Exploring Entrainment Processes and Parameterizations in Stratocumulus Clouds Using ARM SGP Observations

Description

Observations from an upward-pointing Doppler cloud radar are used to examine entrainment processes and parameterizations in a continental stratocumulus cloud using ARM observations. The vertical velocity variance and energy dissipation rate (EDR) terms in a parameterized turbulence kinetic energy (TKE) budget are applied to the entrainment zone of a continental stratocumulus cloud. The observations from a solid deck observed over the Southern Great Plains (SGP) at the DOE ARM site for a continuous 15-hour period give vertical velocity and spectrum width observations from an upward pointing millimeter cloud radar (MMCR). Spectrum width observations give energy dissipation rates directly in the entrainment zone and the vertical velocity observations provide vertical velocity variance for one-hour periods. For the 15 hours of observations, the variance term correlates strongly to the dissipation rates in the entrainment zone. However, the ratio of the variance term to the dissipation decreases at night due to an apparent decoupling of the boundary layer. Entrainment rates are estimated from an inversion height budget using SGP observations and large-scale model re-analyses. For this budget the local time derivative of boundary depth at the Central Facility is obtained from radar cloud-top height estimates. The horizontal advection term is estimated using wind profiler observations in-conjunction with estimates of the spatial variability of cloud-top heights inferred from ceilometer cloud-base heights and cloud liquid water paths from microwave radiometer measurements at the boundary facilities. The large-scale vertical velocity for the budget is obtained from ECMWF analyses. The budget gives an entrainment rate averaged over the observing period of 0.76±0.15 cm/s and provides a rough estimate of the entrainment rate variability within the observing period. The average entrainment rate for the observing period from the height budget provides estimates of bulk closure parameters using vertical velocity variance, the convective velocity scale w*, and the EDR. The time variation of the entrainment rates from the radar turbulence parameterizations are examined and compared with those from the boundary layer height budget and w*. The results indicate a strong potential for making entrainment rate estimates directly from the radar EDR estimates—a technique that has distinct advantages over other methods for estimating entrainment rates.