The Entrainment Interface Layer of Stratocumulus-Topped Boundary Layers during POST

The Physics of Stratocumulus Top (POST) field campaign took place in July and August of 2008. The CIRPAS Twin Otter research aircraft was deployed on 17 daytime and nighttime flights to collect data on dynamics, thermodynamics, radiation, and microphysics from marine stratocumulus-topped boundary layers off of the coast of Monterey, California.  Our focus is the Entrainment Interface Layer (EIL), in which there is a transition from pure, well-mixed cloud air to pure free-atmosphere air.  During each flight, the aircraft traversed the EIL many times. The nearly co-located, high-rate Ultra-Fast Thermometer (UFT) and Particle Volume Monitor (PVM) mounted on the aircraft enabled measurements of the conserved variable liquid-water potential temperature to be made at very high spatial resolution (~ 50 cm).

By using variables that are conserved for moist adiabatic processes, we can examine the processes that determine the buoyancy of air parcels in the EIL as a function of the mixture fraction of pure free-atmosphere air. For this purpose, we followed the method developed by vanZanten and Duynkerke (2002). We used measurements of temperature, liquid water content, water vapor mixing ratio, and pressure for our mixture fraction analysis. The technique uses the total (vapor plus liquid) water mixing ratio to estimate the mixture fraction of pure free-atmosphere air. We then use the liquid-water potential temperature and liquid water mixing ratio to estimate the buoyancy contributions from mixing, phase changes, and radiation.

We will present our results on the contributors to negative buoyancy near cloud top as a function of the mixture fraction for several flights, some during the day and some at night. The immediate goal of our analysis is to determine how the above processes combine to produce negatively buoyant parcels at cloud top. This should ultimately help to improve the modeling stratocumulus-topped boundary layers.

References

vanZanten, M.C. and P.G. Duynkerke, 2002: Radiative and evaporative cooling in the entrainment zone of stratocumulus – the role of longwave radiative cooling above cloud top. Boundary-Layer Meteorology, 102, 253-280.