Microphysical and radiative development of a cirrus cloud during FIRE: implications for dynamical effects

Data for this study were collected over Minnesota on 19 October, 1986 during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) cirrus field campaign. Broadband radiometers mounted on the top and bottom of the NCAR King Air and Saberliner aircraft were used to measure irradiances. Constant altitude flight legs were flown at various altitudes within cirrus clouds. The data were averaged over the 60-km flight legs and used to calculate radiative heating rate profiles and cloud radiative properties over the resolved layers from 6 to 12 km. Adiabatic vertical velocity w (cooling rate) required to balance (compensate) the observed radiative heating rates was calculated. Values ranged from about 0.40 m s −1 at mid-cloud levels (8.2 km) to 0.05 m s −1 at the 10 km level. In the absence of other processes, the pattern of radiative heating would have generated a unstable thermal stratification ( ∂θ/ ∂z < 0) at the 8.2 km level where a near neutrally stratified layer was observed within the cloud. Observations of the cirrus could ice particle size distribution and habit were used to define the cloud ice water content profile. Assuming steady state supersaturation ( S) with respect to ice, the rate of adiabatic ascent required to maintain constant S against the effects of diffusional growth of the ice particles was calculated. The resulting vertical motions were found to be comparable to, but smaller than, those needed to compensate the effects of radiative heating.