LABORATORY SIMULATIONS OF RADIATIVELY INDUCED ENTRAINMENT IN STRATIFORM CLOUDS

Laboratory experiments have been conducted to simulate entrainment in stratiform clouds. In particular, the case of entrainment across a cap ping temperature inversion and induced by cloud top cooling has been s imulated. This geometry is termed interfacial convection, and its phys ics differ from the more thoroughly studied case of penetrative convec tion. The dimensionless entraininent rate associated with interfacial convection has been found to vary inversely with a bulk Richardson num ber over a broad range of Richardson numbers. A dependence of the entr ainment rate on the diffusivity of the stratifying agent has also been found. This dependence is explained in terms of Taylor layers. A phys ical model for the dynamics of interfacial convection is proposed. In the laboratory case, a stably stratified interface separates two fluid layers. Convection is driven in the upper layer by the deposition of radiation near the interface. After sufficient energy has been deposit ed, buoyant fluid rises and induces formation of entraining cusps at t he interface. The spacing between cusps is determined by equating a bu oyancy instability timescale with a heating timescale. When the depth of the convecting layer is large compared to the distance separating t he cusps, a larger-scale circulation also develops. In such cases, edd ies of size comparable to the depth of the convecting layer advect the cusps horizontally. Despite Reynolds numbers that differ by 4 orders of magnitude or more between the laboratory simulation and the real at mosphere, it is argued that the entrainment dynamics are analogous. Di mensionless entrainment rates measured in the laboratory are within 1 order of magnitude of those measured in the atmosphere for a given Ric hardson number. Thinner Taylor layers and the lack of evaporative effe cts in the laboratory may account for the difference.