Interannual variability in spectrally resolved longwave radiances is quantified at a variety of spatial scales using 5 yr of IASI observations. Maximum variability is seen at the smallest scales investigated (10 degrees zonal means) at northern and southern high latitudes across the center of the 15-mu m CO2 band. As the spatial scale increases, the overall magnitude of interannual variability is reduced across the spectrum and the spectral shape of the variability changes. In spectral regions sensitive to conditions in the upper troposphere, the effect of increasing spatial scale is relatively small and at the global scale these parts of the spectrum show the greatest year-to-year variability. Conversely, the atmospheric window (8-12 mu m), which is sensitive to variations in surface temperature and cloud, shows a marked reduction in interannual variability with increasing spatial scale. Over the 5 yr studied, at global scales the standard deviation in annual mean brightness temperature is less than 0.17K across the spectrum, dropping to less than 0.05K across the window. Spectrally integrating the IASI measurements to create pseudobroadband and window channels indicates a variation about the mean that is higher for the broadband channel than for the window channel at the global and quasi-global scales and over the Southern Hemisphere. These findings are in agreement with observations from CERES Terra over the same period and imply that at the largest spatial scales, over the period considered here, fluctuations in mid-to upper-tropospheric temperatures and water vapor, and not cloud or surface temperature, play the dominant role in determining the level of interannual variability in all-sky outgoing longwave radiation.

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24 μm and 120 μm (85–420 cm −1 ) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer–Tobin-Clough–Kneizys–Davies (MT-CKD) 2.5 model parametrization above 300 cm −1 , but indicates the need to strengthen the parametrization below 300 cm −1 , by up to 50 per cent at 100 cm −1 . Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.

Recent observations and model studies of the earth's radiative energy balance have focused attention on the earth's top of atmosphere (TOA) energy balance. This is the balance between the shortwave energy absorbed by the earth, which is represented by a spatially and temporally averaged absorbed flux (F) over bar (down arrow), and the emitted longwave energy, which is represented by the corresponding averaged emitted flux (F) over bar (up arrow). The TOA average net flux (F) over bar (N) is defined as the difference between the two over the averaged area and time, which may be a local, regional, or global average. A global nonzero net flux represents a measure of imbalance between the energy being absorbed and emitted by the earth for the time interval in question. It is of interest to ask what the natural variability of the net flux might be and whether, during times of climate change, signals of important climate change processes might be detected against this natural background variation; examples of these signals include evidence of ocean heat storage, the effects of El Nino, and the radiative effects of volcanic eruptions. In this paper, the authors review the significance of the net flux, survey the observational evidence from a range of satellite instruments over several decades, and analyze some of the most recent observations from the Clouds and the Earth's Radiant Energy System (CERES) program to determine what signals and what natural variability might be expected in the TOA net flux. Based on this analysis, the use of broadband radiation measurements for global climate change studies can be assessed.

Observed infrared spectral radiances from the Atmospheric Emitted Radiance Interferometer (AERI) during a severe desert dust storm in March 2006 over West Africa were compared to spectrally resolved, radiative transfer calculations. A good quality of fit was observed in the thermal infrared (7-14 mu m) bands however a significantly poorer fit was observed in the mid-near infrared (3-5 mu m) region. The sensitivity of the simulated radiance to the assumed particle shape and mono-modal size distribution was studied. Simulations using the scattering properties of prolate and oblate spheroids were compared to simulations from spheres. However, little sensitivity to shape was observed throughout the thermal and mid-near IR regions. Greater sensitivity to the size distribution shape was found; with residuals between observations and simulations in the 3-5 mu m region reduced when using a size distribution with a smaller width and a higher mode radius. However, the opposite was observed for the thermal IR region, suggesting a bi-modal distribution with different widths and mode radii would produce an Optimum fit in both bands simultaneously.

The paper presents a review of the far‐infrared (FIR) properties of the Earth's atmosphere and their role in climate. These properties have been relatively poorly understood, and it is one of the purposes of this review to demonstrate that in recent years we have made great strides in improving this understanding. Seen from space, the Earth is a cool object, with an effective emitting temperature of about 255 K. This contrasts with a global mean surface temperature of ∼288 K and is due primarily to strong absorption of outgoing longwave energy by water vapor, carbon dioxide, and clouds (especially ice). A large fraction of this absorption occurs in the FIR, and so the Earth is effectively a FIR planet. The FIR is important in a number of key climate processes, for example, the water vapor and cloud feedbacks (especially ice clouds). The FIR is also a spectral region which can be used to remotely sense and retrieve atmospheric composition in the presence of ice clouds. Recent developments in instrumentation have allowed progress in each of these areas, which are described, and proposals for a spaceborne FIR instrument are being formulated. It is timely to review the FIR properties of the clear and cloudy atmosphere, the role of FIR processes in climate, and its use in observing our planet from space.

Resolving uncertainties surrounding the nature of future climate change is currently one of the greatest challenges facing mankind. Validation of global climate model (GCM) predictions of the currently much miss-represented cloud radiative feedback requires measurements made from orbit of the Earth Radiation Budget (ERB), specifically targeted at clouds. The ERB parameters for measure are the scattered solar or short wave (SW, 0.3 5µm) and the emitted thermal or long wave radiance (LW, 5 100µm). Such measurements map out the heat source/sink locations that drive all weather and climate, which acts like a complex network of coupled heat engines. The Clouds and the Earth’s Radiant Energy System (CERES), as part of NASA’s Earth Observing System (EOS), uses thermistor bolometer detectors to provide global high spatial resolution ERB measurements from Low Earth Orbit (LEO) space platforms. However, comprehensive validation of GCM prediction of cloud processes requires ERB measurements on a sub-hourly timescale to sample throughout the cloud formation/dissipation process. This is not practically possible with the use of LEO platforms, so ERB measurements are needed from synchronous orbits such as geostationary in order to increase the frequency of measurements. The Geostationary Earth Radiation Budget (GERB) experiment is a European Space Agency (ESA) project on board the spin stabilized Meteosat second Generation (MSG) platform. Location in geostationary orbit and the use of an array of thermopile detectors enables sampling of ERB radiances from the entire Earth disc at an optimum 15 minute temporal resolution. This study describes the instrumentation and sampling capabilities of the current GERB mission. Given its success it is proposed that efforts be made to expand the project and place GERB-like instruments on multiple high orbit platforms. This will provide climatologists with much needed global ERB data at sub-hourly time resolution, making it more likely that model predictions of future climate change can be properly validated.

The influence of atmospheric humidity and clouds on the Earth’s climate system is one of the major uncertainties in our present understanding of how the climate system works. Recent work has explained in detail how the infrared cooling to space of the Earth’s surface and cloud-free atmosphere occurs, as a function of altitude and of spectral frequency. Clough et al, 1992, showed that a significant contribution to the outgoing longwave radiation (OLR) originated from the upper troposphere in the far IR between 100cm and 500cm. Further theoretical work by Sinha and Harries (1997), have looked into the proportion of the OLR that can be attributed to the pure rotation band, concluding that 3070% of the OLR originates from this band, depending on latitude. The spectrally resolved heating rate is key to our understanding of heat loss from the atmosphere and is defined as:

Nature 410, 355; 2001 In Fig. 1a of this paper, the labels for the two curves were inadvertently switched. The grey curve represents IMG and the black curve represents IRIS.

Recent studies have highlighted the important contribution made by the far i.r. component of the water vapour spectrum to present-day greenhouse forcing, and the possible influence of this region upon future climate. In order to further assess these findings at high spectral resolution a method allowing the calculation of longwave clear sky fluxes and cooling rates based on the GENLN2 line-by-line radiative transfer code has been implemented, and studies have been carried out of the sensitivity and response of the outgoing planetary radiation to changes in atmospheric composition. Comparisons with other line-by-line model show typical cooling rate agreement of within 0.05 K day−1 at all pressure levels. Taking two model atmospheres representative of tropical (TRP) and sub-arctic winter (SAW) conditions, several experiments have been performed to assess both the present-day greenhouse effect, and the impact of perturbations in carbon dioxide and water vapour amounts on the greenhouse trapping of each atmosphere. The results reinforce the importance of the far i.r. with, for the present-day conditions, one third and one quarter of the total absolute greenhouse forcing calculated to occur in this region for the SAW and TRP atmospheres respectively. The sensitivity of the normalised greenhouse effect to water vapour concentrations is greatest in the far i.r. for the SAW atmosphere, and in the window region for the TRP; this sensitivity differing most between atmospheres in the far i.r. The sensitivity to vertical variations in water vapour perturbation shows a peak response located at mid-tropospheric levels within the window region for the TRP case, and within the far i.r. at upper tropospheric levels for the SAW atmosphere. The carbon dioxide perturbation experiments indicate the increasingly important role played by the weaker carbon dioxide bands as concentrations increase, and provide a further illustration of the fallacy of the suggestion of a `CO2 saturated' atmosphere.

Measurements of the transmission of solar infrared radiation through the earth's atmosphere, by the HALOE experiment on the UARS spacecraft, were made at high southern latitudes during October 1991. These observations are direct measurements of atmospheric transmission, and do not need to be passed through a composition or temperature retrieval process; they are, therefore, amenable to direct interpretation. During October 1991 the profiles of transmittance versus height in the atmosphere show clear evidence for the arrival (from more northerly latitudes) of layers of some material which absorbs infrared radiation, at heights of up to 28 km. The spatial structure of the absorbing material shows considerable variability with longitude at a given latitude. The spectral properties of the detected absorption, measured at the various HALOE wavelengths, are consistent with absorption by sulphate aerosol, and clearly implicate the volcanic eruptions from Mts Pinatubo and Hudson during 1991. These results provide direct and detailed evidence for the arrival of layers of what appears to be sulphate aerosol during the 1991 southern spring, at latitudes as high as 80 degrees south.

Measurements of stratospheric composition have now been made by the Halogen Occultation Experiment (HALOE) on board NASA's Upper Atmospheric Research Satellite (UARS) since October 1991. Amongst the parameters measured are water vapor, H2O, and methane, CH4. These species comprise the dominant components of the total hydrogen budget in the lower stratosphere, but not so at higher levels, where the molecular hydrogen, H2, component is significant, and at high altitudes is dominant. This paper reports on measurements of the water vapor and hydrogen fields in the stratosphere and mesosphere, and on studies of the derivation of molecular hydrogen in the mesosphere.

The degree to which the Southern Hemisphere polar vortex is isolated against horizontal (isentropic) mixing is investigated using data from the Halogen Occulation Experiment (HALOE), U.K. Meteorological Office (UKMO) potential vorticity (PV), and contour advection diagnostics. Measurements of methane and water vapor taken by HALOE during a disturbed period in the Southern Hemisphere springtime (21 September - 15 October 1992) are interpreted in light of the prevailing synoptic meteorology. Daily fields of winds and PV are shown to be essential in the interpretation of the data. A climatological high pressure region is responsible for a distorted vortex, and a substantial ''vortex stripping'' event is present, associated with the early stages of vortex breakdown. This leads to significant temporal, zonal, and altitudinal variations in the distribution of tracers. The authors point out the difficulties this presents for the interpretation of solar occultation data, especially with regard to the use of zonal average time series. Longitude-height methane distributions from two days during the period are examined. Both days show substantial variations in abundance around a latitude circle. In particular the authors investigate HALOE measurements at 77-degrees-S on 15 October 1992, which indicate an abundance of methane in the height region 600-2000 K (approximately 30-1 mb) that is more typical of midlatitude air. Similar distributions, observed in the 1991 HALOE data, have previously been interpreted as evidence for the penetration of midlatitude air into the vortex. Gradients of potential vorticity and contour advection diagnostics are employed to examine whether the UKMO winds are consistent with this hypothesis in 1992. Although midlatitude air is able to penetrate poleward of the main jet core by advection processes alone, an essentially intact inner core of vortex air remains, which does not mix to any great extent with air from lower latitudes. The authors show that the high-latitude HALOE abundances that are typical of midlatitude air were observed in a region of extensive filamentation and mixing, rather than within the inner, more isolated, core.

HALOE observations of water vapor and methane during the period 21 September–15 October 1992 are used to examine the role of Antarctic drying in the lower stratosphere. Zonal mean cross‐sections of [2 CH4+H2O] show the probability of transport of Antarctic type dryness to latitudes as distant as 20°N, with major water vapor deficits evident between 10 and 100 mb to 10°S. Examination of monthly mean tropical 100 mb temperatures and of Antarctic temperatures suggests that the observations are consistent with stratospheric dryness being achieved by the combined effects of tropopause freeze‐drying over the Micronesia region during northern winter and drying through the influence of the very low temperatures over Antarctica during southern winter. This paper presents these intriguing new results, and offers a possible explanation.

The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) spacecraft September 12, 1991, and after a period of outgassing, it began science observations October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure with an instantaneous vertical field of view of 1.6 km at the Earth limb. Latitudinal coverage is from 80°S to 80°N over the course of 1 year and includes extensive observations of the Antarctic region during spring. The altitude range of the measurements extends from about 15 km to ≈ 60–130 km, depending on channel. Experiment operations have been essentially flawless, and all performance criteria either meet or exceed specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere.

HALOE observations of O3, CH4, HF, H2O, NO, NO2, and HCℓ collected during the October 1991 Antarctic spring period are reported. The data show a constant CH4 mixing ratio of about 0.25 ppmv for the altitude range from 65 km down to about 25 km at the position of minimum wind speed in the vortex: i.e., the vortex center, and depressions in pressure versus longitude contours of NO, NO2, HF, and HCℓ in this same region. Water vapor, HF, and HCℓ enhancement are also observed in the vortex center region above ∼25 km. Between 10 and 20 km, the expected mixing ratio signatures exist within the vortex, i.e., low ozone and dehydration. The water vapor increased by 50%, and the ozone level doubled inside the vortex between October 11 and 24 in the 15 to 20 km layer. These changes imply a time constant for recovery from ozone hole conditions or 19 and 30 days for O3 and H2O, respectively. The data further show the presence of air inside the vortex between 3 and 30 mb which has mixing ratios characteristic of mid latitudes.