Collocated measurements of boundary-layer cloud microphysical and radiative properties and comparison with satellite retrievals

Data from collocated, helicopter-based measurements of boundary-layer cloud microphysical properties and spectral radiative quantities (optical thickness τ, reflectivity R) gathered in the frame of the EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality Interactions) project are presented. The in situ measurements of the microphysical cloud properties (effective droplet radius R_eff, Liquid Water Content LWC, droplet number size distribution) were collected by instruments installed on the Airborne Cloud Turbulence Observation System (ACTOS). Cloud spectral reflectivity was derived from upwelling radiance measurements using a grating spectrometer combined with an optical inlet which was mounted underneath the helicopter. Correlations between cloud microphysics and reflected radiation are analysed for two distinct cloud cases with different optical and geometrical thicknesses. Common retrieval methods based on radiative transfer modelling are applied to the radiation data to derive τ and R_eff of the two cloud cases.

The results are compared to the collocated in situ measurements and data from the Moderate-Resolution Imaging Spectroradiometer (MODIS). The retrieval of τ and R_eff yields values which are in the range of the measurement uncertainties of the in situ measured data. Results of MODIS retrievals for τ are smaller than the in situ results, while the R_eff values retrieved from MODIS are higher than the in situ measurements. This is most certainly due to cloud spatial inhomogeneity and low cloud fractions encountered during the measurement campaign.