Analysis of co-located MODIS and CALIPSO observations near clouds

T Várnai,A Marshak

doi:10.5194/amt-5-389-2012

Abstract

Abstract. This paper aims at helping synergistic studies in combining data from different satellites for gaining new insights into two critical yet poorly understood aspects of anthropogenic climate change, aerosol-cloud interactions and aerosol radiative effects. In particular, the paper examines the way cloud information from the MODIS (MODerate resolution Imaging Spectroradiometer) imager can refine our perceptions based on CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) lidar measurements about the systematic aerosol changes that occur near clouds. The statistical analysis of a yearlong dataset of co-located global maritime observations from the Aqua and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellites reveals that MODIS's multispectral imaging ability can greatly help the interpretation of CALIOP observations. The results show that imagers on Aqua and CALIPSO yield very similar pictures, and that the discrepancies – due mainly to wind drift and differences in view angle – do not significantly hinder aerosol measurements near clouds. By detecting clouds outside the CALIOP track, MODIS reveals that clouds are usually closer to clear areas than CALIOP data alone would suggest. The paper finds statistical relationships between the distances to clouds in MODIS and CALIOP data, and proposes a rescaling approach to statistically account for the impact of clouds outside the CALIOP track even when MODIS cannot reliably detect low clouds, for example at night or over sea ice. Finally, the results show that the typical distance to clouds depends on both cloud coverage and cloud type, and accordingly varies with location and season. In maritime areas perceived cloud free, the global median distance to clouds below 3 km altitude is in the 4–5 km range.

Highlights

Aerosol measurements near clouds play an important role in studying two critical yet poorly understood aspects of anthropogenic climate change, aerosol-cloud interactions and aerosol radiative effects (e.g. Loeb and Schuster, 2008)
While combining data from different satellites offers new possibilities, it poses new challenges. One such challenge is that since CALIPSO flies slightly behind Aqua, CALIOP sees the same scene 72 s later than MODIS. This implies that even if MODIS encounters a cloud at a given spot, the cloud drifting with the wind may move away by the time CALIOP arrives, and so solar reflectances for the cloud may be combined with lidar returns from a clear sky column
We estimate the impact on near-cloud reflectances by dividing the center portion of WFC imagery to 51 km by 51 km segments, and testing what shifting of the geographically co-registered images can maximize the covariance of WFC and MODIS pixel values

Summary

Introduction

Aerosol measurements near clouds play an important role in studying two critical yet poorly understood aspects of anthropogenic climate change, aerosol-cloud interactions and aerosol radiative effects (e.g. Loeb and Schuster, 2008). Loeb and Schuster, 2008) Such measurements indicate that clouds are surrounded by a wide transition zone, in which aerosol optical properties and particle size change systematically Charlson et al, 2007; Koren et al, 2008; Redemann et al, 2009), and both aerosol and cloud particles can be present in the atmospheric column or volume we observe (Liu et al, 2009). This leads to difficulties in the correct interpretation of satellite measurements and, as a result, to bigger uncertainties in remote sensing retrievals

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