Journal: Tellus A: Dynamic Meteorology and Oceanography

Abbreviation

Tellus, Ser. A, Dyn. meteorol. oceanogr.

Publisher

Taylor & Francis

Journal Volumes

ISSN

0280-6495
1600-0870

Description

Filters

2015 - 201932020 - 20211
Reset filters

Search Results

Publications1 - 4 of 4
  • Localizing the Ensemble Kalman Particle Filter
    Item type: Journal Article
    Robert, Sylvain; Künsch, Hans R. (2017)
    Tellus A: Dynamic Meteorology and Oceanography
    Ensemble methods such as the Ensemble Kalman Filter (EnKF) are widely used for data assimilation in large-scale geophysical applications, as for example in numerical weather prediction. There is a growing interest for physical models with higher and higher resolution, which brings new challenges for data assimilation techniques because of the presence of non-linear and non-Gaussian features that are not adequately treated by the EnKF. We propose two new localized algorithms based on the Ensemble Kalman Particle Filter, a hybrid method combining the EnKF and the Particle Filter (PF) in a way that maintains scalability and sample diversity. Localization is a key element of the success of EnKF in practice, but it is much more challenging to apply to PFs. The algorithms that we introduce in the present paper provide a compromise between the EnKF and the PF while avoiding some of the problems of localization for pure PFs. Numerical experiments with a simplified model of cumulus convection based on a modified shallow water equation show that the proposed algorithms perform better than the local EnKF. In particular, the PF nature of the method allows to capture non-Gaussian characteristics of the estimated fields such as the location of wet and dry areas.
  • Mechanisms underlying temperature extremes in Iberia: a Lagrangian perspective
    Item type: Journal Article
    Santos, João A.; Pfahl, Stephan; Pinto, Joaquim G.; et al. (2015)
    Tellus A: Dynamic Meteorology and Oceanography
    The mechanisms underlying the occurrence of temperature extremes in Iberia are analysed considering a Lagrangian perspective of the atmospheric flow, using 6-hourly ERA-Interim reanalysis data for the years 1979–2012. Daily 2-m minimum temperatures below the 1st percentile and 2-m maximum temperatures above the 99th percentile at each grid point over Iberia are selected separately for winter and summer. Four categories of extremes are analysed using 10-d backward trajectories initialized at the extreme temperature grid points close to the surface: winter cold (WCE) and warm extremes (WWE), and summer cold (SCE) and warm extremes (SWE). Air masses leading to temperature extremes are first transported from the North Atlantic towards Europe for all categories. While there is a clear relation to large-scale circulation patterns in winter, the Iberian thermal low is important in summer. Along the trajectories, air mass characteristics are significantly modified through adiabatic warming (air parcel descent), upper-air radiative cooling and near-surface warming (surface heat fluxes and radiation). High residence times over continental areas, such as over northern-central Europe for WCE and, to a lesser extent, over Iberia for SWE, significantly enhance these air mass modifications. Near-surface diabatic warming is particularly striking for SWE. WCE and SWE are responsible for the most extreme conditions in a given year. For WWE and SCE, strong temperature advection associated with important meridional air mass transports are the main driving mechanisms, accompanied by comparatively minor changes in the air mass properties. These results permit a better understanding of mechanisms leading to temperature extremes in Iberia.
  • Eady edge waves, frontal wave-trains and type-B cyclogenesis
    Item type: Journal Article
    Davies, Huw C. (2021)
    Tellus A: Dynamic Meteorology and Oceanography
    Eady edge waves (EEWs) propagate on surface baroclinic zones and they resemble synoptic and sub-synoptic scale frontal wave-trains. The wave-trains are frequently depicted on surface charts, and occur per se on a front with a local baroclinicity extremum. The possible linkage of EEWs to frontal waves is explored within the framework of so-called 'surface quasi-geostrophic dynamics' by establishing the properties of free and forced EEWs propagating on an ambient flow comprising a laterally confined baroclinic zone. It is shown that the dispersion properties of the free EEWs modes are influenced significantly both by the zone's baroclinicity extremum and by key features that are akin to those of cold and warm fronts. In particular the larger-scale waves are nondispersive and a local wave-packet can retain coherency, the leading modes on a pseudo-cold front possess a phase speed related to the baroclinicity extremum, whereas waves on a pseudo-warm front propagate in the reverse direction. Also theoretical considerations demonstrate that forced modes can undergo resonant secular growth. It is further shown that transient or sustained forcing of a front by an upper-tropospheric potential vorticity anomaly (sic. short-wave trough) can serve to instigate features akin to frontal wave-trains and type-B cyclogenesis. More generally the study hints at the significance for NWP of a front's structure and its dispersion properties, and that an element of predictability is introduced by the latter features due to dynamical scale selection.
  • Modelling economic losses of historic and present-day high-impact winter windstorms in Switzerland
    Item type: Journal Article
    Welker, Christoph; Martius, Olivia; Stucki, Peter; et al. (2016)
    Tellus A: Dynamic Meteorology and Oceanography
Publications1 - 4 of 4