Recently, Nakamura and Huang (2018) have described a mechanism for the onset of atmospheric blocking in an idealised framework of the midlatitude jet stream. Their setup is analogous to a traffic flow problem with blocked regimes taking the place of congestion. Finite-amplitude local wave activity (LWA), a potential vorticity-based measure of waviness rooted in wave-mean flow interaction theory, is at the center of this mechanism. It was formulated first by Huang and Nakamura (2016) for the barotropic and quasi-geostrophic framework and extended to the primitive equations by Ghinassi et al. (2018) here in the dynamical meteorology group. This enables us to compute LWA fields directly from forecast and reanalysis data.
We have shown that LWA is suitable to diagnose Rossby wave packets (Ghinassi et al. 2018) and will now use it to investigate the Nakamura and Huang theory outside of the idealised setting it originates from. Our chosen tool is ensemble sensitivity analysis (e.g. Hakim and Torn 2008), applied to operational ensemble forecasts in situations where a subset of members predicted a transition into a blocked regime while others did not. We want to quantify the regime transition with an index developed by Grams et al. (2017) and correlate this index with LWA fields for a range of lead times. The intention is to create a library of case studies, each with a comprehensive analysis of the regime transition in terms of precursor Rossby wave packets and properties of the waveguide based on ensemble statistics and reanalysis data.
- Ghinassi, P., Fragkoulidis, G. and Wirth, V. (2018). Local finite-amplitude wave activity as a diagnostic for Rossby wave packets. Monthly Weather Review, 146(12), 4099-4114. [Link to online version]
- Grams, C., Beerli, R., Pfenninger, S. et al. (2017). Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Clim Change, 7, 557-562.[Link to online version]
- Hakim G.J. and Torn R.D. (2008). Ensemble Synoptic Analysis. In: Bosart L.F., Bluestein H.B. (eds) Synoptic—Dynamic Meteorology and Weather Analysis and Forecasting. Meteorological Monographs, vol 33, No. 55. American Meteorological Society, Boston, MA
- Huang, C. S., and Nakamura, N. (2016). Local finite-amplitude wave activity as a diagnostic of anomalous weather events. Journal of the Atmospheric Sciences, 73(1), 211-229. [Link to online version]
- Nakamura, N. and Huang, C. S. (2018). Atmospheric blocking as a traffic jam in the jet stream. Science, 361(6397), 42-47. [Link to online version]