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A. J. Prata

Abstract

Further evidence is presented for a wavenumber 1 eastward traveling 4-day oscillation in the brightness temperatures of the polar winter stratosphere as measured by satellite. A wavenumber 2 eastward travelling 2-day oscillation is also observed, and appears to be phase locked to the 4-day wave. It is suggested that these oscillations am manifestations of a single propagating “warmpool” which obeys nonlinear dynamic.

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A. J. Prata

Abstract

Observations from the Nimbus–6 Pressure Modulator Radiometer (PMR) are presented showing evidence of a westward traveling wave of period 5 to 6 days. The wave is observed throughout the 20 month time span of the dataset. The amplitude ranges from 0.5 K at 45 km altitude to about 1.0 K at 85 km. The horizontal structure resembles that of the gravest symmetric wavenumber 1 free Rossby wave and the vertical structure is similar to that of a Lamb wave. There is no evidence of a vertically trapped wave in the summer mesosphere, although some asymmetry is noticeable. The PMR data have been supplemented by using data from the Nimbus- 5 Selective Chopper Radiometer (SCR) to investigate vertical structure and seasonal modulation of wave period. The results show that the wave amplitude grows with height but decreases at 62 km. The period is closer to 6 days in the spring and autumn and about 5.2 days in winter and summer.

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J. R. Garratt and A. J. Prata

Abstract

Previous work suggests that general circulation (global climate) models have excess net radiation at land surfaces, apparently due to overestimates in downwelling shortwave flux and underestimates in upwelling long-wave flux. Part of this excess, however, may be compensated for by an underestimate in downwelling longwave flux. Long term observations of the downwelling longwave component at several land stations in Europe, the United States, Australia, and Antarctica suggest that climate models (four are used, as in previous studies) underestimate this flux component on an annual basis by up to 10 W m−2, yet with low statistical significance. It is probable that the known underestimate in boundary-layer air temperature contributes to this, as would low model cloudiness and neglect of minor gases such as methane, nitrogen oxide, and the freons. The bias in downwelling longwave flux, together with those found earlier for downwelling shortwave and upwlling long-wave fluxes, are consistent with the model bias found previously for net radiation. All annually averaged fluxes and biases are deduced for global land as a whole.

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A. J. Prata and I. J. Barton

Abstract

A method is presented for determining the infrared optical depth of semitransparent clouds from satellite measurements. The technique employs cloud measurements at two infrared wavelengths and two angles. Using a simple but accurate model it is shown that the cloud optical depths at both wavelengths can be uniquely determined. Results of simulation studies are presented. The method will be used on data from the Along Track Scanning Radiometer on the first European remote-sensing satellite (ERS-1), which has the capability of providing multiangle multichannel measurements globally.

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I. J. Barton, A. J. Prata, and R. P. Cechet

Abstract

The Along Track Scanning Radiometer (ATSR) was launched on the ERS-1 satellite on 17 July 1991. During the following six months, a concentrated effort was made to validate the sea surface temperature (SST) derived from data supplied by this new generation radiometer. Ship and aircraft radiometers collected “ground truth” data that were coincident with ATSR measurements and thus allowed a comparison of the surface and space measurements. A large proportion of the early validation data was obtained during four research vessel cruises in Australian waters, and a detailed analysis of those results is presented here. Ancillary data were collected to support the shipborne radiometer measurements and to allow further analyses beyond the important validation task. These data included the standard surface meteorological data, bulk SST, and, in most cases, ship-launched radiosondes. Four different algorithms derived using a theoretical atmospheric transmission model were applied to the ATSR data to provide estimates of SST, and these estimates were compared to the surface-based measurements. All the algorithms gave reasonable agreement with each other as well as agreement with the surface data. The algorithm using all six infrared measurements gave the lowest standard deviation but showed a warm bias of 0.2 K when compared to the temperature of the skin layer of the ocean. The validation results show that the ATSR instrument can provide SST within the design accuracy of 0.3 K. The results of the validation presented here are in good agreement with those reported elsewhere using other datasets. Further improvements in SST accuracy, perhaps to 0.2 K, may be expected with a more rigorous analysis of the data.

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B. K. McAtee, A. J. Prata, and M. J. Lynch

Abstract

Radiometric measurements of the angular distribution of infrared (8–12 μm) radiation from a field site in a semiarid sandy soil location are reported. The aim of this study is to investigate the variation of emitted infrared radiation with zenith view angle. The motivation arises through the need to correct wide-field-of-view satellite measurements of land surface temperature for surface-induced zenith-angle effects. A new self-calibrating, scanning radiometer is used to acquire near-continuous measurements for zenith view angles up to 70°. The results suggest that, during the day, the measured brightness temperature is evidently affected by a relation between solar illumination and viewing angle through differential heating and shading. During the nighttime, a significant angular variation (up to 8%) in surface emissivity is discerned. The angular behavior of surface emission at night stemming from this variation is accurately modeled using the δ-Eddington approximation. Implications for satellite measurements of land surface temperature from the Advanced Very High Resolution Radiometer (AVHRR)-2, Geostationary Meteorological Satellite (GMS)-5, Moderate-Resolution Imaging Spectroradiometer (MODIS), and Along-Track Scanning Radiometer (ATSR)-2/Advanced Along-Track Scanning Radiometer (AATSR) are discussed.

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I. F. Grant, A. J. Prata, and R. P. Cechet

Abstract

The correction of a land surface albedo estimate made at one solar zenith angle (SZA) from a polar-orbiting satellite to a standard SZA or to a daily mean albedo requires knowledge of the dependence of the albedo on SZA. This paper uses ground-based measurements of the clear-sky albedo at a uniform grassland site at Uardry (34.39°S, 145.30°E) in southeastern Australia to investigate the accuracy to which the daily mean albedo can be inferred from the albedo at 1030 LST, given knowledge of the SZA dependence of albedo to various levels of detail. During nine months in which the daily mean albedo varied from 0.20 to 0.27, the albedo always had the expected minimum near noon but the strength of the albedo’s SZA dependence varied greatly. For a few months, albedos were up to 0.04 higher in the afternoon than in the morning, and variations on finer timescales of up to 0.02 also appeared in the diurnal albedo cycle for days or weeks. These features of the diurnal variation were all seen at two or three surface points separated by up to 750 m and so are expected to appear at the ∼1-km resolution of many satellite sensors. For the Uardry grassland site, the error in estimating the daily mean albedo from the 1030 LST, albedo can be up to 0.03, which is 15% of an albedo of 0.20, if the albedo is assumed to be constant through the day. The maximum error is reduced to about 0.02 if a simple model of the SZA dependence is used with even an approximate value for the parameter that controls the strength of the dependence, and to 0.01 or less if the strength of the dependence is appropriate to the state of the vegetation on the day. Afternoon–morning asymmetry in the albedo can contribute almost 0.01 to the error in inferring a daily albedo from a morning measurement.

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J. R. Garratt, A. J. Prata, L. D. Rotstayn, B. J. McAvaney, and S. Cusack

Abstract

An updated evaluation of the surface radiation budget in climate models (1994–96 versions; seven datasets available, with and without aerosols) and in two new satellite-based global datasets (with aerosols) is presented. All nine datasets capture the broad mean monthly zonal variations in the flux components and in the net radiation, with maximum differences of some 100 W m−2 occurring in the downwelling fluxes at specific latitudes. Using long-term surface observations, both from land stations and the Pacific warm pool (with typical uncertainties in the annual values varying between ±5 and 20 W m−2), excess net radiation (R N) and downwelling shortwave flux density (S o↓) are found in all datasets, consistent with results from earlier studies [for global land, excesses of 15%–20% (12 W m−2) in R N and about 12% (20 W m−2) in S o↓]. For the nine datasets combined, the spread in annual fluxes is significant: for R N, it is 15 (50) W m−2 over global land (Pacific warm pool) in an observed annual mean of 65 (135) W m−2; for S o↓, it is 25 (60) W m−2 over land (warm pool) in an annual mean of 176 (197) W m−2.

The effects of aerosols are included in three of the authors’ datasets, based on simple aerosol climatologies and assumptions regarding aerosol optical properties. They offer guidance on the broad impact of aerosols on climate, suggesting that the inclusion of aerosols in models would reduce the annual S o↓ by 15–20 W m−2 over land and 5–10 W m−2 over the oceans. Model differences in cloud cover contribute to differences in S o↓ between datasets; for global land, this is most clearly demonstrated through the effects of cloud cover on the surface shortwave cloud forcing. The tendency for most datasets to underestimate cloudiness, particularly over global land, and possibly to underestimate atmospheric water vapor absorption, probably contributes to the excess downwelling shortwave flux at the surface.

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P. W. Webley, D. Atkinson, R. L. Collins, K. Dean, J. Fochesatto, K. Sassen, C. F. Cahill, A. Prata, C. J. Flynn, and K. Mizutani
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