Thursday, 6 June 2002
Simulations of the visible and near-infrared radiative properties of mixed-phased stratocumulus cloud over the sea
Aircraft experiment has been carried out for wintertime boundary-layer clouds over the sea around Japan in January 1999 within the Japanese Cloud and Climate Study (JACCS) program. By using two instrumented aircraft, the microphysical and radiative properties of mixed-phase stratiform clouds have been simultaneously measured. The stratocumulus cloud observed on January 30 was highly heterogeneous vertically and horizontally with different mixing ratios of water droplets and ice particles. On an average over a long flight distance, the stratocumulus cloud with a mean geometrical thickness of 1.3 km absorbed almost 0% and 24 %, and reflected 64 % and 52 %, of the incident solar radiation in the visible (wavelengths < 0.7mm) and near-infrared (> 0.7mm) bands, respectively. This suggests no sign of the so-called anomalous solar absorption for the present case. We made simulation calculations of the radiative properties for various cloud models with the measured cloud parameters. The single scattering properties of the mixed-phase cloud were calculated by the use of Mie theory for water droplets and anomalous diffraction theory for ice particles. A homogeneous, plane-parallel mixed-phase cloud model could not consistently reproduce the observed visible and near-infrared (NIR) radiative properties. When the visible cloud optical thickness was adjusted to fit the calculated visible radiative properties to the measured ones, then the simulation overestimated and underestimated the NIR transmittance and absorptance, respectively, compared with the measured NIR radiative properties. An independent pixel approximation considering the cloud horizontal inhomogeneity could yield better agreement with the averaged radiative properties.