The morning transition between the nocturnal stable boundary layer and the daytime mixed layer is poorly understood, yet the morning transition determines the main characteristics of the ensuing daytime mixed layer. Extensive data have been collected during the morning transition period in the Southern Great Plains Hydrology Experiment by the Canadian NRC Twin Otter aircraft and the NOAA LongEZ and four towers along the El Reno flight track.
During the morning transition, large entrainment often leads to drying of the mixed layer. In this case, the dry air entrainment exceeds the surface evapotranspiration, corresponding to moisture flux divergence and boundary-layer drying. The moisture flux in the upper part of the boundary layer varies dramatically between sequential passes partly because of the event nature of the entrainment flux. However, the entrainment moisture flux is larger than the surface moisture flux for almost all aircraft passes and averages about three times greater than the surface moisture flux. Tower eddy correlation data collected underneath the flight track further document drying during the period of vertical flux divergence observed by the aircraft. Both data composited over all of the flight days as well as case studies will be presented.