Experimental studies of entrainment at the top of the dry atmospheric boundary layer

• Main Author: Claxton, Bernard M.
• Published: University of Manchester 1999
• Subjects: 551.51
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604311

Entrainment, the dynamical incorporation of free air into the boundary layer, is important because it affects the temperature and moisture profiles of the boundary layer. Weather forecast models require a parameterization of entrainment so as to more accurately predict cloud formation at the top of the boundary layer; a process sensitive to small changes in relative humidity. Using a tethered-balloon based observing platform with multiple turbulence probes, observations of a convective planetary boundary layer were made to investigate the process of entrainment at the boundary layer's interface with the free atmosphere. Observations were taken throughout the boundary layer at a vertical resolution of about 100rn, enabling two-dimensional scalar fields to be constructed. Measurements made in situ within the entrainment zone were used to calculate the entrainment flux. By applying a filtering technique it was discovered that 50% of the entrainment flux could be attributed to processes occurring on scales larger than the boundary layer depth, Zi. This emphasised the importance of large-scale motions, e.g. thermals, to entrainment. At the other end of the scale, 20% of the entrainment flux was due to processes occurring on scales < zi/4. Hence, small-scale local mixing processes are also relevant to entrainment. Conditional sampling analysis showed that within the upper part of the entrainment zone, over 80% of the entrainment flux is carried by up draughts. The observations provide evidence for two previously postulated mechanisms for entrainment. Specific engulfment episodes are presented. These support recently reported Large-Eddy Simulation work that has resolved engulfment's within a model. Also observed is an instance of an upwelling thermal impinging on and distorting the inversion layer. This interaction results in a local reduction in stability across the inversion layer, allowing entrainment to occur as a result of wave-breaking.