Representation of Synoptic‐Scale Rossby Wave Packets and Blocking in the S2S Prediction Project Database

Abstract

Rossby wave packets (RWPs) are one of the dominating midlatitude atmospheric phenomena. A realistic representation of RWPs is essential for a skillful prediction of the day‐to‐day weather and its extremes. Using an objective RWP tracking technique, this study verifies the representation of RWPs for all models in the Subseasonal to Seasonal (S2S) Prediction Project Database. Consistent with reanalyses, the reforecasts show one RWP every 4 days. The three models with the coarsest grid spacing tend to overestimate the propagation distance due to a negative RWP decay frequency bias over the Atlantic‐European sector. A verification of atmospheric blocking suggests that this bias is related to a negative blocking frequency bias in the same region. The biases are reduced in models with finer grid spacing. However, that this bias reduction is not only dependent on the grid spacing highlights the effect of the different representation of key physical processes on RWPs and blocking.

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

“…Due to its persistence, blocking might contribute to improved thunderstorm potential predictability on sub-seasonal time scales beyond the classical weather forecast time scale of a few days, and complement current activities that investigate the connection of water vapour transport on the sub-seasonal predictability of extremes (e.g., Lavers et al, 2016aLavers et al, , 2016bPasquier et al, 2018). It is important, however -especially in connection with local-scale phenomena such as deep moist convection -that the blocking position is correctly predicted, which is currently still a challenge in state-of-the-art global numerical weather prediction models (Quinting and Vitart, 2019).…”

“…). It is important, however – especially in connection with local‐scale phenomena such as deep moist convection – that the blocking position is correctly predicted, which is currently still a challenge in state‐of‐the‐art global numerical weather prediction models (Quinting and Vitart, ).…”

“…For example, Rossby wave breaking is one relevant process for blocking occurrence, and thus for the position of blocking and the resulting mid‐tropospheric flow (Masato et al . ; Quinting and Vitart, ). Visual analyses identify two different preferred flow situations during blocking in the BS area.…”

“…The reason for the different signals related to DLS during blocking is due to the synoptic scale flow: the varying location of the western flank of the blocking ridge associated with troughs and fronts upstream. For example, Rossby wave breaking is one relevant process for blocking occurrence, and thus for the position of blocking and the resulting mid-tropospheric flow (Masato et al, 2012;Quinting and Vitart, 2019). Visual analyses identify two different preferred flow situations during blocking in the BS area.…”

New insights into Rossby wave packet properties in the extratropical UTLS using GNSS radio occultations

“…This has important implications for RWP diagnostics, which are usually performed with meridional winds at a fixed pressure level, which is not equidistant from the tropopause over latitude and season. 30 The most widely used pressure level for RWP diagnostics is 300 hPa, either for showcasing RWP envelope reconstruction/tracking methods (Zimin et al, 2003(Zimin et al, , 2006Souders et al, 2014a;Wirth, 2015, 2017), producing RWP clima-tologies/composites (Chang, 1999;Chang and Yu, 1999;Williams and Colucci, 2010;Souders et al, 2014b) or studying their predictability (Quinting and Vitart, 2019). The 250 hPa level was also used by Glatt and Wirth (2014) and Grazzini and Vitart (2015) to study RWP properties and predictability in forecast models.…”