Exploring Stratocumulus Cloud-Top Entrainment Processes Using Doppler Cloud Radar Observations

Observations from a the ARM upward-pointing Millimeter Cloud Radar (MMCR) located at the Southern Great Plains (SGP) site are used to examine the vertical velocity variance and energy dissipation rate at the top of continental stratocumulus clouds. These observations are used to examine terms in the turbulence kinetic energy (TKE) budget in the entrainment zone that are related to the entrainment rate. When this budget (without wind shear) is applied to the entrainment zone, the entrainment rate is proportional to a vertical transport and pressure perturbation term minus a dissipation term and inversely proportional to the strength of the inversion. The transport-pressure term is usually assumed to be proportional to the vertical velocity variance to the 3/2 power and inversely proportional to the boundary layer depth (called the variance term in this study). In this study we use observations from a continental stratocumulus cloud observed over the SGP for a 14-hour period. Clouds were solid during this entire period and had thicknesses of 300-400 meters and tops increasing from 800-1200 m. The turbulence forcing due to surface buoyancy fluxes and radiative cooling at cloud top is obtained from surface flux measurements and radiative transfer calculations based on the cloud characteristics derived from cloud radar and lidar observations. Vertical velocity and spectrum width observations from the upward pointing millimeter cloud radar (MMCR) at the SGP site are used to examine the turbulence in the top 20% (60-80 m) of the cloud, which is define as the entrainment zone. The spectrum width is used to define energy dissipation rates directly in the entrainment zone and the vertical velocity observations are used to define vertical velocity variance for one-hour periods. These quantities are then used to examine the terms in the TKE budget in this layer.