High-resolution measurement of cloud microphysics and turbulence at a mountaintop station

摘要

Mountain research stations are advantageous not only for long-term samplingof cloud properties but also for measurements that areprohibitively difficultto perform on airborne platforms due to the large true air speed or adversefactors such as weight and complexity of the equipment necessary. Somecloud–turbulence measurements, especially Lagrangian in nature, fall intothis category. We report results from simultaneous, high-resolution andcollocated measurements of cloud microphysical and turbulence propertiesduring several warm cloud events at the UmweltforschungsstationSchneefernerhaus (UFS) on Zugspitze in the German Alps. The data gatheredwere found to be representative of observations made with similar instrumentationin free clouds. The observed turbulence shared all features known forhigh-Reynolds-number flows: it exhibited approximately Gaussian fluctuations forall three velocity components, a clearly defined inertial subrange followingKolmogorov scaling (power spectrum, and second- and third-order Eulerianstructure functions), and highly intermittent velocity gradients, as well asapproximately lognormal kinetic energy dissipation rates. The clouds wereobserved to have liquid water contents on the order of 1 g m and sizedistributions typical of continental clouds, sometimes exhibiting longpositive tails indicative of large drop production through turbulent mixingor coalescence growth. Dimensionless parameters relevant to cloud–turbulenceinteractions, the Stokes number and settling parameter are in the rangetypically observed in atmospheric clouds. Observed fluctuations in dropletnumber concentration and diameter suggest a preference for inhomogeneousmixing. Finally, enhanced variance in liquid water content fluctuations isobserved at high frequencies, and the scale break occurs at a valueconsistent with the independently estimated phase relaxation time frommicrophysical measurements.