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I. J. Barton

Abstract

The Selective Chopper Radiometer on Nimbus 5 includes two narrow-band channels in the 2.7 μm absorption bands of carbon dioxide and water vapor. Because of strong absorption by these two constituents, no radiation reflected by the surface or low and mid level clouds is detected by the instrument, but for clouds higher than 6 km, a differential absorption technique can be used to determine cloud heights and reflectances (or amounts). A set of empirical equations is derived and used to analyze two and one-half years of Nimbus 5 data from December 1972 launch. Distributions of upper level clouds are derived on a 10° latitude-longitude grid and average monthly and seasonal maps of cloud cover are given. Reliable distributions are obtained in both the tropics and the temperate latitudes of the summer hemisphere, but in other situations the quality decreases as the relevant path lengths increase due to both higher solar zenith angles and lower “upper level” clouds. The maps are highlighted by the excessive cover in the region of the southeast Asian monsoon and the persistently clear area in the eastern Pacific associated with the descending arm of the Walker Circulation of the Southern Oscillation. As an example o year-to-year variation the monthly distributions for each January are discussed in relation to the 1972–73 El Niño phenomenon.

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I. J. Barton

Abstract

The parameterization of evaporation proposed by Priestley and Taylor (1972) is modified to take account of nonsaturated surfaces. It is suggested that evaporation can be expressed as a function of net radiation, air temperature, a constant related to the nature of the surface, and a variable that is dependent on moisture availability. The constant is equal to 1.26 for water and open grassland surfaces (as in Priestley and Taylor) but is nearer unity for some other surfaces. The variable is found to be directly related to near-surface soil moisture content for both bare and grassland surfaces.

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I. J. Barton

Abstract

A band model of atmospheric transmission is used to investigate some of the multichannel sea surface temperature (MCSST) algorithms that are currently in use. The model is used with a set of atmospheric data to derive algorithms for the AVHRR and VAS radiometers that are on the operational NOAA and GOES satellites respectively. For the AVHRR instrument good agreement is found between the model algorithms and those derived in a recent study by McClain. The model results also show that in most cases it is necessary to take account of the satellite view angle. Poorer agreement is found between the theoretical (model) algorithms for the VAS split-window channels and those obtained by regression analyses of satellite data against a multitude of buoy measurements.

Intensive ground-truth data are used to evaluate the algorithms. A comparison between measured deficits (sea surface temperatures minus satellite brightness temperatures) and model deficits suggests that the water vapor absorption coefficients used in the model may be too small. This suggestion is supported by ground-based radiometer measurements of clear sky emission.

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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 and R. P. Cechet

Abstract

Satellite measurements of sea surface temperature (SST) are regularly available from data supplied by the AVHRR instruments on the NOAA meteorological satellites. In cloudless areas SST is derived from the infrared data using a differential absorption technique to correct for the effect of the atmosphere. For the AVHRR data a multichannel (multiwavelength) approach is used and global operational algorithms are in use. During 1990 a new instrument that has been specifically designed to measure SST will be launched on the European satellite, ERS-1. The Along Track Scanning Radiometer (ATSR) will provide six infrared measurements for each pixel on the earth's surface. Using the same differential absorption techniques, a multitude of algorithms for providing SST will then be possible. In this note a technique is described that will enable the comparison and optimization of SST algorithms and will also aid in the selection of the most appropriate algorithm for ATSR data analysis.

To demonstrate the technique mosaic images were constructed from small areas of cloud-free infrared images of the sea surface as seen by the NOAA-9 AVHRR. Each area was approximately 55 km by 55 km and, by arranging them in order of decreasing mean temperature and increasing mean zenith angle, it was possible to use an image analysis system to compare the relative performance of different algorithms for deriving surface temperature. The images were also used to compare some NOAA-7 SST algorithms.

A second set of mosaic images was constructed using NOAA-10 AVHRR data collected on the same night and for the same surface location. Images of SST derived with theoretical NOAA-10 algorithms were compared with those from an operational NOAA-9 algorithm. Then a simple optimization technique was used to obtain a new algorithm for deriving SST from channels 3 and 4 of the NOAA-10 instrument. This optimization scheme, using an ordered mosaic image that covers a wide range of conditions (location, local zenith angle, or some other parameter), should be applicable to the comparison and optimization of other satellite data products.

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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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C. M. R. Platt, A. C. Dilley, J. C. Scott, I. J. Barton, and G. L. Stephens

Abstract

The infrared properties and structures of some anvils emanating from local thunderstorms were studied by lidar and infrared radiometry at Darwin, tropical Northern Australia. The anvils were typically from 1 to 2 km deep, at altitudes from 7 to 16 km and at temperatures from −15 to −70°C. There was a rough dependence of infrared emittance on temperature, but there was also a dependence on the age of the anvil. The average altitude and calculated wide-band greybody flux emittance were 11 km and 0.65 respectively.

One dense cloud appeared “superblack” when observed from below, due to reflection of upwelling warm radiation from the surface. The magnitude of the effect agreed within experimental error with that predicted from computations on a model cloud of ice cylinders, but was about twice that computed for a model of ice spheres.

Calculated rates of heating in the very cold clouds were very high, reaching 4°C h−1 near cloud base. The survival of these clouds for several hours suggests that the absorbed radiant heat was converted largely into sensible heat in the atmosphere rather than causing evaporation of the crystals.

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C. J. Donlon, S. J. Keogh, D. J. Baldwin, I. S. Robinson, I. Ridley, T. Sheasby, I. J. Barton, E. F. Bradley, T. J. Nightingale, and W. Emery

Abstract

Satellite sea surface skin temperature (SSST) maps are readily available from precisely calibrated radiometer systems such as the ERS along-track scanning radiometer and, in the near future, from the moderate-resolution imaging spectroradiometer. However, the use of subsurface bulk sea surface temperature (BSST) measurements as the primary source of in situ data required for the development of new sea surface temperature algorithms and the accurate validation of these global datasets is questionable. This is because BSST measurements are not a measure of the sea surface skin temperature, which is actually observed by a satellite infrared radiometer. Consequently, the use of BSST data for validation and derivation of satellite derived “pseudo-BSST” and SSST datasets will limit their accuracy to at least the rms deviation of the BSST–SSST difference, typically about ±0.5 K. Unfortunately, the prohibitive cost and difficulty of deploying infrared radiometers at sea has prevented the regular collection of a comprehensive global satellite SSST validation dataset. In response to this situation, an assessment of the TASCO THI-500L infrared radiometer system as a potential candidate for the widespread validation of satellite SSST observations is presented. This is a low-cost, broadband radiometer that has been commonly deployed in the field to measure SSST by several research groups. A comparison between SSST derived from TASCO THI-500L measurements and contemporaneous scanning infrared sea surface temperature radiometer measurements, which are accurate to better than 0.1 K, demonstrates low bias (0.1 K) and rms (0.08 K) differences between the two instruments. However, to achieve this accuracy, the TASCO THI-500L radiometer must be deployed with care to ensure that the radiometer fore-optics are kept free of salt water contamination and shaded from direct sunlight. When this is done, this type of low-cost radiometer system could form the core of a global SSST validation program.

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C. J. Donlon, P. J. Minnett, C. Gentemann, T. J. Nightingale, I. J. Barton, B. Ward, and M. J. Murray

Abstract

A poor validation strategy will compromise the quality of satellite-derived sea surface temperature (SST) products because confidence limits cannot be quantified. This paper addresses the question of how to provide the best operational strategy to validate satellite-derived skin sea surface temperature (SSTskin) measurements. High quality in situ observations obtained using different state-of-the-art infrared radiometer systems are used to characterize the relationship between the SSTskin, the subsurface SST at depth (SSTdepth), and the surface wind speed. Data are presented for different oceans and seasons. These data indicate that above a wind speed of approximately 6 m s−1 the relationship between the SSTskin and SSTdepth, is well characterized for both day- and nighttime conditions by a cool bias of −0.17 ± 0.07 K rms. At lower wind speeds, stratification of the upper-ocean layers during the day may complicate the relationship, while at night a cooler skin is normally observed. Based on these observations, a long-term global satellite SSTskin validation strategy is proposed. Emphasis is placed on the use of autonomous, ship-of-opportunity radiometer systems for areas characterized by prevailing low–wind speed conditions. For areas characterized by higher wind speed regimes, well-calibrated, quality-controlled, ship and buoy SSTdepth observations, corrected for a cool skin bias, should also be used. It is foreseen that SSTdepth data will provide the majority of in situ validation data required for operational satellite SST validation. We test the strategy using SSTskin observations from the Along Track Scanning Radiometer, which are shown to be accurate to approximately 0.2 K in the tropical Pacific Ocean, and using measurements from the Advanced Very High Resolution Radiometer. We note that this strategy provides for robust retrospective calibration and validation of satellite SST data and a means to compare and compile in a meaningful and consistent fashion similar datasets. A better understanding of the spatial and temporal variability of thermal stratification of the upper-ocean layers during low–wind speed conditions is fundamental to improvements in SST validation and development of multisensor satellite SST products.

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I. J. Barton, P. J. Minnett, K. A. Maillet, C. J. Donlon, S. J. Hook, A. T. Jessup, and T. J. Nightingale

Abstract

The second calibration and intercomparison of infrared radiometers (Miami2001) was held at the University of Miami's Rosenstiel School of Marine and Atmospheric Science (RSMAS) during a workshop held from May to June 2001. The radiometers targeted in these two campaigns (laboratory-based and at-sea measurements) are those used to validate the skin sea surface temperatures and land surface temperatures derived from the measurements of imaging radiometers on earth observation satellites. These satellite instruments include those on currently operational satellites and others that will be launched within two years following the workshop. The experimental campaigns were completed in one week and included laboratory measurements using blackbody calibration targets characterized by the National Institute of Standards and Technology (NIST), and an intercomparison of the radiometers on a short cruise on board the R/V F. G. Walton Smith in Gulf Stream waters off the eastern coast of Florida. This paper reports on the results obtained from the shipborne measurements.

Seven radiometers were mounted alongside each other on the R/V Walton Smith for an intercomparison under seagoing conditions. The ship results confirm that all radiometers are suitable for the validation of land surface temperature, and the majority are able to provide high quality data for the more difficult validation of satellite-derived sea surface temperature, contributing less than 0.1 K to the error budget of the validation. The measurements provided by two prototype instruments developed for ship-of-opportunity use confirmed their potential to provide regular reliable data for satellite-derived SST validation. Four high quality radiometers showed agreements within 0.05 K confirming that these instruments are suitable for detailed studies of the dynamics of air–sea interaction at the ocean surface as well as providing high quality validation data. The data analysis confirms the importance of including an accurate correction for reflected sky radiance when using infrared radiometers to measure SST. The results presented here also show the value of regular intercomparisons of ground-based instruments that are to be used for the validation of satellite-derived data products—products that will be an essential component of future assessments of climate change and variability.

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