Infrared transmission by water vapour

Summary The central problem in radiative flux calculation is the coordination and integration of laboratory data, spectroscopic data and theory and infrared transmission theory. In the case of water vapour, the general applicability of a random spectral model permits a simple “universal” plot of transmission data which is widely valid. By consideration of a similar plot for a regular spectral model, a technique is developed for a spectrum which is neither purely regular nor purely random, which permits a simple graphical presentation and analysis of laboratory transmission data. An extension of the Curtis-Godson approximation permits the application of such a universal curve to arbitrary atmospheres, both for intensity transmission and flux transmission. Special problems arise in the case of spectral window transmission, and suitable techniques are devised to treat this case, although it is necessary to lean more heavily in this spectral domain on theory in order to calculate flux transmission for an arbitrary atmosphere. A number of subsidiary problems are also considered in this paper, such as the effects of multiple absorbents and of experimental slit widths, and techniques designed specifically for the use of spectrally-integrated transmissions on a radiation chart.

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Zusammenfassung Das Hauptproblem bei der Berechnung von Strahlungsstr?men bildet die Koordinierung und Zusammenfassung von Laboratoriumsresultaten, von spektroskopischen Daten und von Ergebnissen der Theorie der Energieleitung durch infrarote Strahlen. Im Falle des Wasserdampfspektrums gestattet die Annahme eines „Zufallsmodells” der Spektrallinienverteilung eine graphische Darstellung der Transmissionsdaten, die den Tatsachen weitgehend gerecht wird. Durch Betrachtung einer ?hnlichen Darstellung im Falle eines regul?ren Spektralmodells wird eine Methode entwickelt, die auch auf ein Spektrum anwendbar ist, das als weder vollst?ndig regul?r noch als rein zuf?llig anzusehen ist. Diese Methode erlaubt eine einfache graphische Darstellung und Analyse der aus Laboratoriumsversuchen gewonnenen Transmissionsdaten. Eine Erweiterung der Curtis-Godson-N?herung erlaubt dann die Anwendung einer solchen generellen Kurve auf beliebige Atmosph?ren, sowohl für die Intensit?ts- als auch für die Strahlungsstromtransmission. Spezielle Probleme entstehen im Falle einer Transmission durch ein Fenster im Spektrum. Für die Behandlung dieses Falles werden geeignete Methoden entwickelt. Es ist dabei notwendig, sich st?rker an die Theorie zu halten, um die Strahlungstransmission für beliebige Atmosph?ren zu berechnen. In dieser Arbeit werden noch einige Nebenprobleme behandelt, so die Effekte der Mehrfachabsorption und die Effekte der Spaltweiten und Spektralintervalle. Es wird auch eine Methode zur Anwendung des Strahlungsdiagrammes auf die spektral integrierte Strahlungstransmission mehrerer Absorber angegeben.

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Résumé Dans le calcul des flux de rayonnement, le problème principal réside dans la coordination et la synthèse de données de laboratoire, de données spectroscopiques et des résultats de la théorie de la transmission de l'énergie par les rayons infra-rouges. Dans le cas du spectre de la vapeur d'eau, l'utilisation d'un „modèle alétoire” de la répartition des lignes spectrales permet la représentation graphique des valeurs de transmission, représentation qui répond en grande partie à la réalité. En considérant une représentation analogue dans le cas d'un spectre régulier, on développe une méthode utilisable également sur un spectre qui ne soit ni absolument régulier, ni complètement d? au hasard. Cette méthode permet une représentation et une analyse graphiques des données de propagation tirées d'essais de laboratoire. L'application étendue des approximations de Curtis-Godson permet alors d'appliquer de telles courbes générales à n'importe quelle atmosphère aussi bien pour la transmission de l'intensité que du flux de rayonnement. Des problèmes spéciaux se posent dans le cas de la transmission au-travers d'une fenêtre du spectre. On a établi des méthodes appropriées à ce cas particulier; il est alors nécessaire de s'en tenir plus étroitement à la théorie afin de calculer la propagation du rayonnement dans une atmosphère quelconque. Dans ce travail, on a encore traité de problèmes accessoires comme les effets de l'absorption multiple et des effets de la largeur de la fente et des intervalles entre les lignes du spectre. Enfin, on indique une méthode permettant d'utiliser les diagrammes de rayonnement en vue de l'intégration spectrale de la propagation du rayonnement dans le cas de plusieurs milieux absorbants successifs.

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With 12 Figures

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Published by permission of the Director, Meteorological Branch, Department of Transport, Canada, and dedicated to Dr.W. M?rikofer on the occasion of his 70th birthday.     

Tropospheric water vapour soundings by lidar at high Arctic latitudes

Ground-based lidars can provide continuous observations of tropospheric humidity profiles using the Raman scattering of light by water vapour and nitrogen molecules. We will present specific humidity profiles obtained at the high Arctic location Ny-Ålesund (Spitsbergen, 79°N). Under nighttime conditions the observations cover a range from about 500 m altitude up to the upper troposphere. Daylight limits the observations to the lower troposphere, depending on atmospheric transmission and the water vapour content. In a case study on 29 January, simultaneous observations of humidity and aerosol extinction show distinct differences in the various altitudes during the advection of aerosol-rich air masses. In the boundary layer, the aerosol is less affected by the humidity. In the free troposphere, the lidar ratio was observed to be up to 60 sr with some evidence for the uptake of water vapour by the aerosol particles. In another case study from 28 February 2002, the influence of the mean wind direction and the orography on the water vapour concentration near the ground and in the free troposphere will be discussed. During wintertime, a humidity inversion up to about 1.5 km altitude with dry air near the ground has frequently been found with wind from the southeast. Such local effects and small-scale structures observed by stationary lidar mostly cannot be resolved by other sounding methods like passive satellite soundings.     

A multifractal analysis of lidar measured water vapour

Accurate and fast-response measurements of space-time observations of specific humidity were made above a drying land surface at the University of California at Davis, using the Los Alamos water Raman-lidar. In an attempt to quantify the space-time intermittency features of turbulent flows in the lower atmosphere, a multifractal analysis of these water vapour measurements was performed. The structure of the specific humidity, (x, t), was analyzed quantifying a scalar gradient measure both in time and space, for all possible one-dimensional cuts, i.e. and . The results confirm the multifractal nature of this scalar gradient measure (a type of scalar dissipation rate) and show that humidity measurements at fixed times (x) are more intermittent (e.g. have less entropy dimension) than those at fixed locations in space (t). Similar multifractal behaviour of the spatial data, with and without a transformation from the observed wind velocities, supports the validity of Taylor's hypothesis for the studied fields.     

A new way of quantifying GCM water vapour feedback

The water vapour feedback probably makes the largest contribution to climate sensitivity, and the second-largest contribution to its uncertainty, in the sense of disagreement between General Circulation Models (GCMs, the most physically detailed models of climate we have). Yet there has been no quantification of it which allows these differences to be attributed physically with the aim of constraining the true value. This paper develops a new breakdown of the non-cloud LW (longwave) response to climate change, which avoids the problems of the conventional breakdown, and applies it to a set of 4 GCMs. The basic physical differences are that temperature is used as the vertical coordinate, and relative humidity as the humidity variable. In this framework the different GCMs’ feedbacks look more alike, consistent with our understanding that their water vapour responses are physically very similar. Also, in the global mean all the feedback components have the same sign, allowing us to conveniently attribute the overall response fractionally (e.g. about 60% from the “partly-Simpsonian” component). The systematic cancellation between different feedback components in the conventional breakdown is lost, so now a difference in a feedback component actually contributes to a difference in climate sensitivity, and the differences between these GCMs in the non-cloud LW part of this can be traced to differences in formulation, mean climate and climate change response. Physical effects such as those due to variations in the formulation of LW radiative transfer become visible. Differences in the distribution of warming no longer dominate comparison of GCMs. The largest component depends locally only on the GCM’s mean climate, so it can in principle be calculated for the real world and validated. However, components dependent on the climate change response probably account for most of the variation between GCMs. The effect of simply changing the humidity variable in the conventional breakdown is also examined. It gives some of this improvement—the loss of the cancellations that leave the conventional breakdown of no use to understand differences between GCMs’ climate sensitivities—but not the link to mean climate.     

Quantifying the water vapour feedback associated with post-Pinatubo global cooling

There is an ongoing important debate about the role of water vapour in climate change. Predictions of future climate change depend strongly on the magnitude of the water vapour feedback and until now models have almost exclusively been relied upon to quantify this feedback. In this work we employ observations of water vapour changes, together with detailed radiative calculations to estimate the water vapour feedback for the case of the Mt. Pinatubo eruption. We then compare our observed estimate with that calculated from a relatively large ensemble of simulations from a complex coupled climate model. We calculate an observed water vapour feedback parameter of –1.6 Wm^–2 K^–1, with uncertainty placing the feedback parameter between –0.9 to –2.5 Wm^–2 K^–1. The uncertain is principally from natural climate variations that contaminate the volcanic cooling. The observed estimates are consistent with that found in the climate model, with the ensemble average model feedback parameter being –2.0 Wm^–2 K^–1, with a 5–95% range of –0.4 to –3.6 Wm^–2 K^–1 (as in the case of the observations, the spread is due to an inability to separate the forced response from natural variability). However, in both the upper troposphere and Southern Hemisphere the observed model water vapour response differs markedly from the observations. The observed range represents a 40%–400% increase in the magnitude of surface temperature change when compared to a fixed water vapour response and is in good agreement with values found in other studies. Variability, both in the observed value and in the climate models feedback parameter, between different ensemble members, suggests that the long-term water vapour feedback associated with global climate change could still be a factor of 2 or 3 different than the mean observed value found here and the model water vapour feedback could be quite different from this value; although a small water vapour feedback appears unlikely. We also discuss where in the atmosphere water vapour changes have their largest effect on surface climate.     

Direct human influence of irrigation on atmospheric water vapour and climate

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm^–2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.     

Some implications of a new approach to the water vapour feedback

The water vapour feedback is the largest physical climate feedback. It also gives the second-largest contribution to the range of uncertainty in climate sensitivity in General Circulation Models (GCMs). Tracing these differences back to their physical causes in the hope of constraining climate sensitivity requires an appropriate quantification. Yet the Intergovernmental Panel on Climate Change judge that the conventional diagnosis of a “water vapour feedback” and a “lapse rate feedback” provides little insight into differences between GCMs’ climate sensitivities. We show that the conventionally diagnosed water vapour feedback is in fact formally useless for investigating differences between GCMs’ climate sensitivities—the anticorrelation between conventional “water vapour feedback” and “lapse rate feedback” makes the correlation between the “water vapour feedback” and their sum insignificant: i.e. statistically, knowing this “feedback” allows one to conclude nothing about the sum and thence about climate sensitivity. This follows primarily from how little relative humidity (RH) changes with climate change in GCMs. A more detailed physical analysis concludes that the overall mean decrease of RH on warming seen in GCMs is robustly physically based. This and other physical arguments then suggest that the stronger the positive “water vapour feedback”, the less sensitive climate can be expected to be. A diagnosis based on the “partly-Simpsonian” model of water vapour feedback avoids these problems. On the conventional view of the water vapour feedback, naive extrapolation of variations within present-day climate suggests that parts of our planet are close to locally reaching conditions that would allow a run-away water vapour greenhouse effect once they were extensive enough. Of course this has never occurred in geological history, and is not seen in Earth-like GCMs. Again, the “partly-Simpsonian” approach provides a simple qualitative explanation, by showing that the water vapour feedback can only cancel part of the basic Planck’s Law negative feedback.     

The effect of water vapour broadening on methane eddy correlation flux measurements

A high resolution tunable diode laser absorption spectrometer (TDLAS) was used to measure the broadening effect of water vapor and other gases (dry air, nitrogen, oxygen, hydrogen and helium) on three methane lines in the v₄ fundamental. The effects on methane eddy correlation flux measurements amount to a few percent for the least broadened line for expected H₂O fluxes, to 10% for the most broadened line for higher H₂O and lower CH₄ fluxes likely to be encountered. The broadening coefficients of methane measured for air, N₂, O₂, and He are in good agreement with recently published values.     

GPS water vapour tomography: preliminary results from the ESCOMPTE field experiment

Water vapour plays a major role in atmospheric processes but remains difficult to quantify due to its high variability in time and space and the sparse set of available measurements. The GPS has proved its capacity to measure the integrated water vapour at zenith with the same accuracy as other methods. Recent studies show that it is possible to quantify the integrated water vapour in the line of sight of the GPS satellite. These observations can be used to study the 3D heterogeneity of the troposphere using tomographic techniques. We develop three-dimensional tomographic software to model the three-dimensional distribution of the tropospheric water vapour from GPS data. First, the tomographic software is validated by simulations based on the realistic ESCOMPTE GPS network configuration. Without a priori information, the absolute value of water vapour is less resolved as opposed to relative horizontal variations. During the ESCOMPTE field experiment, a dense network of 17 dual frequency GPS receivers was operated for 2 weeks within a 20×20-km area around Marseille (southern France). The network extends from sea level to the top of the Etoile chain (700 m high). Optimal results have been obtained with time windows of 30-min intervals and input data evaluation every 15 min. The optimal grid for the ESCOMTE geometrical configuration has a horizontal step size of 0.05°×0.05° and 500 m vertical step size. Second, we have compared the results of real data inversions with independent observations. Three inversions have been compared to three successive radiosonde launches and shown to be consistent. A good resolution compared to the a priori information is obtained up to heights of 3000 m. A humidity spike at 4000-m altitude remains unresolved. The reason is probably that the signal is spread homogeneously over the whole network and that such a feature is not resolvable by tomographic techniques. The results of our pure GPS inversion show a correlation with meteorological phenomena. Our measurements could be related to the land–sea breeze. Undoubtedly, tomography has some interesting potential for the water vapour cycle studies at small temporal and spatial scales.     

Recent fluctuations of tropospheric temperature and water vapour content in the tropics

Summary During the last decades the average temperature of the tropical troposphere (200/850 hPa layer) has steadily increased, between 1965 and 1984 by about 0.8°C in the whole equatorial belt. Data series from a section of individual stations verify this trend as seasonally constant, but decreasing from the equator towards both hemispheres. Further evidence is presented by selected mountain stations and glacier retreat in all equatorial mountains.Above the equatorial Pacific, the same stations indicate an increase of moisture content in the middle troposphere (500/700 hPa layer) expressed in precipitable water as well as in relative humidity. This coincides with increasing sea surface temperature in the area around Indonesia and northern Australia. Above Africa the trend is (if real) quite patchy. Due to the short residence time of water vapour in the atmosphere the horizontal (zonal ) distances between its sources and sinks remain near 2000 km, which may explain, in addition to instrumental differences, large regional deviations.With 7 Figures     

大气水汽的输送和收支及其对副热带干旱的影响

本文利用1979年9月至1984年8月欧洲中期天气预报中心(ECMWF)每日四次的十三层分析资料,研究不同尺度的大气运动对水汽输送和收支的贡献。指出Hadley环流对水汽从副热带向赤道输送和从冬半球向夏半球的输送中起着决定性作用。热带行星波则把水汽从热带输向副热带。在中高纬的水汽向极输送中,天气尺度的波动比行星尺度的波动更重要。在南半球,天气尺度的波动支配着水汽的输送过程。分析和数值试验表明,副热带干旱受到nadley环流和热带行星波异常的强烈影响。     

Influence of leaf water potential on diurnal changes in CO2 and water vapour fluxes

Mass and energy fluxes between the atmosphere and vegetation are driven by meteorological variables, and controlled by plant water status, which may change more markedly diurnally than soil water. We tested the hypothesis that integration of dynamic changes in leaf water potential may improve the simulation of CO₂ and water fluxes over a wheat canopy. Simulation of leaf water potential was integrated into a comprehensive model (the ChinaAgrosys) of heat, water and CO₂ fluxes and crop growth. Photosynthesis from individual leaves was integrated to the canopy by taking into consideration the attenuation of radiation when penetrating the canopy. Transpiration was calculated with the Shuttleworth-Wallace model in which canopy resistance was taken as a link between energy balance and physiological regulation. A revised version of the Ball-Woodrow-Berry stomatal model was applied to produce a new canopy resistance model, which was validated against measured CO₂ and water vapour fluxes over winter wheat fields in Yucheng (36°57′ N, 116°36′ E, 28 m above sea level) in the North China Plain during 1997, 2001 and 2004. Leaf water potential played an important role in causing stomatal conductance to fall at midday, which caused diurnal changes in photosynthesis and transpiration. Changes in soil water potential were less important. Inclusion of the dynamics of leaf water potential can improve the precision of the simulation of CO₂ and water vapour fluxes, especially in the afternoon under water stress conditions.     

Surface fluxes of heat and water vapour from sites in the European Arctic

Summary  Measurements of the surface fluxes of heat and water vapour were taken at four sites across the European Arctic as part of the EU funded LAPP project. The sites cover a range of latitudinal, altitudinal and climatic conditions. The most northerly site is near Ny-?lesund, Svalbard, a polar semi-desert with continuous permafrost. A second permafrost site is a fen area in the Zackenberg valley, East Greenland. Finally two sites in northern Finland, Skalluvaara and Kaamanen are on the southern boundary of the region affected by permafrost. At all sites measurements were made of the turbulent fluxes of heat and water vapour using eddy correlation equipment for at least one active season. The net radiation totals for July and August are similar at all sites. At the sites with permafrost a substantial proportion (over 20%) of the net radiation goes into soil heat flux, to thaw the soil moisture in the top metre. Of the remaining energy just over half is used for evaporation. At the Finnish sites the vegetation is largely deciduous and this is seen in the record with higher evaporative ratios in July and August, after the vegetation becomes green. The Finnish sites tend to have higher surface resistance to evaporation; however, the evaporative demand is greater leading to slightly higher evaporation rates. The two Finnish sites have a similar seasonal pattern determined by the water table and seasonality of the vegetation. The two northern sites show a pattern that is determined primarily by the variation of water table only. It is concluded that the water balance through the active season is influenced primarily by the history of snow cover. The seasonality of the vegetation, the permafrost and the depth of water table are also important influences. Received November 1, 1999 Revised April 17, 2000     

Surface exchange of water vapour between an open sphagnum fen and the atmosphere

Water loss by evapotranspiration (ET) is a principal component of the hydrologic cycle in wetlands. Using micrometeorological techniques, we measured ET from a Sphagnum-dominated open fen in northcentral Minnesota (U.S.A.) from May to October in 1991 and 1992. The daily ET rate ranged from 0.2–4.8 mm d^-1 with a growing season average of 3.0 mm d^-1. The evapotranspiration rate of the fen was near the potential rate of open water evaporation when the vascular plants were actively growing and the water table level was within or above the rooting zone. Using a dual-source modification of the Penman-Monteith equation (Massman, 1992), we partitioned the measured ET into evaporation from the non-vascular Sphagnum surfaces and transpiration from vascular plants. The analysis indicated that about two thirds of the water vapour flux to the atmosphere was from evaporation when the Sphagnum surface was wet. Such an evaporative flux was expected because of vertical distribution of vascular plant leaves which had a small leaf area index (0.4–0.7) and intercepted only about 30% of net radiation (R _n ) during the day. The remainder of R _n was thus available for evaporation from Sphagnum. Evaporation significantly decreased as the Sphagnum surface dried out. When the water table was within the rooting zone (0–0.4 m), the vascular plants absorbed Sphagnum-generated sensible heat, which amounted up to one third of their transpiration energy flux. Under these conditions, the total water vapour flux remained near its potential rate owing to the enhanced transpiration from vascular plants. A drop in water table of 0.15–0.2 m below the hollow bottom during vascular plant senescence resulted in ET rates lower than the potential rates by 5–65%.     

Heat and water vapour fluxes and scalar roughness lengths over an Antarctic ice shelf

We present eddy-correlation measurements of heat and water vapour fluxes made during the Antarctic winter. The surface layer was stably stratified throughout the period of observation and sensible heat fluxes were always directed downwards. However, both upward and downward water vapour fluxes were observed. Their magnitude was generally small and the latent heat flux was not a significant fraction of the surface energy budget. The variation of heat and water vapour fluxes with stability is well described by Monin-Obukhov similarity theory but the scalar roughness lengths for heat and water vapour appear to be much larger than the momentum roughness length. Possible explanations of this effect are discussed.     

实时水情信息传输途径

1水情信息传输历程1.1电报网传输期解放初期始,我国采用5位数字编码利用电报网络解决水情信息传输问题,并制定颁发了《水文情报预报拍报办法》。电报网主要覆盖邮电部门,水情站相对邮电部门较远,水情信息传输时效性受到影响,收齐水情信息需要1~3 d,无法满足防汛抢险工作需要。1     

Seasonal variations in the isotopic composition of near-surface water vapour in the eastern Mediterranean

Although the isotopic composition of precipitation is widely used in global climate change studies, use of water vapour isotopes is considerably more limited. Here we present the results from 9 yr of atmospheric vapour measurements in the Eastern Mediterranean, at a site in Israel. The measurements show a strong mean seasonal cycle of about 4‰ in ¹⁸O (peaking around July). This seasonality could not be adequately explained by changes in surface interactions or in air mass trajectories, as usually invoked for variations in local precipitation. We could explain this cycle only as a combination of three components: (1) rainout effects; (2) temperature and relative humidity control of the initial vapour and (3) seasonal variations in the vertical mixing across the top of the planetary boundary layer. This last component is emphasized in the current study, and it was shown to be a significant factor in the seasonal cycle features. The measurements were also compared with an isotope-enabled GCM (CAM2) run, which exhibited a markedly different seasonal cycle. Such comparisons with vapour isotopes data could help in constraining models better.