35 P 28 A blue sun observation-an interplay between cloud and aerosol microphysics, and optics

During a SAGE II tangent path intercept on 19 April 1991 over New Mexico, the NASA DC-8 aircraft inadvertently entered a cirrus cloud at 33000 feet pressure altitude. During a turning climb to get back into clear air, a blue sun at 2.3° elevation was observed near the top of the cloud at 35000 feet. In situ microphysical instruments aboard the aircraft showed, at the time of the blue sun observation, the appearance of three major modes in the particle size range 0.03 < R < 20 μm: An aerosol mode (lognormal parameters N0 = 18.6 cm−3, rg = 0.06 μm, σg = 1.5), a cloud-aerosol mode (N0 = 8.2 cm−3, rg = 0.27 μm, σg = 1.2) and a cloud mode (N0 = 1.1 cm−3, rg = 1.8 μm, σg = 1.9). Mie-calculations showed the atmospheric optics being dominated by the cloud and cloud-aerosol modes. Of these two, only the cloud mode alone shows a slight increase of light extinction with wavelength in the visible, which is a necessary condition for the appearance of a blue sun. In order to explain the blue sun observation, we had to combine actual measurements with a model (Horvath et al., 1993) by which cloud-aerosol interactions as a function of saturation with respect to ice explain the cloud-aerosol mode as a residue of a dissipating cloud mode. This observation points to the importance of multidisciplinary approaches to atmospheric observations, and it is an example of the occasional difficulty of reconciling in situ with remote sensing measurements.