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J. E. Harries

Abstract

A description is given of a program of observations, carried out over a three-year period, of the sub-millimeter (far-infrared) emission spectrum of the stratosphere. From the discrete emission lines which occur in this spectral region it is possible to measure the total concentration of H2O, and a new technique for doing this is described. The values of stratospheric humidity which have been obtained are presented and discussed as a function of time and of meteorological conditions. An analysis indicates interesting seasonal and longer term trends which should be investigated further.

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H. E. Brindley and J. E. Harries

Abstract

A recent comparison between data taken by two different satellite instruments, the Interferometric Monitor of Greenhouse Gases (IMG) that flew in 1997 and the Infrared Interferometer Spectrometer (IRIS) that flew in 1970, showed evidence of a change in the clear-sky greenhouse radiative forcing due to the increase in greenhouse gas concentrations between those years. A possibly even more intriguing question is whether the data can be used to extract unambiguous information about the radiative feedback processes that accompany such a change of forcing, especially cloud feedback. This paper is an investigation of this question, with particular reference to the uncertainties introduced into the differences between IMG and IRIS spectra due to their different patterns of temporal and spatial sampling. This has been approached by modeling the sampling problem, using high-resolution proxy scenes of top-of-the-atmosphere 11-μm brightness temperature, T B11, taken from International Satellite Cloud Climatology Project (ISCCP) data, sampled according to the characteristics of IRIS and IMG, respectively. The results suggest that while the sampling pattern of the IRIS instrument is sufficiently well distributed and dense to generate monthly regional mean brightness temperatures that are within 1.5 K of the true all-sky values, the IMG sampling is too sparse and yields results that differ from the true case by up to 6.0 K. Under cloud-free conditions the agreement with the true field for both instruments improves to within a few tenths of a kelvin. Comparisons with the observed IMG–IRIS difference spectra show that these uncertainties due to sampling presently limit the conclusions that can be drawn about climatically significant feedback processes. However, further analysis using the sampling characteristics of the Advanced Infrared Sounder (AIRS) instrument suggests that as climate change progresses, spectral measurements may be able to pick out significant changes due to processes such as cloud feedback.

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J. A. Griggs and J. E. Harries

Abstract

The observation of changes in the earth’s spectrally resolved outgoing longwave radiation (OLR) provides a direct method of determining changes in the radiative forcing of the climate system. An earlier study showed that satellite-observed changes in the clear-sky outgoing longwave spectrum between 1997 and 1970 from the Infrared Interferometer Spectrometer (IRIS) and Interferometic Monitor of Greenhouse Gases (IMG) instruments could be related to changes in greenhouse gas composition. The authors present a new study that extends this to 2003, through the first use of a new, independent source of global atmospheric infrared spectra, from the Atmospheric Infrared Sounder (AIRS) experiment. AIRS is a dispersion grating spectrometer, while the other two were Fourier transform spectrometers, and this is taken into account in the analysis. The observed difference spectrum between the years 2003 and 1970 generally shows the signatures of greenhouse gas forcing, and also shows the sensitivity of the signatures to interannual variations in temperature. The new 2003 data support the conclusions found in the earlier work, though, interestingly, the methane (CH4) Q branch centered at 1304 cm−1 exhibits more complex behavior, showing a decrease in intensity in the difference spectrum between 1997 and 2003. Sensitivity analysis indicates that this is due to changes in temperature structure, superposed on an underlying increase in CH4. Radiative transfer calculations based on reanalysis data are used to simulate the changes in the OLR spectrum; limitations in such data and possible variations that could account for several observed effects are discussed.

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J. M. Futyan, J. E. Russell, and J. E. Harries

Abstract

The high degree of cancellation between longwave (LW) and shortwave (SW) cloud radiative forcing (CRF) observed in the Pacific warm pool region has generally been assumed to be a property of all convective regions in the Tropics. Analysis of the (Earth Radiation Budget Experiment) ERBE-like data from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra satellite reveals that a similar degree of cancellation occurs over the African convective region at monthly and longer time scales, but only in the area average. In the Atlantic intertropical convergence zone (ITCZ), the degree of cancellation is lower, particularly during the summer months, where the area-average SW forcing typically exceeds the LW forcing by more than 20 W m−2. This behavior is similar to that found previously for the eastern Pacific ITCZ region, which is consistent with the similarity in dynamics between these two regions.

Over Africa, substantial seasonal and spatial variations in net CRF occur, with significant departures from cancellation within the convective region. These are explained here by a combination of surface albedo and cloud effects. In particular the large negative values of net CRF found in the summer months result from the inclusion of the radiative effects of low cloud present during the course of the month in the monthly mean cloud forcings. This work highlights the limitations of monthly mean radiation budget data for studies of rapidly evolving processes such as convection, indicating the need for studies at a higher time resolution.

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Harry E. Brown and Russell J. Younkin

The worst east coast storm of 1971–72 developed very rapidly and reached maturity at extreme southerly latitudes. To a large degree, the development was forecast more than two days in advance by use of the National Meteorological Center 6-layer primitive equation (P.E.) model. However, the P.E. prognosis contained systematic errors. Most were eliminated by the limited area fine mesh model (LFM) and NMC forecasters. NMC's total performance with the storm was nearly the best possible with the current state-of-the-art. The successful forecast of cyclogenesis led to equally successful forecasts of precipitation quantities, heavy snow, and storm surges, among other things. Examples are presented.

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A. J. Geer, J. E. Harries, and H. E. Brindley

Abstract

The use of multivariate fingerprints and spatial pattern correlation in the detection and attribution of climate change has concentrated on radiosonde temperature fields. However, the large body of radiance data from satellite-borne instruments includes contiguous datasets of up to 17 yr in length and in future years will present the most well-calibrated and large-scale data archive available for climate change studies. Here the authors give an example of the spatial correlation technique used to analyze satellite radiance data. They examine yearly mean brightness temperatures from High Resolution Infrared Spectrometer (HIRS) channel 12, sensitive to upper-tropospheric water vapor and temperature. Atmospheric profiles from a climate change run of the Hadley Centre GCM (HADCM2) are used to simulate the pattern of brightness temperature change for comparison to the satellite data. Investigation shows that strong regional brightness temperature changes are predicted in the Tropics and are dominated by changes in relative humidity in the upper troposphere. At midlatitudes only small changes are predicted, partly due to a compensation between the effects of temperature and relative humidity. The observational data showed some significant regional changes, especially at 60°S, where there was a trend toward lower brightness temperatures. The pattern similarity statistics revealed a small trend between 1979 and 1995 toward the predicted climate change patterns but this was not significant. The detection of any trend is complicated by the high natural variability of HIRS-12 radiances, which is partly associated with the El Niño–Southern Oscillation.

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H. E. Brindley, A. J. Geer, and J. E. Harries

Abstract

Several recent studies have highlighted the potential of utilizing statistical techniques to pattern match observations and model simulations in order to establish a causal relationship between anthropogenic activity and climate change. Up to now these have tended to concentrate upon the spatial or vertical patterns of temperature change. Given the availability of contiguous, global-scale satellite observations over the past two decades, in this paper the authors seek to employ an analogous technique to spatially match model predictions to directly measured radiances. As part of the initial investigations, the technique to channel 1 of the Stratospheric Sounding Unit, sensitive to stratospheric temperature and carbon dioxide concentrations, is applied. Over the majority of the globe the observations show a negative trend in brightness temperature, with significant decreases occurring throughout the Tropics. The influence of the volcanic eruptions of El Chichón and Mount Pinatubo can also be clearly identified. Simulated brightness temperature fields, against which the satellite data are compared, are calculated using atmospheric temperature profiles from a transient climate change run of the Hadley Centre GCM. The modeled change pattern also indicates a global reduction in brightness temperature but with an altered spatial distribution relative to the observations. This tendency is reflected in the trends seen in the correlation statistics. One, dominated by the spatial mean change, shows a significant positive trend; while the other, influenced by patterns around this mean, exhibits a reducing correlation with time. Possible reasons for this behavior are discussed, and the importance of both improving model parameterizations and performing additional“unforced” simulations to assess the role of natural variability is stressed.

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R. J. Bantges, H. E. Brindley, X. H. Chen, X. L. Huang, J. E. Harries, and J. E. Murray

Abstract

Differences between Earth’s global mean all-sky outgoing longwave radiation spectrum as observed in 1970 [Interferometric Infrared Spectrometer (IRIS)], 1997 [Interferometric Monitor for Greenhouse Gases (IMG)], and 2012 [Infrared Atmospheric Sounding Instrument (IASI)] are presented. These differences are evaluated to determine whether these are robust signals of multidecadal radiative forcing and hence whether there is the potential for evaluating feedback-type responses. IASI–IRIS differences range from +2 K in the atmospheric window (800–1000 cm−1) to −5.5 K in the 1304 cm−1 CH4 band center. Corresponding IASI–IMG differences are much smaller, at 0.2 and −0.8 K, respectively. More noticeably, IASI–IRIS differences show a distinct step change across the 1042 cm−1 O3 band that is not seen in IASI–IMG comparisons. This step change is a consequence of a difference in behavior when moving from colder to warmer scenes in the IRIS data compared to IASI and IMG. Matched simulations for the relevant periods using ERA reanalyses mimic the spectral behavior shown by IASI and IMG rather than by IRIS. These findings suggest that uncertainties in the spectral response of IRIS preclude the use of these data for quantitative assessments of forcing and feedback processes.

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M. Bithell, L. J. Gray, J. E. Harries, J. M. Russell III, and A. F. Tuck

Abstract

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°S on 15 October 1992, which indicate an abundance of methane in the height region 600–2000 K (∼30-l 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.

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Thijs Heus, Gertjan van Dijk, Harm J. J. Jonker, and Harry E. A. Van den Akker

Abstract

Mixing between shallow cumulus clouds and their environment is studied using large-eddy simulations. The origin of in-cloud air is studied by two distinct methods: 1) by analyzing conserved variable mixing diagrams (Paluch diagrams) and 2) by tracing back cloud-air parcels represented by massless Lagrangian particles that follow the flow. The obtained Paluch diagrams are found to be similar to many results in the literature, but the source of entrained air found by particle tracking deviates from the source inferred from the Paluch analysis. Whereas the classical Paluch analysis seems to provide some evidence for cloud-top mixing, particle tracking shows that virtually all mixing occurs laterally. Particle trajectories averaged over the entire cloud ensemble also clearly indicate the absence of significant cloud-top mixing in shallow cumulus clouds.

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