Ian F. Grant

Observations and modeling of correlated Pi B magnetic and auroral luminosity pulsations

A comparison of observations of Pi B magnetic pulsations at the ground with wide-angle photometer measurements of the accompanying auroral luminosity fluctuations has found several cases of well-correlated magnetic and optical pulsations, with periods in the range 15-50 s. This finding strongly suggests that precipitation fluctuations drive at least some of those components of the Pi B signal with periods in this range. The authors found no luminosity fluctuations associated with the 3-s enhancement of Pi B pulsations. They modeled the magnetic perturbation under a patch of enhanced E region conductivity as it drifts or as its conductivity changes, for circular and strip patch shapes. These idealized models can reproduce some features of the Pi B signature. Most notable, a monotonic variation in the model parameters can produce a reversal in the sense of polarization rotation. A moderate enhancement in the conductivity of a 1-km-wide strip can produce magnetic perturbations with amplitudes typical of Pi B pulsations. 49 refs., 16 figs.

Angular corrections to satellite data for estimating earth radiation budget

Monitoring the spatial and temporal variations of the earths radiation budget (ERB) is essential for our understanding of climate change. Estimating the ERB using satellite data has proven to be a promising avenue for research in recent years. One of the key issues is to convert radiance that is observed at a specific direction to flux, an integrated quantity over all viewing directions. In many cases it is also desirable to convert the derived flux at one specific illumination angle to the daily average value that is integrated over all illumination angles. This article begins with reviewing several representative angular distribution models (ADMs) for converting top‐of‐atmosphere (TOA) radiance to flux in both shortwave and longwave spectrum. In the surface radiation budget part, we primarily focus on land surface albedo, including narrowband to broadband conversion, diurnal cycle and snow albedo. Research issues are also discussed.

Five years of shortwave radiation budget measurements at a continental land site in southeastern Australia

A new data set of near-continuous, high-quality measurements of the components of the land surface radiation budget (SRB) at a uniform site in Australia is described. Its intended uses include the validation of SRB parameters predicted by general circulation models (GCMs), the validation of SRB parameters remotely sensed by satellites, and the study of surface processes. The site at Uardry (145.30°E, 34.39°S) near the town of Hay is the first in Australias Continental Integrated Ground-truth Site Network. It is located in extremely flat and homogeneous natural terrain, with grassland cover that is characteristic of much of the Australian continent, beneath an atmosphere that is characterized by low amounts of water vapor and aerosol. The measurements include the downwelling and upwelling shortwave flux densities. The upwelling measurements are made at several points across the site in order to sample the variability on a scale of 1 km, which is approximately the resolution of many current and planned satellite-based sensors. The paper describes the shortwave component of the data set, shows some internal quality checks, presents some statistical summaries, and makes comparisons with the albedos used in GCMs. It then discusses features of the temporal variation of the albedos derived from the data set in terms of their impact on the remote sensing of land albedo. In particular, significant morning-afternoon asymmetry is present in the diurnal variation on some days, and changes of 20% in albedo occur over a few days. Both of these characteristics violate common assumptions of algorithms used to retrieve land reflective properties from satellite measurements. Consideration of their effect is merited.

First observation of the hot spot from space at sub-degree angular resolution using POLDER data

We show that the Polarization and Directionality of the Earths Reflectances (POLDER)/ADEOS two-dimensional imaging concept permits measurement of the innermost few degrees of the hot spot (the enhancement of land surface reflectance when Sun and view directions coincide) with an angular resolution of approximately 0.3@. The analysis approach assumes that the hot-spot characteristics (amplitude, width) are homogeneous over regions of about 50 km in extent. Examples of the hot spot in red and near-infrared bands are shown. The five hot spot examples presented show a variation of less than a factor of two in both amplitude and width, despite sampling a wide variety of land cover including grassland, woodland and dense eucalypt forest. POLDER offers the potential for the first systematic global mapping of hot-spot characteristics, and hence a possible new source of quantitative information on vegetation structure.

Lunar calibration of geostationary visible-band imagers

The U.S. Geological Survey and Northern Arizona University have established a program of lunar photometry with the dedicated Robotic Lunar Observatory (ROLO). This program has the potential to calibrate any past or future visible-band image from a geostationary satellite that contains the moon. As an early application of this technique, a visible-band image of the moon has been obtained from Japans Geostationary Meteorological Satellite (GMS-5) under viewing and illumination conditions that allow it to be directly compared with a calibrated multi-band image of the moon taken nearly simultaneously from the ground by ROLO. The results demonstrate that the lunar method can potentially calibrate the sensor to an accuracy of a few percent over a wide range of the radiances typical of cloud-free land scenes. However, the inadequate calibration and probable cloud contamination of the particular ground image used preclude an accurate absolute calibration in this case. The merits of this new technique relative to other techniques of calibrating current operational visible-band sensors are discussed and the spectral considerations peculiar to the calibration of broadband sensors are pointed out.

Hot spot measurements at high angular resolution using POLDER data over Australia

The hot spot is an enhancement in the optical reflectance of land surfaces at view directions close to the antisolar (exact backscatter) direction. The two-dimensional imaging capability of POLDER on ADEOS has previously been exploited to measure the hot spot on homogeneous land surfaces from space with an angular resolution of 0.3/spl deg/, by using the variation in view direction with spatial position across a single image frame. This paper extends that approach to inhomogeneous areas by estimating the non-hot-spot component of the reflectance from views of the scene at directions far from the hot spot direction. The technique is robust enough to be applied to essentially any POLDER image in which the hot spot falls on cloud-free land. On a single orbit down eastern Australia, the half-width of the hot spot at 670 nm has the rather restricted range of 1/spl deg/-2/spl deg/ despite sampling a broad range of land cover types.

Lunar calibration of the GMS-5 visible band

Calibration of the visible band of the VISSR radiometer on the Geostationary Meteorological Satellite GMS-5 is necessary for quantitative analysis of the images it produces. Applications include the retrieval of quantities such as aerosol amount, surface albedo and cloud properties. The lunar photometry program conducted by the US Geological Survey with its Robotic Lunar Observatory (ROLO) is establishing the Moon as a radiometric calibration target for space-based Earth observation sensors. The occasional appearance of the Moon in the operational GMS-5 images near the times of ROLO observations has allowed the acquisition of several pairs of images with almost identical view and illumination geometry at ROLO and GMS-5. The lunar disk presents a range of radiances and has a diameter of a few hundred pixels in both the ROLO and GMS-5 images. This allows, from a single image pair, calibration over most of the GMS-5 dynamic range used by clear terrestrial land scenes. An absolute calibration of the GMS-5 sensor to within several percent has been derived from one of the ROLO-GMS image pairs. The advantages of lunar calibration over other calibration methods are discussed.

BRDF modelling of AVHRR data validated with POLDER data

Modelling the effects of the surface bidirectional reflectance distribution function (BRDF) on AVHRR data improves the consistency of time series of land surface measurements produced from AVHRR and is necessary to retrieve surface albedo from AVHRR. An approach based on fitting a BRDF model to AVHRR measurements within a sliding window has been tested. Several BRDF models have been compared for their performance in normalising the measurements to a standard view-illumination geometry, and the choice of window length investigated, at Australian sites spanning a wide range of land cover. This paper highlights the suitability of POLDER measurements for the evaluation of the performance of BRDF modelling of AVHRR data at the top of the atmosphere.

Towards a Climatology of Australian Land Surface Albedo for use in Climate Models

This paper describes the motivation and an approach for deriving a time series of albedo maps of Australia from historical Advanced Very High Resolution Radiometer (AVHRR) data. Polarization and Directionality of the Earth’s Reflectances (POLDER) measurements will be used to test the angular correction algorithm. Some initial results from a survey of POLDER directional reflectance signatures of Australian land cover are presented. Those results show that, while there is much correspondence between the spatial patterns of directional signatures and land cover types, there is a large spread of signatures within each land cover type. However, the similarity of two of the kernels of the bidirectional reflectance distribution function (BRDF) model used to parameterise the directional signatures can produce spurious variations in the model parameters. Finally, some field measurements of grassland albedo are used to make the point that for the greatest accuracy in the estimation of land surface albedo from satellites, it is necessary to account for the detailed shape of the diurnal variation and the effect of the cloudiness on albedo.

Short-wave calibration of ATSR-2 at Australian land sites

This paper presents results from the ATSR-2 atmospheric Correction EXperiment (ACEX), a collaborative campaign between CSIRO and the University of Nottingham held at two uniform land sites in Australia in April and May 1997 in which the surface-leaving radiance was measured in the three VIS/NIR bands of the ATSR-2 sensor as it passed over aboard the ERS-2 satellite. The sites, which are permanently instrumented to monitor the surface radiation budget components, and the campaign instruments, measurement procedure and analysis are described. The results agree with other calibrations of the ATSR-2 shortwave channels to within a few percent.