The Coherence Time of Midtropospheric Wind Features as a Function of Vertical Scale from 300 m to 2 km

Abstract

The coherence between vertical wind profiles separated by a time lag is measured as a function of vertical scale from Doppler radar wind profiler data. Each profile covers altitudes from 6811 m to 16?261 m and is Fourier transformed over a vertical wavenumber (inverse scale) range from 0 to 3.33 ? 10?3 m?1. Time lags between profiles of 0.083, 0.25, 0.5, 1.0, and 2.0 h are used. A correction for instrument noise is derived and is applied to the results. An empirical formula for the coherence as a function of lag and scale is presented and evaluated. The ?coherence time? is defined as the value of time lag beyond which the coherence decays below a chosen value at a given scale. A relation between coherence time and vertical scale is derived. This relation provides a measure of the lifetime of wind features in the midtroposphere as a function of their vertical scale for application to space vehicle wind loads.