Hurricane kinetic energy spectra from in situ aircraft observations

Abstract

Just over 30 years ago, Nastrom and Gage (1985) completed their seminal paper on aircraft-derived power spectra. Since then, significant strides have been made in understanding the wavenumber distribution of energy in Earth's atmosphere and its implications on the mathematical limits of forecasting. In recent decades, the meteorological community has enjoyed a substantial degree of success in improving forecast skill, but hurricane intensity predictions have persistently lagged behind. Limited intrinsic predictability is believed to be partially responsible for the failure to appreciably increase tropical cyclone (TC) intensity skill. In this study, aircraft data is analyzed from over 1200 missions carried out by the National Oceanic and Atmospheric Administration (NOAA) and Air Force Reserve Command (AFRC) Hurricane Hunters. Each mission is parsed into distinct flight legs, and legs meeting a specified set of criteria are retained for spectral analysis. Power spectra composites for each category of the Saffir-Simpson scale are produced and reveal relationships between spectral slope and intensity. As hurricane intensity increases, it is found that 1) spectral slope becomes steeper across scales from 10 km to 160 km and 2) the transition zone where spectral slope begins to steepen in value (usually about 400 km; Nastrom and Gage, 1985) shifts downscale. Possible explanations for steepening spectral slope and shifting transition zone are briefly postulated as the product of strong rotation