Microphysical and radiative properties of boundary layer stratiform clouds deduced from ground-based measurements

Two methods for retrieving the microphysical and radiative properties of marine and continental boundary layer stratiform clouds from ground-based measurements are implemented. The first method uses measurements of the cloud liquid water path and the cloud nadir radiance at 1 μm to infer the cloud optical depth, cloud droplet effective radius, and cloud droplet concentration. In the second method a δ2-stream radiative transfer model is used to retrieve the microphysical and radiative properties of stratiform clouds from measurements of the cloud liquid water path, the cloud geometric thickness, the downward shortwave irradiance at the surface, and atmospheric profiles of temperature, pressure, and relative humidity. In all of the retrievals the cloud droplets are assumed to have a lognormal size distribution with a logarithmic width of 0.35; the cloud droplet modal radius and cloud droplet concentration are free parameters. Data obtained in the Azores and Oklahoma from marine and continental boundary layer stratiform clouds, respectively, are used in the study. Results of the analysis demonstrated that the retrieved cloud droplet effective radii from the marine clouds (13 μm) were generally larger than the values from continental clouds (7 μm), while cloud droplet number concentrations were much less in marine clouds (150 cm^-3) than in continental clouds (500 cm^-3). The retrieved cloud droplet effective radius had a strong negative correlation with the cloud droplet concentration. The effects of uncertainties in the measurements, the cloud droplet distribution width, and the surface albedo on the retrieved properties were evaluated by using the δ2-stream model. The errors in the retrieved cloud radiative properties were generally less than 5%, while the errors in the retrieved cloud microphysical properties were considerably larger.