A combination of CIMEL radiometer and MODIS measurements are used to correct surface albedo models. In particular,
we show through an analysis of hyperspectral high resolution Hyperion data that the correlation coefficient assumption
underestimates ground albedo resulting in an overestimate of the VIS optical depth and operational collect 5 surface
model shows an incorrect trend between the MVI index and the surface correlations. Preliminary radiative transfer
calculations based on the same model show that this mechanism can help explain the observed overestimation and the
corrected models have been implemented for NYC and Mexico City with significantly improved AOD.
A combination of CIMEL radiometer and MODIS measurements are used to correct surface albedo models. In
particular, we show through an analysis of hyperspectral high resolution Hyperion data that the correlation coefficient
assumption underestimates ground albedo resulting in an overestimate of the VIS optical depth and operational collect 5
surface model shows an incorrect trend between the MVI index and the surface correlations. Preliminary radiative
transfer calculations based on the same model show that this mechanism can help explain the observed overestimation
and the corrected models have been implemented for NYC and Mexico City with significantly improved AOD.
In this paper, we focus on the use of simultaneous MODIS and AERONET sky radiometer data to refine
the surface albedo models regionally and improve on the current AOD operational retrieval. In particular,
over New York City, we show that the correlation coefficient assumption used in the MODIS Collection
(5) model between the VIS and MIR channels used for surface reflection parameterization are still severely
underestimated in comparison with high spatial imagery data from Hyperion thereby leading to an
underestimate in the VIS ground albedos and explaining the subsequent overestimate of the VIS optical
depth. Furthermore, we find that the VIS/MIR ratios depend only weakly on the scattering geometry
allowing us to generate a regional VIS/MIR surface reflectance correlation coefficient map at spatial
resolutions down to 1.5km. When applying the new VIS/MIR surface reflectance ratio model, we show the
MODIS and AERONET derived optical thickness agreement is significantly improved for the operational
10km resolution product. Moreover, we also show the high resolution surface model allows us to improve
the resolution of the retrieved AOD to 3km. Although direct comparisons for a given day can only be made
at the AERONET site, we find the AOD spatial variability from the improved MODIS retrievals is in far
better agreement with temporal statistics seen in the AERONET time series retrievals. In addition to that,
we also process and validate with another urban area, Mexico City, and the result is also significantly
improved by using refined regional VIS/MIR surface reflectance ratio model.
Determination of aerosol optical depth from satellite remote sensing measurements is extremely complex due to the large
variability of aerosol optical properties. Significant simplification occurs when measurements are taken over water since
the ocean reflection signal can be taken as negligible in the NIR. Unfortunately, over land, most of the signal can be
attributed to ground reflectance. While conventional approaches look for "dark" pixels in an image to isolate aerosols,
these pixels are subjected to increased noise. In this paper, we focus on the feasibility of the MODIS sensor to determine
aerosol optical depth. In particular, an intercomparison between MODIS and CIMEL shows a significant trend for
MODIS to overestimate optical depth. We show that this may be explained through an inaccurate assumption on the
correlation between the VIS and NIR surface albedos. In particular, we show through an analysis of hyperspectral high
resolution Hyperion data that the correlation coefficient assumption underestimates ground albedo resulting in an
overestimate of the VIS optical depth. Using a series of coincident measurements between aeronet and MODIS, we
estimate a more accurate angle dependant albedo and use it to determine aerosol optical depth. The results show
significantly less overbias. Our efforts to reduce space resolution to 1.5km are also discussed.
The polarization characteristics of the water leaving radiance can provide information on bio-optical properties and composition of coastal water and can be used as well as a tool for the separation of chlorophyll fluorescence from elastic reflectance spectra. We report the results of simulations using a coupled ocean-atmosphere vector radiative transfer code (NASA GISS) to obtain the polarized reflectance for various water compositions typical for coastal zones, as function of wavelength and sun/sensor geometry. Results show that even for sensor orientations outside of the principal scattering plane, the polarization can nearly approach the maximum values observable in the principal scattering plane thereby minimizing undesirable sun glint effects. Simulations are complemented by laboratory and field measurements in Chesapeake Bay and near Sapelo Island, GA. We also report the results of the fluorescence retrieval from reflectance spectra using polarization discrimination as well as fluorescence line height (FLH) algorithms. Performance of these algorithms is analyzed in conjunction with the relationship between fluorescence magnitude and chlorophyll concentrations in the conditions typical for coastal waters.
We examine the potential, range of application, and limiting factors of a polarization selection technique, recently devised by us, which takes advantage of naturally occurring polarization properties of scattered sky light to minimize the detected sky background signal and which can be used in conjunction with linearly polarized elastic backscatter lidars to maximize lidar receiver SNR. In this approach, a polarization selective lidar receiver is aligned to minimize detected skylight, while the polarization of the transmitted lidar signal is rotated to maintain maximum lidar backscatter signal throughput to the receiver detector, consequently maximizing detected signal to noise ratio. Results presented include lidar elastic backscatter measurements, at 532 nm which show as much as a factor of √10 improvement in signal-to-noise ratio over conventional un-polarized schemes. For vertically pointing lidars, the largest improvements are limited to symmetric early morning and late afternoon hours. For non-vertical scanning lidars, significant improvements are achievable over much more extended time periods, depending on the specific angle between the lidar and solar axes. A theoretical model that simulates the background skylight within the single scattering approximation showed good agreement with measured SNR improvement factors. Diurnally asymmetric improvement factors, sometimes observed, are explained by measured increases in PWV and subsequent modification of aerosol optical depth by dehydration from morning to afternoon. Finally, since the polarization axis follows the solar azimuth angle even for high aerosol loading, as demonstrated using radiative transfer simulations, it is possible to conceive automation of the technique. In addition, it is shown that while multiple scattering reduces the SNR improvement, the orientation of the minimum noise state remains the same.
The impact and potential of a polarization selection technique to reduce sky background noise (BGN) for monostatic elastic backscatter lidar measurements is examined. Taking advantage of naturally occurring polarization patterns in scattered sky light, we devised a polarization discrimination technique in which both the lidar transmitter and receiver track and minimize detected sky background noise while maintaining maximum lidar signal throughput. Lidar elastic backscatter measurements, carried out continuously during daylight hours at 532 nm, with a vertically pointing lidar in an urban atmosphere, show that changes of a factor of 10 in detected sky background noise power can occur between different polarization orientations for large solar zenith angles. This translates to over a factor of three improvement in signal to noise ratios (SNR) over conventional un-polarized schemes, depending on inclination of the lidar axis and the solar angle, with potential corresponding improvements in lidar range for elastic backscatter schemes including Raman and DIAL. The experimental measurements show that the diurnal variations in improved signal to noise ratio, including the impact of relative humidity, are consistent with theoretical estimates employing Radiative Transfer (RT) and using an urban aerosol model based on adjacent and simultaneous CIMEL measurements.
Previously, it was shown that it is possible to separate the elastic scattering from the chlorophyll fluorescence signal using a polarization discrimination technique. The separation procedure depends however fundamentally on the degree of polarization of the water leaving radiance. In this paper, we compare polarization effects and efficiency of the fluorescence retrieval by simulating the total and polarized reflectance of waterleaving radiances originating from elastic scattering in case 1 and case 2 waters using radiative transfer and multiple component Mie scattering programs. This is done by superimposing upon these reflectances the contribution of known fluorescence spectra, and by using our polarization discrimination procedure to invert the resulting data back into fluorescence spectra. It is shown that a Mie scattering code which does not take into account multiple scattering effects can strongly overestimate the degree of polarization for case 1 and case 2 waters as well as reflectances in the green part of the spectra. Making use of these results we propose and evaluate an improvement for the traditional height over baseline fluorescence extraction method which strongly overestimates fluorescence value for case 2 waters. For our approach we use inverse absorption spectra of water and chlorophyll a scaled up and fitted to the reflectance spectra in the red and near IR bands with the subsequent retrieval of the fluorescence spectrum and chlorophyll a concentration. Results and potential of this approach are discussed.
Previously, it was shown that it is possible to separate the elastic scattering from the chlorophyll fluorescence signal using a polarization discrimination technique. The separation procedure depends however fundamentally on the degree of polarization of the water leaving radiance. In this paper, we study this dependence by simulating the total and polarized reflectance of waterleaving radiances originating from elastic scattering in case 1 and case 2 waters, by superimposing upon these reflectances the contribution of known fluorescence spectra, and by using our procedure to invert the resulting data back into fluorescence spectra. It is shown that the results of this retrieval compare well with the input values of fluorescence spectra for a wide range of underwater light scattering conditions. We show also that height baseline method, which is frequently used for the retrieval of fluorescence, can lead to significant overestimation of fluorescence for coastal zone conditions. The effects of different depths, surface roughness and bottom albedo are also analyzed. It is shown that in most cases fluorescence can be successfully retrieved with acceptable accuracy.
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