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

Abstract High resolution airborne cloud Doppler radars such as the W-Band Wyoming Cloud Radar (WCR) have, since the 1990s, investigated cloud microphysical, kinematic, and precipitation structures down to 30-m resolution. These measurements revolutionized our understanding of fine-scale cloud structure and the scales at which cloud processes occur. Airborne cloud Doppler radars may also resolve cloud turbulent eddy structure directly at 10-meter scales. To date, cloud turbulence has been examined as variances (Schwartz et al. 2019) and dissipation rates (Strauss et al. 2015) at coarser resolution than individual pulse volumes. The present work advances the potential of near-vertical pulse pair Doppler spectrum width 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 and hydrometeor motions have limited airborne radar spectrum width measurements 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 yield spectrum width values that strongly correlate with retrieved mean Doppler variance for a range of non-convective cloud conditions. Further, Doppler spectrum width within turbulent regions of cloud also shows good agreement with in-situ eddy dissipation rate (EDR) and gust probe variance. However, the use of pulse pair estimated spectrum width as a metric for turbulent air motion intensity is only suitable for turbulent air motions more energetic than the magnitude of spectral broadening, estimated to be < 0.4 m s −1 for the WCR in these cases.