Advanced Very High Resolution Radiometer Instruments Research Articles

Infrared Radiative Properties of the Antarctic Plateau from AVHRR Data. Part I: Effect of the Snow Surface

The effective scene temperature, or “brightness temperature,” measured in channel 3 (3.5–3.9 μm) of the Advanced Very High Resolution Radiometer (AVHRR) is shown to be sensitive, in principle, to the effective particle size of snow grains on the Antarctic plateau, over the range of snow grain sizes reported in field studies. In conjunction with a discrete ordinate method radiative transfer model that couples the polar atmosphere with a scattering and absorbing snowpack, the thermal infrared channels of the AVHRR instrument can, therefore, be used to estimate effective grain size at the snow surface over Antarctica. This is subject to uncertainties related to the modeled top-of-atmosphere bidirectional reflectance distribution function resulting from the possible presence of sastrugi and to lack of complete knowledge of snow crystal shapes and habits as they influence the scattering phase function. However, when applied to NOAA-11 and NOAA-12 AVHRR data from 1992, the snow grain effective radii of order 50 μm are retrieved, consistent with field observations, with no apparent discontinuity between two spacecraft having different viewing geometries. Retrieved snow grain effective radii are 10–20-μm larger when the snow grains are modeled as hexagonal solid columns rather than as spheres with a Henyey–Greenstein phase function. Despite the above-mentioned uncertainties, the retrievals are consistent enough that one should be able to monitor climatically significant changes in surface snow grain size due to major precipitation events. It is also shown that a realistic representation of the surface snow grain size is critical when retrieving the optical depth and effective particle radius of clouds for the optically thin clouds most frequently encountered over the Antarctic plateau.

Satellite and ground based monitoring of aerosol plumes

Plumes of atmospheric aerosol have been studied using a rangeof satellite and ground-based techniques. The Sea-viewing WideField-of-view Sensor (SeaWiFS) has been used to observe plumesof sulphate aerosol and Saharan dust around the coast of theUnited Kingdom. Aerosol Optical Thickness (AOT) was retrievedfrom SeaWiFS for two events; a plume of Saharan dusttransported over the United Kingdom from Western Africa and aperiod of elevated sulphate experienced over the Easternregion of the UK. Patterns of AOT are discussed and related tothe synoptic and mesoscale weather conditions. Furtherobservation of the sulphate aerosol event was undertaken usingthe Advanced Very High Resolution Radiometer instrument(AVHRR). Atmospheric back trajectories and weather conditionswere studied in order to identify the meteorologicalconditions which led to this event. Co-located ground-basedmeasurements of PM10 and PM2.5 were obtained for 4sites within the UK and PM2.5/10 ratios were calculatedin order to identify any unusually high or low ratios(indicating the dominant size fraction within the plume)during either of these events. Calculated percentiles ofPM2.5/10 ratios during the 2 events examined show thatthese events were notable within the record, but were in noway unique or unusual in the context of a 3 yr monitoringrecord. Visibility measurements for both episodes have beenexamined and show that visibility degradation occurred duringboth the sulphate aerosol and Saharan dust episodes.

A practical method for simulating AVHRR-consistent NDVI data series using narrow MODIS channels in the 0.5-1.0 μm spectral range

Over the past two decades, a key indicator of climate change has been the long time series of global maps of the normalized difference vegetation index (NDVI), derived from remotely sensed data acquired with a series of NOAA advanced very high resolution radiometer (AVHRR) instruments from space. These NDVI values are calculated from relatively broad AVHRR channels in the red and near-infrared regions. Continuation of this long term data set is extremely valuable for climate-related research, However, sometime in the coming decade, the AVHRR time series measurements will no longer be continued. Instead, the measurements will be made using newer generation satellite instruments having narrower channels and improved spatial resolution. For example, the moderate resolution imaging spectroradiometer (MODIS) onboard the Terra spacecraft has several narrow channels in the 0.4-1.0 spectral range. The NDVI values derived from the MODIS red channel and near-IR channel will be biased compared to those derived from the broader AVHRR channels because of differences in channel positions and widths for the two instruments. The narrow MODIS near-IR channel is only slightly affected by atmospheric water vapor absorption, while the broad AVHRR near-IR channel is strongly affected by water vapor absorption. As a result, the largest bias comes from the near-IR channels on the two instruments. To a lesser extent, the bias also comes from the differences between the red channel positions and the widths of MODIS and AVHRR instruments. In this paper, the authors describe a practical method for simulating AVHRR NDVI values using several narrower MODIS channels in the 0.4-1.0 /spl mu/m spectral range, including the MODIS green channel and the water vapor absorption channel.

Impact of AVHRR filter functions on surface temperature estimation from the split window approach

With the development of long-term remotely-sensed data sets through the National Oceanic and Atmospheric Administration (NOAA) and National Aeronautics and Space Administration (NASA) Pathfinder programme, decadal length studies of the land surface are being conducted. However, data from five different Advanced Very High Resolution Radiometer (AVHRR) instruments and thus five different filter functions for each thermal channel must be used. If variations in sensor filter functions are not taken into account when the split window technique is applied, considerable error in the estimation of land surface temperature can be produced. For example, the Becker and Li (1990) equation was designed to be used with data from the AVHRR instrument on the NOAA-9 satellite. If it is applied to data from an AVHRR sensor other than NOAA-9's, the error in surface temperature can be as large as 2.3 degK.

Development, validation, and potential enhancements to the second‐generation operational aerosol product at the National Environmental Satellite, Data, and Information Service of the National Oceanic and Atmospheric Administration

A revised (phase 2) single‐channel algorithm for aerosol optical thickness, τSATA, retrieval over oceans from radiances in channel 1 (0.63 μm) of the advanced very high resolution radiometer (AVHRR) has been implemented at the National Oceanic and Atmospheric Administration's National Environmental Satellite Data and Information Service for the NOAA 14 satellite launched December 30, 1994. It is based on careful validation of its operational predecessor (phase 1 algorithm), implemented for NOAA 11 in 1989. Both algorithms scale the upward satellite radiances in cloud‐free conditions to aerosol optical thickness using an updated radiative transfer model of the ocean and atmosphere. Application of the phase 2 algorithm to three matchup Sun‐photometer and satellite data sets, one with NOAA 9 in 1988 and two with NOAA 11 in 1989 and 1991, respectively, show systematic error is less than 10%, with a random error of στ≈0.04. First results of τSATA retrievals from NOAA 14 using the phase 2 algorithm, and from checking its internal consistency, are presented. The potential two‐channel (phase 3) algorithm for the retrieval of an aerosol size parameter, such as the Junge size distribution exponent, by adding either channel 2 (0.83 μm) from the current AVHRR instrument, or a 1.6‐μm channel to be available on the Tropical Rainfall Measurement Mission and the NOAA‐KLM satellites by 1997 is under investigation. The possibility of using this additional information in the retrieval of a more accurate estimate of aerosol optical thickness is being explored.

Improved cloud detection for daytime AVHRR scenes over land

Accurate cloud detection in Advanced Very High Resolution Radiometer (AVHRR) data over land is a difficult task complicated by spatially and temporally varying land surface reflectances and emissivities. The AVHRR Split-and-Merge Clustering (ASMC) algorithm for cloud detection in A VHRR scenes over land provides a computationally efficient, scene-specific, objective way to circumvent these difficulties. The algorithm consists of two steps: 1) a split-and-merge clustering of the input data (calibrated channel 2 albedo, calibrated channel 4 temperature, and a channel 3- channel 4 temperature difference), which segments the scene into its natural groupings; and 2) a cluster-labelling procedure that uses scene-specific, joint three-dimensional adaptive labelling thresholds (as opposed to constant static thresholds) to label the clusters as either cloud, cloud free land, or uncertain. The uncertain class is used for those pixels whose signature is not clearly cloud free land or cloud (e.g., pixels at cloud boundaries that often contain subpixel cloud and land information that has been averaged together by the integrating aperture function of the AVHRR instrument). Results show that the ASMC algorithm is neither regionally nor temporally specific and can be used over a large range of solar altitudes. Sensitivity of the segmentation and labelling steps to the choice of input variables also was studied. Results obtained with the ASMC algorithm also compare favorably with those obtained from a wide range of currently used algorithms to detect cloud over land in A VHRR data. Moreover, the ASMC algorithm can be adapted for use with data to be taken by the Moderate Resolution Imaging Spectrometer-Nadir (MODIS-N).

Satellite‐derived sea surface temperatures: Current status

The current status of techniques for deriving sea surface temperatures (SST) from infrared satellite data is reviewed. A short history of the subject is presented, along with a survey of the different techniques employed for obtaining SST from space measurements. The discussion concentrates on the advanced very high resolution radiometer (AVHRR) and similar instruments, as these are found to provide the most useful data for SST estimation. A comparison of different algorithms for deriving SST from the AVHRR instruments shows that there has been little improvement in derivation accuracy over the past decade. The importance of the “first‐guess” principle is introduced and shows that some of the more recent “advances” in this field do not address the residual source of error in SST derivation. New algorithms that include coefficients dependent on the variances of the satellite brightness temperature images are found to be difficult to apply to real satellite data. Other new algorithms that include measurements of total atmospheric water vapor column may give a slight improvement in derivation accuracy, but they do not account for anomalous vertical structure in the atmosphere. Directions for future research to improve the accuracy of satellite‐derived SST are suggested.

Calibration of advanced very high resolution radiometer infrared channels: A new approach to nonlinear correction

A detailed reanalysis of the calibration procedures for the National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) based on thermal‐vacuum test data was performed as part of the National Air and Space Administration/NOAA AVHRR Pathfinder Project. This effort, a followup to work by Brown et al. (1985), was motivated by the finding that the AVHRR instruments on several NOAA platforms have been routinely operated outside the range of thermal‐vacuum test results, and thus one could not interpolate nonlinear corrections directly from earlier methods. These new calibration procedures permit calculation of nonlinear temperature corrections for any AVHRR operating temperature based on a second‐order polynomial regression with a total calibration accuracy relative to an external calibration standard of less than two digital counts (±0.2°C). Such an improvement is quite important to the absolute accuracy of surface thermal fields, which are derived from these data utilizing various multichannel atmospheric water vapor correction schemes. We find systematic differences in the newly derived nonlinear correction results and those reported previously by Weinreb et al. (1990) and the original reference material in the various addenda to NOAA NESS Technical Memorandum 107 (Lauritson et al., 1979). Calibration results for various AVHRR radiometers show instrument‐similar corrections for each band. Radiometers on NOAA platforms 8‐12 demonstrate similar nonlinearities.

Los Angeles fires seen from space

Hundreds of fires were set in Los Angeles following the verdict of Rodney G. King versus the Los Angeles Police Department on April 29, 1992. These fires were of sufficient intensity and extent to be imaged at 1‐km resolution by the Advanced Very High Resolution Radiometer (AVHRR) aboard the polar‐orbiting satellites operated by the National Oceanic and Atmospheric Administration (see VanWoert et al. [1992] for an overview of the characteristics of the AVHRR instrument). Here we present a thermal infrared image taken the first night of the riots, compare it with an image taken several years earlier but typical of this area, and discuss the relationships with the land cover observed in a 20‐m resolution image from the French satellite SPOT (Systeme Probatoire pour l'Observation de la Terre).

Evaluation of a technique for satellite‐derived area estimation of forest fires

Satellite data have been used increasingly during the past few years to examine burning around the globe. One such satellite instrument, the advanced very high resolution radiometer (AVHRR), has been found useful for the location and monitoring of both smoke and fires because of the daily observations, the large geographical coverage of the imagery, the spectral characteristics of the instrument, and the spatial resolution of the instrument. Earlier studies using AVHRR imagery have focused on locating and monitoring fires and studying the characteristics of smoke. This paper will discuss the application of AVHRR data to assess the geographical extent of burning. Methods have been developed to estimate the surface area of burning by analyzing the surface area effected by fire with AVHRR imagery. Characteristics of the AVHRR instrument, its orbit, field of view, and archived data sets are discussed relative to the unique surface area of each pixel. The errors associated with this surface area estimation technique are determined using AVHRR‐derived area estimates of target regions with known sizes. This technique is used to evaluate the area burned during the Yellowstone fires of 1988.

Estimation of the earth's radiation budget from narrowband NOAA satellite observations

Operational satellites provide global data from multispectral narrowband radiometric scanners such as the Advanced Very High Resolution Radiometer (AVHRR) and the High Resolution Infrared Radiation Sounder (HIRS) found on the polar-orbiting NOAA series of satellites. For many years, the narrowband channels have been utilized to estimate the components of the Earth's radiation budget (ERB). Broadband instruments specially designed for ERB measurements have been available only on experimental satellites. Unfortunately, these instruments have been very intermittent in their coverage, with large gap's in time occurring between the failure of an instrument and the launch of its successor. AVHRR instruments, on the other hand, have been present on every operational NOAA satellite since 1979, making it possible for continuous coverage. The evolution of the ERB products generated from the AVHRR over this period will be discussed, as well as the expectations and plans for the future. This will include a brief tutorial on the generation of the current products, a discussion on anticipated improvements to the algorithms, the use of the HIRS in estimating the outgoing longwave radiation flux, and the development of new products that will enhance our ability to monitor climate and global change.

Noaa's avhrr archive

Abstract The Satellite Data Services Division (SDSD) maintains a digital browse file of Advanced Very High Resolution Radiometer (AVHRR) data. The AVHRR instrument is aboard the TIROS‐N series of spacecraft. The instrument has five channels for viewing the earth, two channels are in the visible part of the spectrum and three are in the infrared portion of the spectrum. Selected data from this instrument are converted to digital browse files and loaded on the Image Library and Browse System (1LABS). The ILABS allows SDSD personnel to review data before the digital tapes are sent to satisfy the user's order. It also allows hardcopy images be sent to users. Working in conjunction with the ILABS is the Electronic Catalog System (ECS). The ECS is able to be directly accessed by the user community and it allows the users to find data in their areas of interest. Once the data are found, then that data can be ordered from the SDSD.