Measurements of fine-scale structure at the top of marine stratocumulus

During the Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS) experiment in July-August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marine stratocumulus cloud decks that cap the mixed layer off the coast of southern California. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds - roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 x 10^-3 kg m^-3. An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3-6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does not contain isotropic turbulence.