On entrainment rates in nocturnal marine stratocumulus

The first research flight of the second Dynamics and Chemistry of Marine Stratocumulus field study is analysed. This case attracted our interest because it showed a consistently deepening cloud layer despite macroscopic conditions which previous work has suggested should be an indication of cloud thinning or break-up. Detailed analysis of the flight data shows that despite the cloud-top entrainment instability parameter being well beyond its critical value the cloud did indeed deepen through the night. Our best estimates show little indication of rapid changes in cloud top, while cloud base was found to be lowering at a rate of several metres per hour. This evolution, and independent measurements of trace-gas budgets, imply an entrainment rate of 0.0039 +/- 0.001 m s(-1). This is compared to entrainment rates from recently proposed parametrizations (forced by the observed forcing of the cloud layer) which range from 0.002 to 0.008 m s(-1). Two of the parametrizations we test reproduce the observed entrainment rates within their stated uncertainties, although subsequent tests show that one of these rules exhibits sensitivities to changes in the environmental conditions which are difficult to justify. Large-eddy simulation of the observed case was able to reproduce the macroscopic evolution of the layer, but in doing so had some difficulty in maintaining the observed mixing-line structure at cloud top. A comparison of the observed and simulated turbulent structure show these to be broadly consistent, although there is an indication that the structure of the simulated turbulence differs from the observations near the flow boundaries, particularly at cloud top.