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Global perspectives on the plane-parallel nature of oceanic water clouds using data synergy from MISR and MODIS

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Title: Global perspectives on the plane-parallel nature of oceanic water clouds using data synergy from MISR and MODIS
Author(s): Liang, Lusheng
Director of Research: Di Girolamo, Larry
Doctoral Committee Chair(s): Di Girolamo, Larry
Doctoral Committee Member(s): Rauber, Robert M.; McFarquhar, Greg M.; Nesbitt, Stephen W.
Department / Program: Atmospheric Sciences
Discipline: Atmospheric Sciences
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): atmospheric sciences cloud remote sensing
Abstract: The plane-parallel assumption is commonly used for solving radiative transfer problems in weather and climate research. While a plethora of observational and three-dimensional (3-D) radiative transfer simulations have revealed many shortcomings in the application of the plane-parallel assumption, less attention has been given to providing a global perspective on these shortcomings from observations. This thesis provides this perspective for oceanic water clouds based on data synergy from the Multiangle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS). Eight years of data for the months of January and July were examined to determine (1) the angular anisotropy in the upwelling radiation captured by multi-angle observations and its departure from plane-parallel expectations, quantified by cloud view-angle consistency, and (2) the viewing zenith angle (VZA) dependence of plane-parallel retrieved cloud optical thickness (τ). Cloud view-angle consistencies, relative to their plane-parallel expectations, were defined in bidirectional reflectance factor (BRF), cloud optical thickness and spherical albedo metrics. The probability distribution functions of these metrics reveal that clouds are angularly consistent in BRF, cloud optical thickness and spherical albedo to within 5% of their plane-parallel value 67.6%, 23.0%, and 72.0% of the time, respectively, for January and 61.0%, 23.7%, and 61.3% of the time, respectively, for July. Global maps of these metrics show large spatial variability and solar zenith angle (SZA) dependence, with stratiform regions being more plane-parallel like than cumuliform regions and stratiform regions being less plane-parallel-like when the SZA is greater than 60º. We establish a relationship between the cloud view-angle consistency metrics and a cloud spatial heterogeneity metric (Hσ) that allows us to potentially identify, with a prescribed confidence level, which MODIS microphysical retrievals meet the plane-parallel assumption to within any desired range in view-angle consistency. For example, requiring 96% of the MODIS cloud microphysical retrievals to be angularly consistent in τ to within 15% of their plane-parallel value (i.e., optical thickness metric values < 15%) suggests performing retrievals only where Hσ < 0.072; 22.1% of the domains met this criterion. The cloud view-angle consistency is further examined by the view-angle dependence of plane-parallel retrieved τ. With the unique near-simultaneous multiangle observations from MISR, we are able to overcome many shortcomings in previous observational studies on τ-VZA relationships derived from wide-swath, single-view scanning instruments. Unlike previous studies, we are able to exclude cloud seasonal and latitudinal invariant assumptions, eliminate inconsistency in cloudy scene identification across multiple view-angles and minimize the impact of pixel expansion with viewing obliquity on τ retrievals. Our analysis qualitatively confirms many τ-VZA relationships found in previous studies, while able to characterize these relationships regionally over the globe. However, quantitative comparisons are hard to interpret, given many variables that the bias in plane-parallel retrieved τ depends on, and the different sampling characteristics of the various dataset. Our results show that, under oblique Sun, for example, τ is biased low relative to nadir in the mean by 73% at a VZA = 70.5º in the forward-scatter directions at 47.5ºS-50ºS latitude (SZA = 75º) in July, and τ is substantially biased high in the backscatter directions only for VZA = 70.5º, with a bias as high as 83%. Examining our data for large SZAs (SZA > 68º in January and SZA > 73º in July) and up to VZA = 70.5°, and stratifying the analysis by nadir-τ and cloud spatial heterogeneity, reveal additional complexities not observed before. When VZA = 70.5º, τ is biased higher than nadir in both forward-scatter and backscatter directions even under high Sun (SZA < 40° in both January and July). Under very low Sun (SZA > 68° in January and SZA > 73° in July) and in the forward-scatter direction, optically thinner clouds and heterogeneous clouds are less negatively biased or even positively biased at small VZAs as compared to optically thicker clouds and homogeneous clouds, resulting in a slight τ peak at VZA = 26° when averaged over all clouds. Additionally, stratifying the data by nadir-τ reveals additional 3-D and non-3-D radiative transfer effects that determine the τ-VZA relationships. We demonstrated that to understand the complexity in the τ-VZA relationships requires carefully considering (1) the various 3-D radiative transfer pathways, (2) the increased viewing of more cloud-sides with viewing obliquity, (3) the relative azimuth angle between sun and view, (4) the change in concavity of the radiance-τ non-linear relationship with view-angle, and (5) other non-3-D radiative transfer effects, such as sunglint. Given that a large fraction of water clouds are not plane-parallel to within any reasonable degree in view-angle consistency, and given the great complexity in which the bias in plane-parallel retrieved t depends on sun-view geometry and other factors, we call to the research community to develop new retrieval paradigms for cloud microphysical properties that can properly account for the 3-D radiative transfer found in nature.
Issue Date: 2010-01-06
URI: http://hdl.handle.net/2142/14566
Rights Information: Copyright 2009 Lusheng Liang
Date Available in IDEALS: 2010-01-06
Date Deposited: December 2
 

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