Friday, 8 August 2003: 11:10 AM
Observed Radiative Cooling in Nocturnal Marine Stratocumulus
The Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field program was designed to improve our understanding of the physics and dynamics of the marine stratocumulus. Nighttime aircraft measurements are used to characterize the entrainment velocities at cloud top, large-scale divergence within the boundary layer, and cloud microphysics. Uniform cloud fields topping a well-mixed layer were encountered in almost every flight mission. Analyses of the airborne data, with the help of a thermodynamic diagram show that radiative cooling at cloud top effectively cools the cloud layer resulting a cooler absolute potential temperature (the potential temperature with the consideration of the liquid water in the air parcel) of the upper cloud layer than the air below the cloud base. A hydrostatic instability between the cloud layer and the air below is thus created; convection will take place in the marine boundary layer and a uniform absolute potential temperature throughout the boundary layer would then be reestablished. This radiative cooling effect is important not only in cloud dynamics but also in the estimation of entrainment velocities.