5.9
A new method for estimating the lifetime of atmospheric features as a function of their vertical scale and its advantage over coherence analysis
PAPER WITHDRAWN
Francis J. Merceret, NASA, Kennedy Space Center, FL; and A. Szpiro
Space vehicle launches are sensitive to winds in the mid-troposphere and frequent high-resolution measurements required. Jimspheres balloons have higher vertical resolution than wind profilers, but profilers have higher temporal resolution. To determine the relative importance of spatial and temporal resolution it is necessary to determine the lifetime of atmospheric features as a function of their size.
Coherence analysis is frequently used for estimating lifetime of features as a function of scale. Coherence is computed as a function of vertical wavenumber for ensembles of wind profile pairs separated by specified time intervals. By using multiple time intervals, coherence is determined as a function of wavenumber and time lag. The lifetime is defined by the lag at which the coherence falls to a selected value.
Coherence analysis is widely accepted, but has a limitation in its application to launch wind variability analyses. Coherence measures phase changes, not amplitude changes. For an amplitude modulated sinusoid, the coherence remains unity regardless how much the amplitude changes. This paper presents a new measure that is sensitive to both amplitude and phase.
The measure normalizes the ensemble power spectrum of the difference between members of wind profile pairs rather than the ensemble cross spectrum of the pairs as is done for coherence. The normalizing factor uses the ensemble power spectra of the individual profiles in the pairs. Unlike coherence, this technique responds to changes in either amplitude or phase and thus provides a more complete measure of wind variability as a function of vertical scale.
Application of the technique is demonstrated by analysing wind profiler data to determine the lifetime of atmospheric features in the mid troposphere as a function of their vertical scale.
Session 5, Forecasting and Evaluation/Verification (Parallel with Sessions 6 & 7)
Thursday, 14 September 2000, 8:00 AM-5:49 PM
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