Airborne Radar Doppler Spectrum Width as a Scale-Dependent Turbulence Metric

A potential utility of airborne Doppler (mm-wavelength) cloud radars lies in resolving cloud turbulent eddy structure, previously quantified through variations in the mean Doppler motions at scales much larger than individual pulse volumes. The present work advances this potential, utilizing the near-vertical pulse pair Doppler spectrum width from the Wyoming Cloud Radar onboard the Wyoming King Air Research Aircraft (WCR; Wang et al. 2012) as a metric for turbulent air motion. Doppler spectrum width has long been used to investigate turbulent motions from ground-based remote sensors. However, complexities of airborne Doppler radar and spectral broadening resulting from platform motions have limited spectrum width measurements from airborne cloud radar to qualitative interpretation only. Here we present the first quantitative validation of spectrum width from an airborne cloud radar. Echoes with signal-to-noise ratio greater than 10 dB are demonstrated to provide spectrum width values that strongly correlate with vertical air motion variance retrieved from mean Doppler velocity for a range of non-convective cloud conditions. Further, measurements of Doppler spectrum width from within turbulent regions of cloud also show good agreement with retrieved eddy dissipation rate (EDR) from an in-situ gust probe. We show that this methodology only works for turbulent air motions that are more energetic than the magnitude of spectral broadening—estimated to be about 0.4 m s-1 for the WCR in these cases.

Examples of this newly validated (Majewski et al., In Review) cloud radar turbulence metric are presented as the basis for identifying and characterizing the link between turbulent eddy structures and microphysical growth properties for a few cases.

Majewski, A., J. R. French, and S. Haimov, In Review: Airborne Radar Doppler Spectrum Width as a Scale-Dependent Turbulence Metric. JTech.

Wang, Z. and Coauthors, 2012: Single Aircraft Integration of Remote Sensing and In Situ Sampling for the Study of Cloud Microphysics and Dynamics. BAMS, 93, 653-668.