On the operational predictability of blocking

Abstract

The entire 7‐year archive of ECMWF operational analysis and forecast data is used to assess the skill of the Centre's model in short‐ and medium‐range forecasting of atmospheric blocking. The assessment covers 7100‐day periods, from 1 December to 10 March of all winters from 1980‐81 to 1986‐87, inclusive. A slightly modified version of the Legenäs and Økland objective zonal index is used to quantify both observed and forecast occurrence of blocking. The study is performed on 500 hPa geopotential height and on Euro‐Atlantic and Pacific blocking separately. It is found that blocking frequency is severely underestimated in medium‐range forecasts; the model is, on average, reasonably skilful if the initial conditions are blocked, but blocking onset is poorly represented if it occurs more than a few days into the forecast. This inability in entering the blocking regime has a substantial impact on the systematic error of the model.

Dependence of Present and Future European Temperature Extremes on the Location of Atmospheric Blocking

“…We use a standard two‐dimensional blocking detection algorithm based on the reversal of 500‐hPa geopotential height gradients (Anstey et al, 2013; Rex, 1950; Scherrer et al, 2006; Tibaldi & Molteni, 1990). At each grid point geopotential height gradients to the north (Δ Z N ) and to the south (Δ Z S ) are calculated as $\begin{array}{ccc}right\mathrm{\Delta }{Z}_{\mathrm{normalN}}& left=\frac{Z(\lambda ,\varphi +\mathrm{\Delta }\varphi )-Z(\lambda ,\varphi )}{\mathrm{\Delta }\varphi }& right\\ right\mathrm{\Delta }{Z}_{\mathrm{normalS}}& left=\frac{Z(\lambda ,\varphi )-Z(\lambda ,\varphi -\mathrm{\Delta }\varphi )}{\mathrm{\Delta }\varphi },\end{array}$ where Z is the geopotential height at 500 hPa, Δ ϕ = 15°, and ϕ ranges from 50°N to 75°N.…”

Influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings

“…[10] Here, we make use of a blocking index based on the meridional gradient of geopotential height (Z) at 500 hPa [Tibaldi and Molteni, 1990] as follows: First, geopotential height gradients at higher latitude (GHGH) and lower latitude (GHGL) are computed at each longitude:…”

Trends in Mediterranean gridded temperature extremes and large-scale circulation influences

“…2). Thus, the analysis could be repeated using one or more of the available Atlantic blocking indices (Tibaldi and Molteni, 1990;Scherrer et al, 2006). Moreover, the frequency and persistence of mesoscale patterns may also play a considerable role in the year-to-year variations of indices of extremes as well as their trends (Domonkos et al, 2003;Kyselý, 2008).…”