Climate Observations By Radiometryspectrometry

10.1 Introduction

Quantitative and spectroscopic measurements of radiation are at the heart of observations made to understand and monitor the climate system (Fig. 10.1). The time-dependent complexity of the system under observation requires that the measurements span the globe, including the vertical dimension. Many of the important processes, for example those involved in ozone depletion, are associated with transient dynamical phenomena and occur on local and diurnal scales, calling for high-resolution measurements in space and time. Energy-budget studies of the Earth require measurements at the surface and external to the planet, ideally with global coverage that is comprehensive in angular as well as geographical space, and time. The Earth radiates about the same total energy to space as a 255-K blackbody, with a spectrum rich in molecular vibration-rotation lines that contain information about the physical state of the atmosphere, and continuum fluctuations that represent weak and/or complex atmospheric bands plus surface and cloud emissivity variations. The most pronounced of these spectral features are those caused by absorption and emission in the bands of the principal atmospheric minor constituents, especially water vapour and carbon dioxide. Infra-red spectroscopy and radiometry from satellites observing the reflected and thermally emitted radiation as a function of latitude, longitude, time of day, and season, can yield global fields of vertical temperature and minor constituent distributions (e.g. water and ozone), cloud cover, thickness and height, and ocean temperature.

Individual measurements or profiles typically take a second or less to acquire, and can be repeated as the scene changes below, allowing the parameters to be mapped in three dimensions. Satellites in high-inclination orbits cover the whole planet approximately every day, make it possible to study processes at work in large-scale weather systems, the stratospheric ozone layer, pollution in the lower layers of the atmosphere, and the 'greenhouse' gases that drive global warming. Mineralogy, vegetation (including agricultural products and some of the species in the sea), the icy cryosphere, and volcanism and its products are also accessible to sensors in space. Radar and lidar are rapidly being adopted to investigate land and ice topography, sea state, and to infer wind fields.

In this chapter we discuss the use of radiometry and spectroscopy to measure

FlG. 10.1. An overview of some of the main ways in which climate data are collected using radiation measurements. Broadly speaking, the measurements are of two types: radiometric, to investigate the flux of energy in the climate system, and spectroscopic, to map temperature and composition using remote-sensing techniques for dynamical and process studies. The source of the radiation may be the Sun, viewed directly in the UV, visible or near-infra-red spectrum through the atmosphere or by scattering from atmospheric molecules, cloud particles or the surface, or it may be longwave thermal emission from these components of the climate system. The observing platform may be on the surface, airborne, or orbiting in space.

quantities useful for climate research, focusing on specific real instruments as much as possible. The goal is to define the state-of-the-art in climate measurements, and show how the fundamentals developed in earlier chapters are applied in practice.

10.2 Surface radiation budget: the pyranometer

The Earth-atmosphere system and global climate depend crucially on the surface radiation budget (SRB), i.e. the balance between shortwave and longwave irradiances and the outgoing fluxes at the Earth's surface. The SRB depends on the amount of incident solar radiation at the top of the atmosphere (TOA), the amount of energy absorbed, scattered or reflected towards the surface or back into space within the atmosphere and the reflection, absorption and longwave radiation from the ground at the surface itself. Radiative-transfer processes within the atmosphere depend critically on the concentrations of water vapour, trace

thermal emission

FlG. 10.1. An overview of some of the main ways in which climate data are collected using radiation measurements. Broadly speaking, the measurements are of two types: radiometric, to investigate the flux of energy in the climate system, and spectroscopic, to map temperature and composition using remote-sensing techniques for dynamical and process studies. The source of the radiation may be the Sun, viewed directly in the UV, visible or near-infra-red spectrum through the atmosphere or by scattering from atmospheric molecules, cloud particles or the surface, or it may be longwave thermal emission from these components of the climate system. The observing platform may be on the surface, airborne, or orbiting in space.

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gases (greenhouse gases) and aerosols, while the surface reflectance/emittance properties largely determine radiative processes at the Earth's surface.

The role of Earth's SRB for the global freshwater budget and the impacts of climate change and aerosol loads on a global scale are essential for a number of important issues. A reliable estimate of energies absorbed by the atmosphere and the surface layers of the Earth through an assessment of the SRB in combination with the radiation budget at the TO A will enable the calculation/modelling of heat transports in the different parts of the Earth-atmosphere system under current conditions and will ultimately improve our understanding of climate processes on the global to the regional scale. Such an understanding is an essential prerequisite to the modelling of Earth-atmosphere heat transports in atmospheric general circulation models (AGCMs, §11.4). At present, these models provide the only means of enhancing our understanding of climate processes and of possible future climate developments. The latter is particularly relevant with regard to anthropogenic climate change as a consequence of increased concentrations of atmospheric greenhouse gases.

A long-term SRB is also important because of the many processes within the climate system and the biosphere that are affected by the radiative fluxes. The net surface solar and terrestrial radiations affect the heating/cooling of the surface and act to control sensible and latent heat fluxes. The amount of solar energy and moisture on the Earth's land surface determine its ability to sustain life processes and these life processes affect in turn the net fluxes.

The SRB is also important from an applied point of view, since it governs the use of clean renewable solar energy, e.g. related to photovoltaic arrays and industrial applications such as solar-energy cookers and refrigeration technologies that are important to improving the quality of the environment.

Thus, knowledge of the SRB is required by many fields of science; these include: regional surface climatology, ocean modelling and energy budgets, polar climatology, validation of global climate models (GCMs), plant-growth models, understanding of forest ecosystems and forest fires and regional water resources, to just name a few.

However, the radiation budget not only exerts an influence on the climate system and the Earth's surface. There are important feedbacks that have to be taken into account, when considering the influence of surface and climate processes on the Earth's radiation budget. In particular, rapid changes in surface re-flectance/emittance properties, arising, for example, from a retreating ice/snow layer due to global warming, have profound effects on net fluxes at the surface and may cause regional and global climatic feedbacks. Equally important are changes in cloudiness, aerosol concentrations, and radiatively active trace gas concentrations that regulate the surface fluxes in profound ways.

Thus, quantifying the changes in the SRB arising from changes in surface properties and atmospheric chemical composition is an important pre-requisite in understanding the processes of global change and its impact on ecosystems and societies.

When considering these problems, two issues become particularly important. Clouds, their dynamics and properties remain the major uncertainty in determining the radiation budget (IPCC 2001). The International Satellite Cloud Climatology Project (ISCCP), addresses this problem and provides the most extensive and comprehensive global cloud climatologies.

However, under clear-sky conditions, aerosols become a dominant determinant of the SRB and thus of climate development. Unlike greenhouse gases that drive global climate change though, aerosols are fundamentally different. A major difference is that aerosols have much shorter atmospheric lifetime (from 10~4 days for natural, to tens of days for anthropogenic aerosols) compared with the important greenhouse gases (decades to centuries). This, together with microphysical and mixing processes, results in larger spatial and temporal aerosol variability.

Hatzianastassiou et al. (2004a,b, 2006) have conducted detailed modelling studies to examine globally clear-sky shortwave aerosol forcing over mainly absorbing surfaces (such as oceans and vegetation-covered surfaces) versus low-absorbing/high-reflecting surfaces typical for the snow- and ice-covered polar regions. Their results demonstrate that an increase in relative humidity enhances the planetary cooling effect of aerosols over oceans and low-albedo land areas, whilst over polar regions and highly reflecting land surfaces, the expected warming effect of aerosols changes to a cooling effect. Thus, global warming and an associated increase in relative humidity, would lead to enhanced aerosol cooling worldwide. The sensitivity results also demonstrate that an increase in surface albedo due to, for example, a reduction in land vegetation cover as a result of enhanced desertification would lead to increased atmospheric warming by aerosols. This, in turn, will result in a reduction of cloud formation and a further enhancement of the desertification process.

With regard to the radiative properties of the Earth's surface, the high albedo of snow and ice surfaces in the polar regions are particularly important. However, the arctic sea-ice cover, an important component in this regard, is apparently shrinking in area and decreasing in mean thickness. This can be considered the beginning of a positive feedback process where decreasing sea-ice surfaces lead to increasing heat uptake by the ocean, which results in further atmospheric warming. Thus, there are possible interlinkages between the SRB, climate change and a deteriorating Arctic sea-ice cover.

The Baseline Surface Radiation Network (BSRN) and the Global Energy Balance Archive (GEBA) are important examples of global data acquisition and archiving systems for data on observed climatologies, radiation and other fluxes

FlG. 10.2. The Eppley pyranometer is typical of the instruments used for the measurement of global, reflected, and diffuse shortwave radiation. The detector is a differential thermopile with the hot-junction receivers blackened and the cold-junction receivers whitened. Temperature-compensation circuitry frees the instrument from the effects of ambient temperature changes. A precision ground optical glass hemisphere of Schott glass (WG295) uniformly transmits energy from 0.285 to 2.800 ¡xm over zenith angles from 0-70°. The sensitivity is 11 microvolts per W m~2 with a response time of 5 s. The size is 5 3/4 in. diameter, 3 3/4 in. high and the weight is 2 lbs.

FlG. 10.2. The Eppley pyranometer is typical of the instruments used for the measurement of global, reflected, and diffuse shortwave radiation. The detector is a differential thermopile with the hot-junction receivers blackened and the cold-junction receivers whitened. Temperature-compensation circuitry frees the instrument from the effects of ambient temperature changes. A precision ground optical glass hemisphere of Schott glass (WG295) uniformly transmits energy from 0.285 to 2.800 ¡xm over zenith angles from 0-70°. The sensitivity is 11 microvolts per W m~2 with a response time of 5 s. The size is 5 3/4 in. diameter, 3 3/4 in. high and the weight is 2 lbs.

at the Earth's surface. BSRN has measuring sites covering major climatic zones between the Arctic and the Antarctic. At these sites the main radiative fluxes, direct solar, diffuse sky, global radiation (Fig. 10.2), reflected shortwave radiation, longwave in- and outgoing radiation are measured at 1 Hz frequency with 1-min statistics. Many stations have spectral measurements of direct solar radiation for the estimation of the aerosol optical depth, total ozone, and precipitable water vapour. Most stations have synoptic and radiosonde observations. This is the only network of radiation measurements with a defined accuracy with collocated atmospheric observations.

GEBA is a database with monthly means of energy fluxes at the Earth's surface including direct solar, diffuse sky, global radiation, reflected shortwave radiation, longwave in- and outgoing radiation, net radiation, sensible heat flux, latent heat flux, subsurface heat flux, and heat of fusion. At present, GEBA possesses about 250,000 monthly means for about 1600 sites, including monthly mean irradiances transferred from the World Radiation Data Center in St. Petersburg. The oldest data in the archive is the August 1922 global radiation thermoelectrically measured at University of Stockholm by A. Angstrom, followed by that from the IGY in the late 1950s.

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FlG. 10.3. The ACRIM II instrument contains two nominally identical conical cavities back-to-back, as shown here. The one on the left faces the Sun and is heated by radiation; the one on the right is blanked off and is heated electrically until the temperatures of the two cones, monitored by sets of sensitive thermistors, are identical. The power supplied to the electrically heated cone, divided by the area of the aperture of the radiation-heated cone, is an accurate measure of the solar irradiance.

10.3 Solar irradiance: ACRIM

One of the first applications of radiometry from space was the measurement of the solar constant, that is to say the total amount of energy from the Sun falling upon the Earth at all wavelengths. Obviously, this is a fundamental quantity that is difficult to measure from the surface because of atmospheric absorption and scattering from clouds and aerosols. The most successful technique is the 'active cavity method', in which the heating effect of the radiation is compared very precisely to the electrical energy required to maintain an identical cavity at the same temperature.

Figure 10.3 shows the details of the ACRIM II (Active Cavity Radiometer Irradiance Monitor) device that operates on this principle and that has been used to obtain a long series of measurements of solar constant since the late 1960s. An absolute accuracy of 99.9% and a long-term precision of a few parts per million is claimed for this sensor. It operates by controlling both the active and the reference cavities at the same relative temperature using an electronic servo. A shutter then repeatedly admits and excludes solar radiation from the active cavity for periods of about one minute each, and derives a value for the energy in the solar flux by comparing the difference between the asymptotic values of the electrical power drawn during the two periods. While conceptually simple, it will be clear that obtaining the high precision required to track the small changes in the solar output (typically around 0.1%) that occur under normal conditions requires very precise manufacturing tolerances in the blackness of the cavities, the temperature sensors, and the size of the aperture. Note that there is no transmitting component in the optical path, a key factor in permitting the desired precision to be achieved.

10.4 TOA radiation budget: ERB, CERES and GERB

Instruments on satellites in high-inclination orbits can measure the net reflected and thermally emitted radiation as a function of latitude, longitude, time of day, and season, covering the whole planet. The 'top of atmosphere' (TOA) radiative energy budget of the Earth, on both a global and regional basis, can then be built up from the balance between the measured ingoing and outgoing fluxes of radiation. Since the heating by the Sun and the cooling by the Earth are separated in wavelength, falling to the short- and long-wave sides, respectively, of about 4 pm, in principle the sensor need have only minimal spectral resolution, provided, however, that a high degree of radiometric accuracy is achieved.

Apart from the ubiquitous problems of absolute calibration in the hostile environment of space where the gradual deterioration of reference targets cannot be checked, real instruments cannot obtain a uniform spectral response over the wide wavelength ranges (roughly 0.4 to 4.0 pm for solar and 4.0 to 100 pm for thermal fluxes) required to measure the energy balance. If the radiation from the Earth or calibration target changes its distribution with wavelength, as well as its overall intensity, these are difficult to separate when analysing the data. Usually it is found necessary to break the wavelength range into segments that are measured and calibrated separately.

While blackened cavities, cones or grooved plates make reasonable reference sources for thermal-IR measurements, it is much more difficult to come up with a reliable and stable source of near-IR radiation of known intensity. Standard lamps and plates that diffuse the reflected light of the Sun into the instrument are commonly used, but are seldom completely reliable, requiring redundant sources to allow for failure or changes due to aging. In addition, integration of the measured radiances over a 2n solid angle is required, and where reflected sunlight is involved these may have strong directional components that are only imperfectly sampled due to the limitations of the observing geometry achievable from a single orbit. This limitation can be partially overcome by using angular scans that later can be integrated into hemispherical fluxes, and by fitting the data to empirical models of the reflectance properties of different regions of the Earth and integrating the model, with a corresponding addition to the error budget.

The first successful meteorological instrument on an orbiting satellite made radiation-budget measurements. Explorer 7, launched on October 13, 1959 carried a simple radiometer with five hemispherical thermistor detectors and crude wavelength selection by the use of black, white and gold coatings with differing sensitivities to solar and terrestrial radiation. With continuous data as the satellite circled the Earth at an inclination of 50° to the equator, it was possible to see for the first time how the energy from the Sun that is absorbed by the Earth is redistributed within the climate system, and finally re-emerges as longwave infra-red radiation.

NASA's Earth Radiation Budget Experiment (ERBE) scanners have three broad spectral channels, covering the wavelength ranges from 0.2 to 5.0 ¡m (i.e. effectively the entire solar spectrum), 5 to 50 ¡m (most of the Earth's thermal infra-red spectrum) and 0.2 to 50 ¡m (both). ERBE sensors flew on two Sun-synchronous polar orbiting satellites (NOAA-9 and N0AA-10) and on the Earth Radiation Budget Satellite (ERBS). The first of an improved version known as CERES (Clouds and the Earth's Radiant Energy System) was launched into a 35° inclination orbit in 1998 and the second in 2000 on Terra, part of the EOS series of satellites discussed further below. In Europe, radiation-budget sensors have been included on the geostationary platform Meteosat, using a field of view that covers the same part of the Earth at all times of day.

The error in the individual measurements of each component of the radiation budget in a single direction, place and time has been estimated at about ±5 W m~2, or about 2%, but the uncertainty in the overall balance of the planet is considerably larger than this, perhaps ±10% at best. By comparison, the so-called cloud-radiative forcing, a useful quantity in climate studies that is defined as the flux averaged over all conditions minus the clear-sky flux, is typically about 10 to 20% of the total. Thus, useful measurements of the local energy balance are achievable but global measurements with adequate precision are still beyond the current state-of-the-art, especially since our most important goal is to detect and understand changes in the global energy balance.

Measurements of the surface-radiation budget plays a key role in determining the surface temperature and evaporation and hence the hydrological cycle, with practical applications in weather forecasting, hydrology and agriculture over a wide range of time-scales. However, for climate monitoring and for evaluating climate models, high quality measurements of the radiation balance at the top of the atmosphere as well as at the surface must be available. In fact, algorithms exist for deriving the surface-radiation budget from the top-of-atmosphere radiationbudget measurements from satellites in both the solar and thermal regimes. Within the context of climate impact research, the distinct contributions from clear and cloudy skies can be investigated and identified.

Not only are the ERB at TOA and at the surface important, but also the difference between them, the atmospheric radiation budget. The concept of budgeting the radiative components of the Earth-atmosphere system is generally used to investigate the forcing exerted by changes in aerosol contents or variable or cloud cover and the vertical profile of the radiation divergence that determines the available potential energy for the dynamic processes of the atmosphere, allowing intercomparison with climate-model results and a better understanding of dynamical atmospheric processes. The main contribution to the profile comes from clouds and aerosols; for example, large plumes of Saharan dust and of smoke from biomass burning are frequently seen in satellite imagery. These have a significant impact on the radiation budget and on the retrieval of surface temperatures that need to be quantified and understood.

The available observations include detailed chemical composition measurements, aerosol physical characterization (size-distribution measurements), aerosol optical properties (scattering and absorption) and results from lidar profiles. Satellite data from the new Meteosat Second Generation satellites includes improved images that give a global climatology of cloud parameters, allowing studies of the diurnal cycle of convective clouds at an unprecedented temporal resolution, and, over the decadal lifetime of the MSG programme, an exciting insight into the interannual variability of this cycle and its influence on precipitation.

Clouds, their dynamics and properties remain the major uncertainty in determining the radiation budget. The precise effects of cloud feedback in climate trends are uncertain enough to completely overwhelm the forcing. The International Satellite Cloud Climatology Project (ISCCP), addresses this problem and provides the most extensive and comprehensive global cloud climatologies to provide information on how the cloud field is changing with time, either due to natural variability or to changed greenhouse forcing. Our understanding of the occurrence of cirrus clouds as well as of their microphysical and optical properties is incomplete. Furthermore, there are indications that the steadily growing air traffic may significantly increase high-level cloudiness. The availability of MSG observations will allow the existing uncertainties on the radiative forcing of natural as well as on aircraft-induced cirrus to be reduced. The combination of the instruments SEVIRI and GERB on MSG will be ideally suited to investigate the radiative forcing of cirrus clouds, because it provides the necessary information to infer cirrus optical and microphysical parameters from SEVIRI and to determine accurately and independently the radiative fluxes from GERB.

Aerosols affect the Earth's radiation budget both directly and indirectly, inducing an average global negative radiative forcing, a cooling effect, which may counteract global warming, positive forcing due to increases of the well-mixed greenhouse gases, estimated to be 2.43 W m~2 (IPCC 2001). Aerosols exhibit strong temporal and spatial variability, producing globally uneven patterns of radiative forcing (compared to greenhouse gases). Besides, there is a large uncertainty in the crucial aerosol radiative parameters (e.g. optical thickness and single-scattering albedo), which along with the small residence time of atmospheric aerosols (few days to weeks), and their variable concentrations, makes difficult the characterization of aerosols as climate-forcing agents, and the quantification of their radiative forcing difficult.

Although significant progress has been achieved in better characterizing the radiative forcing by different types of aerosols (e.g. sulphate but also carbonaceous or organics), the level of scientific physical understanding of climate forcing due to aerosols is still very low and remains one of the largest uncertainties in climate variability and climate-change studies. Relative-humidity changes affect aerosol liquid-water content, size and number distribution, total concentration, and composition, and hence change the extinction coefficient and refractive indices and thus the aerosol microphysical, optical, and radiative properties. The forcing is highly variable in consequence. The effect of reduced cloud cover due to the indirect effect of aerosols is to increase the magnitude of aerosol forcing, since this implies an increased clear-sky fraction, and hence, clear-sky direct aerosol forcing at TOA, within the atmosphere, and at the surface. Thus, there is a larger clear-sky effect of aerosols on the outgoing solar radiation at TOA (higher planetary cooling), more atmospheric warming and surface cooling, i.e. reduced downward solar radiation at the surface.

Finally, satellite data allow an assessment of current sea-ice variability and change in the polar regions and its possible modification by, and feedbacks to, climate change, as well as ramifications for the radiation budget.

The ERB instrument flown on Nimbus 7 is shown in Fig. 10.4. A special feature of this type of instrument is the response function for the thermal infra-red passband, which has to be wide enough to include effectively all of the energy from a 240 K blackbody, and also spectrally flat over the whole range (approximately 0.2 pm to 50 pm). This is achieved in the ERB design by using reflecting optics, except for a filter made of diamond. The thermal and reflected solar components are separated by measuring the latter separately with channels that include a filter made of Suprasil-W fused silica, which cuts off at about 3.8 pm, and taking the difference to get the longwave flux. The two bands are separated using a reflecting chopper, arranged so that a reference blackbody fills the beam during the part of the cycle when the target is not in view. Note the extensive baffling of the primary optics to cut out stray light.

The ERB instrument was calibrated before flight against active cavity radiometers similar to ACRIM, for the solar channels, and a specially fabricated cavity blackbody with an emissivity in vacuum of 0.995 and an operating range of 180 to 390 K, for the thermal component. The solar calibration is particularly difficult because the Earth's atmosphere modifies the wavelength distribution of the energy reaching the detector, so that if the instrument response function is not perfectly flat, as in practice must be the case, the calibration against the cavity instrument can be in error. This error is small in a well-designed instrument, and can be further reduced by calibrating with several targets (in this case solar simulators and tungsten bulbs) with different effective temperatures, or by using filters to check the calibration in different segments of the wavelength range.

flG. 10.4. Schematic of the ERBE (Earth Radiation Budget Experiment) instrument. The modulator or 'chopper' is a rotating blade, polished to be highly reflecting on both sides. The two sets of infra-red detectors, arranged symmetrically as shown, alternate between looking out at the Earth or at an internal reference black-body every 20 ms as the blade rotates. The different spectral channels are selected by filters in front of the detectors; here, only one detector-filter pair is shown on each side for clarity.

flG. 10.4. Schematic of the ERBE (Earth Radiation Budget Experiment) instrument. The modulator or 'chopper' is a rotating blade, polished to be highly reflecting on both sides. The two sets of infra-red detectors, arranged symmetrically as shown, alternate between looking out at the Earth or at an internal reference black-body every 20 ms as the blade rotates. The different spectral channels are selected by filters in front of the detectors; here, only one detector-filter pair is shown on each side for clarity.

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