Abstract
Abstract. Two flights of the UK Meteorological Office's Hercules aircraft through daytime frontal cirrus around Scotland have been analysed using wavelet analysis on the vertical velocity time-series from the horizontal runs. It is shown that wavelet analysis is a useful tool for analysing the turbulence data in cirrus clouds. It finds the largest scales involved in producing turbulence, as does Fourier analysis, such as the 2-km spectral peaks corresponding to convective activity during flight A283. Wavelet spectra have the added advantage that the position is shown, and so they identify smaller-scale, highly localised processes such as the production of turbulent kinetic energy by the breaking of Kelvin-Helmholtz waves due to the vertical shear in the horizontal wind. These may be lost in Fourier spectra obtained for long time-series, though they contribute something to the average spectral density at the appropriate scale. The main disadvantage of this technique is that only octave frequency bands are resolved.
Highlights
Cirrus clouds which form in the upper troposphere are composed mainly of ice and can cover extensive areas
Associated with gravitational waves and the vertical shear of the horizontal wind, which are neither continuous in time nor spatially homogeneous
These are shown as grey-scale images, with the largest wavelet amplitudes shown in white
Summary
Cirrus clouds which form in the upper troposphere are composed mainly of ice and can cover extensive areas. It is necessary to understand better the dynamics of these clouds in order to improve their representation in general circulation models because of the important role they play in the earth's radiation budget. Smallscale turbulence has an important inuence on the cloud structure and on the spatial distribution of the optical properties. In the upper troposphere the thermal strati®cation is stable, and consumes turbulent kinetic energy (TKE). The sources of turbulence are instabilities associated with gravitational waves and the vertical shear of the horizontal wind, which are neither continuous in time nor spatially homogeneous. In the presence of clouds, turbulence is generated by the release of latent heat and radiation eects
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