Inferring aerosol optical property and aerosol height above clouds from clear-sky satellite observations

An accurate detection and quantification of Aerosol Above Clouds (AAC) is of utmost importance for aerosol-climate studies under all-sky conditions, both on regional and global scales [Sakaeda et al., 2011]. Several passive spaceborne sensors are, in principle, able to separate the aerosol and cloud information in the radiation measured at the top of the atmosphere. For example, the Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer (MODIS) [Torres et al., 2012 and Jethva et al., 2013, respectively] retrieve COD and AAC aerosol optical depth (AOD), but must assume aerosol and cloud microphysical properties and vertical distribution. The passive polarimeter POLarization and Directionality of the Earth's Reflectances (POLDER) can retrieve AAC AOD and Ångström Exponent, but a priori constraints have to be applied to the vertical distribution of both clouds and aerosols [Waquet et al., 2013]. Validating products such as the OMI or POLDER AAC AOD is challenged by the lack of suitable validation data sets. Our goal is to infer the aerosol amount and vertical height above clouds on the basis of combined Cloud Aerosol LIdar with Orthogonal Polarization (CALIOP) and near-by clear-sky satellite observations.

We propose to combine the standard global CALIOP AAC retrievals [Winker at al., 2009, Vaughan et al., 2009, Liu et al., 2010, Young and Vaughn, 2009; Kacenelenbogen et al., 2014] with the alternate CALIOP AAC measurements based on water cloud depolarization ratio (DR) [Hu et al ., 2007a] during the year 2007. Our study will help answer the following questions: (1) How do the standard and alternate CALIOP AAC observations compare seasonally, regionally, and as a function of the underlying cloud horizontal extent? (2) How fast is the AOD decreasing at a given aerosol height as a function of horizontal distance to cloud edge and underlying cloud horizontal extent? (3) Is there continuity in the vertical distribution of aerosol above cloud and in adjacent clear-sky regions? and (4) Can we successfully predict the AAC AOD and height of the aerosols above clouds knowing clear-sky AOD, aerosol and cloud vertical height and cloud horizontal extent?