System Predictability Assessed by Low Wavenumber Fourier Components and Analogue Pair Progression of Geopotential Height

Abstract

Following Lorenz’s work using analogue pairs for establishing 10-to-14-day predictability limits for synoptic weather regimes, predictability limits for the Rex block, the long-wave wintertime ridge over the eastern Pacific Ocean and the western United States, have been estimated. This was accomplished by using mid-latitude geopotential height reanalysis data over a period of 38 years, 1979–2016, and associated 90-day winters (DJF). The metric used to define analogue pairs is the RMS difference assessed for the hemispheric 850, 500, and 200 hPa geopotential height fields. The resultant set of analogue pairs was used to estimate predictability with respect to both a single latitude circle (40° N) that passes through the Rex Block and for a multi-latitude swath (20–80° N). Our methods showed a range of results, by choice of Fourier component wavenumbers 2 through 8. These results indicate system predictability for low wavenumber components to exceed the 10–14-day limit imposed by Lorenz’ results. The results to 21 days, the maximum predictability limit value allowed by our method, do not preclude the possibility of a greater range of system predictability past 21 days. The unique aspect of this work is determination of predictability limits as a function of geopotential wave structure found through Fourier decomposition.