Wednesday, 5 June 2002
A novel parameterization of turbulent transport in the stratocumulus-topped boundary-layer
Stratocumulus clouds are a prominent feature of the subtropical high pressure systems, forming over cold waters, under large-scale subsidence, covering broad areas over the eastern Pacific and Atlantic oceans and hence influencing the planet's radiative budget. It is recognized that turbulence, mostly driven by cloud-top radiative cooling, is essential to maintain long-lived stratocumuli, although turbulent transport in cloudy boundary-layers is only poorly represented in most atmospheric models. In the present work, a new parameterization for turbulent transport in stratocumulus-topped boundary-layers was developed, in which the eddy viscosity is proportional to the vertical velocity variance times the wavelength corresponding to the maximum in the kinetic energy spectrum. The parameterization was implemented in a single-column model and tested for real ACE2 and ASTEX cases as well as idealized cases. In most cases, the modeled cloud fraction, cloud water mixing ratio, rainwater mixing ratio, turbulent kinetic energy, buoyancy flux and vertical velocity variance compared very well with their observed counterparts (or with corresponding fields from large-eddy simulations shown in the literature). Finally, the parameterization was compared with three other turbulence closure schemes. Most schemes showed similar results for coupled situations, in which a solid stratocumulus layer sat atop of a well-mixed boundary-layer. However, only the present turbulence sub-model was capable of producing realistic single-column model profiles for drizzling, uncoupled stratocumulus layers and stratocumulus-to-cumulus transition cases.