P1.6
Turbulence Structure of the ASTEX First Lagrangian Boundary and Cloud Layer
Michael Tjernström, Stockholm University, Stockholm, Sweden; and A. Rune
The vertical mean and turbulence structure of the ASTEX 1st Lagrangian PBL is analyzed. The analysis of the mean structure is based on composites of height-scaled slant aircraft profiles and on porpoise runs, while the turbulence analysis utilize all aircraft data, including slant profiles and horizontal flight legs. Turbulence statistics as well as spectra and cospectra are presented in scaled form.
The turbulence analysis indicates that the cloud layer is decoupled most of the experiment, and that the decoupling occurs near the cloud base. The cloud layer scales with convective scaling, utilizing the depth of the cloud layer and the buoyancy flux at the cloud top. The boundary-layer turbulence scales with surface layer scaling. The turbulence scales of the cloud layer is analyzed using the peak of the velocity spectra from composite averages of all scaled spectra. A dissipation length scale is also cal-cu-lat-ed for both layers, and is found to increase linearly with height in the boundary layer, but to be a function of cloud depth only in the cloud layer.
The analysis of the mean structure of the cloud layer utilize a corrected cloud liquid water, as it is believed that the measured cloud liquid water on the two aircraft, the Met Office MRF C-130 and the NCAR Electra, were not consistent. This changes the temporal structure of the cloud compared to previous studies. The use of only slant profiles also indicates that synoptic scale sub-sidence prevailed throughout the experiment, in contrast to earlier findings. The cloud and below-cloud transition-layer, that develops between the boundary layer and the cloud layer as the latter is lifted, warms and dries much faster than the boundary layer, indicating entrainment to be the primary factor in the cloud's development. The boundary-layer humidity remains relatively constant, in spite of a significant increase in surface temperature.
The fact that the humidity in the lowest layers remain constant while the layer is heated from below, and that the transition layer warms and dries out with time more than the boundary layer, indicate mixing processes on a larger scale, which is not contained within classical turbulence analysis.
Poster Session 1, Cloudy and Convective BLs
Monday, 15 July 2002, 2:00 PM-2:00 PM
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