AN OBSERVATIONAL AND THEORETICAL-STUDY OF HIGHLY SUPERCOOLED ALTOCUMULUS

Two altocumulus clouds, which formed at a temperature of -30-degrees-C, were sampled using the NCAR King Air aircraft and coincident lidar during the 1986 FIRE cirrus experiment in Wisconsin. The clouds were structurally and thermodynamically similar to stratocumulus, with extensive cloudtop entrainment, a capping temperature inversion, and a dry layer above. The microphysical and radiative properties of both clouds were characterized and modeled numerically. Calculations of droplet concentration and mean diameter profiles compare favorably with the measurements when entrainment effects are incorporated in the model. Radiative transfer calculations suggest radiation played an important role in driving convection in the more dynamically unstable of the two clouds. A simple model shows that radiative cooling causes sufficient negative buoyancy in cloudtop parcels to produce convective instability and to reproduce the observed downdraft velocities. Entrainment of warmer, drier air near cloudtop is shown to partially counteract the radiatively induced negative buoyancy in the downdrafts.