Radiative Properties of Mid-latitude Frontal Ice-Clouds Observed by the Shortwave and Longwave Radiometer-Sondes

In the Japanese Cloud and Climate Study (JACCS) cirrus experiment, simultaneous measurements of cloud radiative and microphysical properties were conducted by using the combined-sonde (radiometersonde + hydrometeor-video-sonde (HYVIS)) observation system at the Meteorological Research Institute (MRI), located at (36.05°N, 140.13°E) in Tsukuba, Japan, during early summer seasons from 1995 to 1999. We have analyzed the radiative properties of frontal ice-clouds observed by the shortwave and longwave radiometer-sondes (Asano et al. 2004). To interpret the observed radiative flux profiles, we have also performed radiative transfer calculations for horizontally homogeneous atmospheric models, where the single-scattering properties of ice-clouds were computed by anomalous diffraction theory for ice-crystals observed by HYVIS. On an average of the observed frontal ice-clouds, the shortwave re- flectance, transmittance and absorptance were estimated to be 0.41±0.03, 0.51±0.06, and 0.08±0.09, respectively, for the averaged ice-cloud layer with a mean visible optical thickness of 4.6 and a mean geometrical thickness of 5.4 km (mean volume extinction coefficient of 0.85 km-1). The ice-clouds were significantly heated by absorption of solar radiation in daytime. On the other hand, the mean effective emittance was estimated to be about 0.86±0.37, showing that the frontal ice-clouds never acted as blackbody for longwave radiation. The lower parts of ice-cloud layers were heated by absorption of longwave radiation from the surface and the atmosphere below the ice-clouds, while the upper parts were cooled by emission of longwave radiation to space. The shortwave and longwave heating profiles could make the daytime ice-cloud layers thermodynamically unstable.