Sunday, 10 August 2003

A radar method to measure turbulence in clouds.

The method is based on the Doppler velocity and spectral width measurements in two adjacent radar volumes (say 1 and 2) and in a radar volume comprises these two volumes, 1&2. For the comprised volume, the spectral width can be computed from the time series data recorded in volumes 1 and 2.The turbulent contribution to spectral width is a function of the size of the radar volume so that the spectral width in bin 1&2 is wider than in bin 1 or bin 2. The turbulence dissipation rate can be expressed as a function of the spectral widths measured in volumes 1 (or 2) and 1&2 and some part of the difference of the Doppler velocities measured in volumes 1 and 2. The differential Doppler velocity consists of the wind shear contribution and turbulent pulsations. The separation of these contributions is discussed. Turbulence dissipation rates obtained with the method in widespread precipitation and thunderstorms are presented.
One of the main characteristics of turbulence is its outer scale. Usually, radar measurements of turbulence are based on an assumption that the outer scale of turbulence is larger than the maximal size of the radar volume. The method can be applied to estimate the outer scale. For widespread precipitation, the outer turbulent scales obtained were less than 250 m. In analysed thunderstorms, the estimated outer scales were about 500 m in regions of high dissipation rates.

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