶的光学厚度、谱分布以及折射率的变化特性。     

巴丹吉林沙漠北缘拐子湖近地层湍流能谱特征分析

利用2013年6月巴丹吉林沙漠拐子湖地区流沙下垫面的陆气通量观测资料,计算并分析了该研究区不同大气稳定层结条件下的湍流速度各分量谱和温度谱及湍流的局地各向同性特征。结果表明:该研究区风速各分量的湍流强度随平均风速的增加而逐渐减小,当平均风速2 m/s时,风速各分量的湍流强度逐渐稳定且基本限定在0.5以内。在不同大气稳定度下,湍流速度和温度能谱曲线在惯性副区内逐渐有合并趋势且遵循Kolmogorov提出的-2/3定律,速度各分量谱在高频段均满足各向同性且符合低频限制理论。随着稳定度的增加,风速分量的能谱曲线逐渐降低且向高频端移动,风速分量和温度能谱对应的谱峰长度则逐渐减小。该研究区水平湍流尺度范围为9.0~600 m,垂直湍流谱峰波长为10.79~75 m。该结果介于草地和森林下垫面之间,与塔克拉玛干沙漠腹地的试验结果较为接近。     

Strange attractors in atmospheric boundary-layer turbulence

The possible chaotic nature of the turbulence of the atmospheric boundary layer in and above a decidious forest is investigated. In particular, this work considers high resolution temperature and three-dimensional wind speed measurements, gathered at six alternative elevations at Camp Borden, Ontario, Canada (Shawet al., 1988). The goal is to determine whether these time series may be described (individually) by sets of deterministic nonlinear differential equations, such that: (i) the data's intrinsic (and seemingly random) irregularities are captured by suitable low-dimensional fractal sets (strange attractors), and (ii) the equation's lack of knowledge of initial conditions translates into unpredictable behavior (chaos). Analysis indicates that indeed all series exhibit chaotic behavior, with strange attractors whose (correlation) dimensions range from 4 to 7. These results support the existence of a low-dimensional chaotic attractor in the lower atmosphere.     

Statistics of atmospheric turbulence within and above a corn canopy

Two three-dimensional split-film anemometers were used to measure turbulence statistics within and above a corn canopy. Normalised profiles of mean windspeed, root-mean-square velocity, momentum flux, and heat flux were constructed from half-hourly averages by dividing within-canopy measurements by the simultaneous canopy-top measurement. With the exception of the heat flux, these profiles showed consistent shape from day to day. Time series of the three velocity components were recorded on magnetic tape and subsequently analysed to obtain Eulerian time and length scales and the power spectrum of each component at several heights. The timescale was found to have a local minimum value at the top of the canopy. However the length scale L _wformed from the timescale and the root-mean-square vertical velocity varied with height as L _w 0.1 z. The power-spectra were non-dimensionalised to facilitate comparison of spectra at different heights and times. All spectra had -5/3 regions spanning at least two decades in frequency.     

Analysis and trends of atmospheric turbidity parameters over Cairo

Summary This work deals with the Linke turbidity factor, based on total spectrum observations of the direct solar beam and aerosol turbidity parametersa _a , , and based on observations in broad spectral bands. Diurnal and seasonal variations of these turbidity parameters were analyzed for the period 1975 to 1991.Annual variations of these parameters show low values in winter and high values in both spring and summer. The extinction coefficients decrease with increase of both wavelength and optical airmass. Trend analysis shows an increase in aerosol extinction coefficient below 0.63 m, and a slight decrease for longer wave-lengths.Linear regression relations are also constructed to estimate botha _a and whenT _L is available. The relations show thata _a can be estimated with errors below 20%. The relation with the parameter, may give better results when it is estimated by assigning a fixed value of .Nomenclature AV Monthly and total average of the measured parameter - a Atmospheric optical thicknes - a _a Aerosol optical thickness - a _r Mean value of optical thickness of. Rayleigh atmosphere over all wavelengths - a _o Ozone optical thickness - a _o Ozone absorption coefficient - a _w Water vapor optical thickness - COR Correlation coefficient of the linear relation in percentage - Ex₁, Ex₂, Ex₃ Aerosol extinction coefficients in the bands .2–.53, .53–.63, .63–.695, respectively - I _(o) Normal incident direct solar radiation, under clear sky condition - I _o Extraterrestrial insolation at normal incidence - m _r Relative (optical) air mass - NO Number of the observations used in either making the relation or the verification - RMSE Root mean square error of the linear relation - RMSE% Percentage value of the root mean square error relative to the average measured value AV - T _L Linke turbidity factor - T Dry bulb temperature in °C - u _o Ozone layer thickness, cm - Z Zenith angle - Ångström wavelength exponent - Ångström turbidity coefficient - Wavelength - Y The year number after 1975 With 5 Figures     

我国短期气候预测的物理基础及其预测思路

短期气候预测依据大气科学原理,运用气候动力学、统计学等手段,在研究气候异常成因的基础上对未来气候趋势进行预测。虽然目前我国短期气候预测的水平还不高,但短期气候预测是国家经济发展和防灾减灾的迫切需求,提高预测准确率是气象科研和业务人员的重要任务。该文从海洋、积雪等外强迫信号及大气环流大尺度变动等大气内部特性等角度概述了短期气候预测的物理基础,简要回顾了近60年来我国短期气候预测的发展历程,并介绍了作者近十几年来研制短期气候预测客观统计学及统计与动力学相结合预测模型的主要思路。     

Observations of a multi-level turbulence structure in a very stable atmospheric boundary layer

Boundary-layer measurements conducted at the Marsta site in Sweden from a winter-time situation (23–25 Feb.) with stable stratification have been analysed. The data comprise wind and temperature profile measurements up to 30 m, turbulence measurements at 2, 6 and 30 m and Doppler acoustic sounder data up to about 150 m. The upwind fetch at the site is flat and free from obstacles to a distance of ca 5 km for the particular sector chosen for the experiment.During the night, a two-layer vertical structure developed. Analysis of power spectra, co-spectra and variances in a shallow and very stable turbulent boundary layer near the ground show that the turbulence is fully developed and follow the universal behaviour.Above, at a height of 30 m, another turbulent layer is produced by increased wind shear near a low-level jet. This turbulent upper layer can be regarded as a layer of free shear flow. At this height, there also exist wave-turbulence interactions at low frequencies which sometimes cause a countergradient heat flux.     

Changes in integral and aerosol atmospheric turbidity in Trans-Baikal and Central Siberia

The analysis of time series of the Linke turbidity factors is performed for the atmosphere mass m = 2 (T ₂) and atmospheric aerosol optical thickness for the wavelength λ₀ = 0.55 μm (AOT) from the data of 14 actinometrical stations of Central Siberia and Trans-Baikal territory. It is shown that over the period from 1976 to 2006, the increased atmospheric transparency is observed in the region. Quantitative estimates of changes in multiyear mean annual variations of T ₂ and AOT at different periods of averaging and for different time periods are derived.     

Experimental investigation of atmospheric boundary layer turbulence

This paper describes how to measure turbulence in the atmospheric boundary layer (ABL) in order to address certain problems in modern atmospheric physics. These problems mainly relate to the Earth's energy budget (including the hydrological cycle) and biogeochemical cycles. Starting from the main characteristic numbers and the basic equations of atmospheric turbulent flow, we show what turbulence parameters are important to measure. Special attention is given to the various methods used to compute the turbulent fluxes. We analyse the range of scales which has to be measured to properly capture the eddies contributing to the turbulent transfers. This range of scales determines what sensors can be used in the atmospheric surface layer and in the ABL. We describe the most widely used instruments and their performances. The principal platforms used to deploy these instruments are examined. Aircraft are described in more details, because they allow a thorough exploration of the ABL. In the last section, some examples of ABL turbulence signals measured in various conditions are presented. These examples illustrate horizontally homogeneous turbulence as well as inhomogeneous signals for which standard analysis techniques cannot be used. We show how some recent techniques, like wavelet transforms, can help to investigate this kind of signal. At the end, we present what would be interesting to do in the near future for the study of ABL turbulence.     

Propeller anemometers as sensors of atmospheric turbulence

Even though propeller anemometers are found to give outputs which deviate from the desired cosine relationship by an amount which varies with wind speed, their overall performance is consistent with many atmospheric requirements. Their output per unit wind speed is a function of angle of attack, such that when used as sensors of the vertical or horizontal cross-wind components in the atmosphere, calibration factors may differ by as much as 30 % from those obtained in a normal wind-tunnel calibration procedure (in which wind velocity is parallel to the anemometer shaft). These characteristics are sufficiently important that great care should be taken in using these devices inu-v-w orthogonal arrays.For use in eddy-correlation equipment, it appears that it is best to vane-mount the horizontal sensor to ensure that the appropriate calibration factor is employed.The response lengths of propeller anemometers also vary with angle of attack. Near=0 °, the axially-referred response length appears to depend linearly on cos, but near=90 ° a dependence on cos^1/2 fits the data. No strong effect of wind speed is found.Due to their limited response characteristics, these anemometers give rise to underestimates of the Reynolds stress measured near the surface. The extent of the loss is about 8 % when anemometers in good condition are employed at a height of 5m. Operation at a greater height would allow this error to be reduced. After exposure in the atmosphere for some time, the anemometers tend to respond more slowly and greater losses (of the order 25 %) can occur. Some improvement in performance is possible by the choice of a suitable spatial separation of the sensors.     

The relationship between turbidity and spectral optical depth

Abstract

Spectral and spectrally integrated direct beam irradiances measured in Hamilton during the summer of 1981 were used to examine the relationship between the aerosol optical depth τ_a and Unsworth and Monteith's turbidity coefficient τ_A, which is a measure of spectrally averaged optical depth. Good correlation was obtained between τ_a at a wavelength of 0.67 μm and τ_A, thus confirming experimentally Blanchet's (1982) conclusion from a numerical study.     

Search for finite dimensional attractors in atmospheric turbulence

The question is investigated whether the dynamics of turbulent wind fields in the atmospheric boundary layer can be satisfactorily described by a low-dimensional deterministic system. Special emphasis is laid on the detection of a possibly existing, underlying strange attractor. Fast response wind measurements of an ultrasonic anemometer with a sampling rate of 21 Hz were carried out over periods of several days in the near surface boundary layer. The correlation dimension of the resulting time series, several million data points long, is estimated by means of the Grassberger-Procaccia algorithm. No sign of a low dimensional attractor can be detected. By comparison with different types of random noise, the existence of an attractor with dimension lower than six can be excluded in the present data sets.     

Statistics of atmospheric turbulence within a natural black spruce forest canopy

Turbulence statistics were measured in a natural black-spruce forest canopy in southeastern Manitoba, Canada. Sonic anemometers were used to measure time series of vertical wind velocity (w), and cup anemometers to measure horizontal wind speed (s), above the canopy and at seven different heights within the canopy. Vertical profiles were measured during 25 runs on eight different days when conditions above the canopy were near-neutral.Profiles of s and of the standard deviation ( _w ) of w show relatively little scatter and suggest that, for this canopy and these stability conditions, profiles can be predicted from simple measurements made above the canopy. Within the canopy, a negative skewness and a high kurtosis of the w-frequency distributions indicate asymmetry and the persistence of large, high-velocity eddies. The Eulerian time scale is only a weak function of height within the canopy.Although w-power spectra above the canopy are similar to those in the free atmosphere, we did not observe an extensive inertial subrange in the spectra within the canopy. Also, a second peak is present that is especially prominent near the ground. The lack of the inertial subrange is likely caused by the presence of sources and sinks for turbulent kinetic energy within our canopy. The secondary spectral peak is probably generated by wake turbulence caused by form drag on the wide, horizontal spruce branches.     

ENSO variability and atmospheric response in a global coupled atmosphere-ocean GCM

The interannual variability associated with the El Ni?o/Southern Oscillation (ENSO) cycle is investigated using a relatively high-resolution (T42) coupled general circulation model (CGCM) of the atmosphere and ocean. Although the flux correction is restricted to annual means of heat and freshwater, the annual as well as the seasonal climate of the CGCM is in good agreement with that of the atmospheric model component forced with observed sea surface temperatures (SSTs). During a 100-year simulation of the present-day climate, the model is able to capture many features of the observed interannual SST variability in the tropical Pacific. This includes amplitude, lifetime and frequency of occurrence of El Ni?o events and also the phase locking of the SST anomalies to the annual cycle. Although the SST warming during the evolution of El Ni?os is too confined spatially, and the warming along the Peruvian coast is much too weak, the patterns and magnitudes of key atmospheric anomalies such as westerly wind stress and precipitation, and also their eastward migration from the western to the central equatorial Pacific is in accord with observations. There is also a qualitative agreement with the results obtained from the atmospheric model forced with observed SSTs from 1979 through 1994. The large-scale dynamic response during the mature phase of ENSO (December through February) is characterized by an eastward displacement and weakening of the Walker cell in the Pacific while the Hadley cell intensifies and moves equatorward. Similar to the observations, there is a positive correlation between tropical Pacific SST and the winter circulation in the North Pacific. The deepening of the Aleutian low during the ENSO winters is well captured by the model as well as the cooling in the central North Pacific and the warming over Canada and Alaska. However, there are indications that the anomalies of both SST and atmospheric circulation are overemphasized in the North Pacific. Finally, there is evidence of a coherent downstream effect over the North Atlantic as indicated by negative correlations between the PNA index and the NAO index, for example. The weakening of the westerlies across the North Atlantic in ENSO winters which is related to a weakening and southwestward displacement of the Icelandic low, is in broad agreement with the observations, as well as the weak tendency for colder than normal winters in Europe. Received: 31 October 1995 / Accepted: 29 May 1996     

Resolved-scale turbulence in the atmospheric surface layer from a large eddy simulation

Large eddy simulation has encountered difficulties in handling turbulence in the atmospheric surface layer due to deficiencies in sub-grid scale models. This paper addresses the possibility of resolving the turbulence in the upper part of the surface layer by a low-aspect ratio of grid spacing. Results show that resolved-scale shear stresses dominate over the sub-grid scale components so that effects due to the sub-grid scale model can be ignored in this region. The effects of the lower boundary condition on the resolved-scale turbulence in the upper part of the surface layer are discussed. It is concluded that the normalized mean velocity shear and resolved turbulence in the upper part of the surface layer are not affected by the specification of the lower boundary condition. In addition, the present work proposes a new independent model parameter, the Smagorinsky Reynolds Number (Re_SM), and demonstrates that this number determines the resolved turbulence in the upper part of the surface layer.     

Thermal plumes and turbulence spectra in the atmospheric boundary layer

Experimental observations on the temperature and wind fields above flat grassy terrain have been obtained with an instrumented 92-m tower during intervals of strong insolation about midday. The turbulence characteristics of the air confirm that free convection prevailed at heights between 16 and 48 m, with some tendency for departure at higher levels. The spectra of temperature and vertical velocity contain gaps at wave numbers in the range 0.01–0.025 m^–1. These are attributed to natural thermal plumes that act as sources of extra energy input to the Kolmogorov-Obukhov-Corrsin scheme of turbulence in or at the low-wave number limit of the inertial subrange. Modified forms of the K-O-C spectral laws for thermally unstable air are derived which agree with the observed spectra over the whole range of wave numbers examined, and which contain the spectral gap at wave numbers corresponding to the thermal plume diameters.     

Spatio-temporal variability of atmospheric rivers and associated atmospheric parameters in the Euro-Atlantic region

We study the spatio-temporal variability of Atmospheric Rivers (ARs) and associated integrated water vapor and atmospheric parameters over the Euro-Atlantic region using long-term reanalysis datasets. Winds, temperature, and specific humidity at different pressure levels during 1979–2018 are used to study the water vapor transport integrated between 1000 and 300 hPa (IVT300) in mapping ARs. The intensity of ARs in the North Atlantic has been increasing in recent times (2009–2018) with large decadal variability and poleward shift (~ 5° towards the North) in landfall during 1999–2018. Though different reanalysis datasets show similar spatial patterns of IVT300 in mapping ARs, bias in specific humidity and wind components led to IVT300 mean bias of 50 kg m⁻¹ s⁻¹ in different reanalysis products compared to ERA5. The magnitude of winds and specific humidity in the lower atmosphere (below 750 hPa) dominates the total column water vapor and intensity of ARs in the North Atlantic. Reanalysis datasets in the central North Atlantic show an IVT300 standard deviation of 200 kg m⁻¹ s⁻¹ which is around 33% of the ARs climatology (~ 600 kg m⁻¹ s⁻¹). Though ARs have a higher frequency of landfalling over Western Europe in winter half-year, the intensity of IVT300 in winter ARs is 3% lower than the annual mean. The lower frequency of ARs in the summer half-year shows 3% higher IVT300 than the annual mean. While ARs in the North Atlantic show a strong decadal change in frequency and path, the impact of the North Atlantic Oscillation (NAO) and Scandinavian blocking on the location of landfall of ARs are significant. Furthermore, there is a strong latitudinal dependence of the source of moisture flux in the open ocean, contributing to the formation and strengthening ARs.