Group seminar on 30. April, 14:15
Benefits of initializing equatorial waves on accuracy of extratropical forecasts
Chen Wang
Uncertainties in the tropical initial state in global numerical weather prediction (NWP) models are one cause of forecast-error growth in medium- and extended-range forecasts in the extratropics. It has been demonstrated that a more reliable initialization of tropical Rossby and non-Rossby waves benefits from wind observations and wave-aware data assimilation. It is important to understand how the assimilation of wind and mass observations in the tropics affects the accuracy of equatorial wave analysis and influences extratropical predictability. To address these questions, we investigate the effects of assimilating observations within the tropics using a reduced-complexity model and a perfect-model assimilation framework. Our model and variational data assimilation method consider equatorial waves as both prognostic and control variables. We assess assimilation benefits in terms of spatial scales in the initial conditions and forecasts. Forecast benefits are quantified along latitude circles for wind and geopotential height. We show that the northern hemisphere has a larger MSE reduction than the southern hemisphere because of the asymmetrical jet in the background. The MSE reduction and background energy spectra peak at different wavenumbers, showing that propagating wave signals interact with the zonal mean flow. Initial large-scale MSE reduction in the tropics is important for mid-altitude large-scale MSE reduction, while initial small-scale MSE reduction in the tropics barely have impact in extratropical regions over 40 degrees.