Systematic model forecast error in Rossby wave structure

Abstract

Diabatic processes can alter Rossby wave structure; consequently, errors arising from model processes propagate downstream. However, the chaotic spread of forecasts from initial condition uncertainty renders it difficult to trace back from root‐mean‐square forecast errors to model errors. Here diagnostics unaffected by phase errors are used, enabling investigation of systematic errors in Rossby waves in winter season forecasts from three operational centers. Tropopause sharpness adjacent to ridges decreases with forecast lead time. It depends strongly on model resolution, even though models are examined on a common grid. Rossby wave amplitude reduces with lead 5 days, consistent with underrepresentation of diabatic modification and transport of air from the lower troposphere into upper tropospheric ridges, and with too weak humidity gradients across the tropopause. However, amplitude also decreases when resolution is decreased. Further work is necessary to isolate the contribution from errors in the representation of diabatic processes.

Using satellite and reanalysis data to evaluate the representation of latent heating in extratropical cyclones in a climate model

“…These results are consistent with case study analysis of the relationship between diabatic processes in the WCB and upper-level flow (e.g., Stoelinga 1996;Grams et al 2011;. The representation of the upper-tropospheric circulation around ETCs has implications for downstream development of ETCs, Rossby waves and blocking (e.g., Chagnon et al 2012;Rodwell et al 2013;Gray et al 2014;Martínez-Alvarado et al 2015;Pfahl et al 2015). Any biases in this circulation may impact the ability of HiGEM to simulate ETCs at the eastern end of the storm tracks, such as those ETCs which affect Europe.…”

Rossby wave propagation on potential vorticity fronts with finite width

“…It should be noted that this value of jet maximum represents a local wind speed value; nonlinear waves of realistic amplitude considerably reduce the zonal mean wind speed. A typical isentropic gradient of Ertel PV at the troposphere-stratosphere boundary to the northern side of wintertime ridges in analyses is 1.30 PVU per 100 km, and in 5-day forecasts is 1.05 PVU per 100 km (Gray et al 2014). Taking a typical PV contrast across the tropopause of 4 PVU therefore gives typical widths of the tropopause front on an isentropic surface of r 0 = 308 km and r 0 = 381 km in analyses and 5-day forecasts, respectively.…”

“…The magnitude of the smoothing error has recently been evaluated for several state-of-the-art weather forecast models by Gray et al (2014). They found that the decrease in the isentropic gradient of PV at the tropopause typically occurs during the 790 B. J. Harvey, J. Methven and M. H. P. Ambaum (0) from (3.34) and the phase and group speeds from (3.35) with wavenumber kL R = 1.…”

“…They found that the decrease in the isentropic gradient of PV at the tropopause typically occurs during the 790 B. J. Harvey, J. Methven and M. H. P. Ambaum (0) from (3.34) and the phase and group speeds from (3.35) with wavenumber kL R = 1. The values of r 0 are taken from Gray et al (2014) and all other parameter values are as described in the text.…”

“…Furthermore, the isentropic gradient of PV at the tropopause is found to be systematically too smooth in current global numerical weather prediction (NWP) models, with the gradient typically exhibiting a reduction of approximately 20 % from its initial value over the first three days of a forecast (Gray et al 2014). It is therefore important to understand how the behaviour of Rossby waves on a PV front with small but finite width differs to those on an infinitely sharp PV front.…”