Combined multispectral/hyperspectral remote sensing of tropospheric aerosols for quantification of their direct radiative effect
Date
2013
Authors
McGarragh, Gregory R., author
Stephens, Graeme, advisor
Kreidenweis, Sonia, committee member
Vonder Haar, Thomas, committee member
Bartels, Randy, committee member
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Journal ISSN
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Abstract
Scattering and absorption of solar radiation by aerosols in the atmosphere has a direct radiative effect on the climate of the Earth. Unfortunately, according to the IPCC the uncertainties in aerosol properties and their effect on the climate system represent one of the largest uncertainties in climate change research. Related to aerosols, one of the largest uncertainties is the fraction of the incident radiation that is scattered rather than absorbed, or their single scattering albedo. In fact, differences in single scattering albedo have a significant impact on the magnitude of the cooling effect of aerosols (opposite to that of greenhouse gasses) which can even have a warming effect for strongly absorbing aerosols. Satellites provide a unique opportunity to measure aerosol properties on a global scale. Traditional approaches use multispectral measurements of intensity at a single view angle to retrieve at most two aerosol parameters over land but it is being realized that more detail is required for accurate quantification of the direct effect of aerosols, in particular its anthropogenic component, and therefore more measurement information is required. One approach to more advanced measurements is to use not only intensity measurements but also polarimetric measurements and to use multiple view angles. In this work we explore another alternative: the use of hyperspectral measurements in molecular absorption bands. Our study can be divided into three stages the first of which is the development of a fast radiative transfer model for rapid simulation of measurements. Our approach is matrix operator based and uses the Padé approximation for the matrix exponential to evaluate the homogeneous solution. It is shown that the method is two to four times faster than the standard and efficient discrete ordinate technique and is accurate to the 6th decimal place. The second part of our study forms the core and is divided into two chapters the first of which is a rigorous sensitivity and optimal estimation based information content study that explores the use of measurements made by a MODIS type instrument combined with measurements made by an instrument similar to GOSAT TANSO-FTS which supplies hyperspectral measurements of intensity and polarization in the O2 A-band and the 1.61- and 2.06-μm CO2 bands. It is found that the use of the hyperspectral bands provides a means to separate the effects of the surface and aerosol absorption from effects related to aerosol single scattering parameters. The amount of information increases significantly when the CO2 bands are included rather than just the more traditional O2 A-band, when polarization measurements are included, and when measurements are made at multiple view angles. We then present a retrieval using co-located observations of MODIS and GOSAT TANSO-FTS which are both also co-located with AERONET sites for validation purposes. We introduce an optimal estimation retrieval and perform this retrieval on our co-located observations. We choose a complete state vector to maximize the use of the information in our measurements and use an a priori constraint and regularization to arrive at a stable solution. In addition to the retrieved parameters, we also calculate a self contained estimation of the retrieval error. Validation with AERONET, for retrievals using MODIS plus TANSO-FTS measurements of intensity and polarization in all three bands indicate accuracies within 15% for optical thickness, 10% for fine mode mean radius, 35% for coarse mode mean radius, 15% for the standard deviation of fine mode mean radius, 25% for the standard deviation of the coarse mode mean radius, 0.04 for the real part of the index of refraction, and 0.05 for single scattering albedo. In addition to the retrieved parameters, we also validate the estimated retrieval error and find that the estimations have distributions that are tighter and within the broader distributions of real errors relative to AERONET. The third part of our study uses the retrieval results to calculate radiative fluxes, errors, and sensitivities at solar wavelengths along with aerosol radiative effect and effect efficiency. In addition, we outline how to propagate the errors in the retrieval through the flux calculations to provide an error estimation of the fluxes. These results are then validated against the corresponding AERONET products. It was found that the flux results were most sensitive to single scattering albedo while the size distribution and real part of the index of refraction also have significant effects. Relative to AERONET our fluxes are less accurate than an independent AERONET validation, due to uncertainties in our satellite based retrieval with accuracies within 13 Wm-2 for TOA upward, 9 Wm-2 for BOA upward, and 30 Wm-2 for BOA downward. The estimated errors also contained uncertainties but were in fact more conservative than the actual errors.
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Subject
atmospheric aerosols
remote sensing
radiative transfer