Airborne measurements of spectral shortwave radiation in cloud and aerosol remote sensing and energy budget studies

Abstract

Space-borne observations of clouds and aerosols are currently undergoing important developments. A new generation of passive imagers follows in the footsteps of proven instrumentation akin to AVHRR (Advanced Very High Resolution Radiometer: Cracknell, 1997) and MODIS (Moderate Resolution Imaging Spectroradiometer: King et al., 1992). At the same time, novel approaches are diversifying the instrumental infrastructure and thus extending the observable parameter space: radar and lidar explore the vertical distribution of clouds and aerosols; polarimeters help untangle aerosols and clouds, and complement non-polarized imagery for ice and mixed-phase clouds. Curiously, the spectral information in the shortwave (solar) wavelength range has remained largely underutilized for cloud and aerosol remote sensing, whereas the infrared and microwave spectral ranges are extensively used for sounding techniques - particularly for water vapor. Solar spectral imagers such as AVIRIS (Airborne Visible/InfraRed Imaging Spectrometer) are routinely flown in geological surveys, recently in the aftermath of the May 2010 Gulf of Mexico oil spill (Clark et al., 2010). Ecosystem mapping (Pignatti et al., 2009) and ocean color retrievals (Liew and Kwoh, 2003) with Hyperion onboard the NASA satellite EO-1 are examples of space-borne spectral cartography in biology and ocean chemistry. In all of these applications, the atmosphere between the surface and the sensor is a factor that needs to be removed via correction algorithms. The spectral signal from the atmosphere itself is mainly used for fingerprinting trace gases based on differential optical absorption spectroscopy. In addition to gas-phase spectroscopy, the European Space Agency℉s SCIAMACHY (scanning imaging absorption spectrometer for atmospheric cartography) on ENVISAT and GOME (global ozone monitoring experiment) on ERS-2 provide limited information about aerosols and clouds, which introduce biases in trace gas retrievals due to enhanced scattering and absorption or spatial heterogeneity effects (Wagner et al., 2008). However, derived parameters such as the absorbing and scattering aerosol indices (de Graaf and Stammes, 2005; Penning de Vries et al., 2009) or effective cloud fraction (Grzegorski et al., 2006) remain somewhat quantitative or are limited to certain wavelength bands with strong gas absorption lines (Koelemeijer et al., 2002) or Fraunhofer lines (Ring effect, Joiner and Bhartia, 1995).