Some Remarks on the Climatology of Blocking Action

Brezowsky, H; Elohn, H.; Hess, Peter

doi:10.3402/tellusa.v3i3.8630

Cited by 19 publications

(4 citation statements)

References 4 publications

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“…Blocking frequency is determined as the ratio of the number of blocked days at a given longitude to the total number of days of the season. Two main regions of maximum frequency are observed: one over the Pacific in winter and the second one over the Atlantic and Europe in winter and spring extending to Siberia, in agreement with the early works of Rex [1950] and Brezowski et al [1951]. High values are also found in summer over the Pacific, as described by Lupo and Smith [1995].…”

Section: Tm Indexsupporting

confidence: 84%

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

2002

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[1] The sensitivity of Northern Hemisphere blocking features to the use of two different blocking indices is investigated using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses (ERA) and data from a climate simulation carried out with the atmospheric general circulation model Action de Recherche, Petite Echelle, Grande Echelle ARPEGE. The seasonal cycle of blocking frequency shows a good agreement between both indices, for both analyses and model, when the seasonal cycle of the 500-hPa geopotential height is previously removed from the data. Furthermore, the blocking frequency seasonal cycle agrees well with the 5-to 45-day bandpass-filtered standard deviation (a proxy indicator of blocking activity) seasonal cycle. In contrast to previous results, this demonstrates that a consensus estimate of blocking features may be obtained with two different blocking indices. Nevertheless, some differences in the results from the two indices have been detected, particularly for the Euro-Atlantic blocking. Regional differences suggest that on some occasions, an index may detect blocking events that the other index misses, and vice versa. An important consequence is that uncertainty in blocking interannual variability estimates, which depend on the index used, is expected to be high. The study shows that the model underestimates the blocking frequency regardless of the index used. However, the model proves to be able to simulate realistic blocks when removing an estimate of the geopotential time-mean bias. Blocking drawbacks in the model are interpreted as a consequence of a specific zonal wind forcing associated with a decrease in the frequency of large-scale ridges over the main blocking regions. These results strengthen the interest of analyzing model blocking as a complex phenomenon with regional dynamical differences and tight links with other atmospheric phenomena.

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Section: Tm Indexsupporting

confidence: 84%

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

2002

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“…This flow excited internal gravity waves that were able to propagate up into the stratosphere. The correlation of blocking highs [Brezowsky et al, 1951] with the appearance of PSCs above northern Scandinavia was noted early on by Dietrichs [1950].…”

Section: Discussionmentioning

confidence: 94%

Evidence for inertia gravity waves forming polar stratospheric clouds over Scandinavia

et al. 2002

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At three successive days at the end of January 2000 the Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) airborne lidar Ozone Lidar Experiment explored mountain‐wave‐induced polar stratospheric clouds above the Scandinavian mountain ridge. Global analyses and mesoscale modeling are applied to explain their complex internal structure and their day‐to‐day variability. Depending on the synoptical‐scale meteorological conditions, stratospheric temperature anomalies of different amplitude and horizontal extent are generated by the upward propagating mountain waves. Short‐term excitation of about 6 hours resulted in localized stratospheric temperature anomalies directly above the mountain ridge as for 25 January 2000. In this case, the elevation of the observed clouds differed not much from the synoptic‐scale clouds upstream above the Norwegian Sea. On the other hand, long‐lasting flow past the Scandinavian mountain ridge formed huge 400‐km horizontally extending stratospheric ice clouds in altitudes as much as 5 km above the elevation of the upstream clouds just 1 day later. Inertia gravity waves with horizontal wavelengths of about 350 km are responsible for their formation. For the first time a predicted temperature minimum far downstream of the mountains could be proofed by the observation of an isolated stratospheric ice cloud above Finland. The observed particles are classified in terms of their measured optical properties such as backscatter ratio and depolarization. In all cases, mountain waves generated ice clouds. In contrast to the nitric acid trihydrate tail of the ice cloud on 25 January the same classification results in a tail of liquid supercooled ternary solutions droplets 1 day later. The particle structure downstream of the mountains is very complex and needs detailed microphyical modeling and interpretation.

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“…Rex's study of blocking situations served as a model for several others: the more significant ones are Brezowsky et al (1951) and Sumner (1954). Using 1949-52 data Sumner confirmed the spatial distribution of Rex and found a mode between 55 and 60°N in the latitudinal distribution extending from 100°W to 60°E.…”

Section: Introductionmentioning

confidence: 92%

Blocking action in the northern hemisphere: A Climatological study

Treidl¹,

Birch²,

Sajecki³

1981

Atmosphere-Ocean

103

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Using criteria developed from scientific studies, the blocking situations observed in the Northern Hemisphere during the period 1945-1977 are subjectively assessed and statistically analysed. Earlier findings are largely confirmed while new results are presented on seasonal and secular blocking trends, and the probability of multiple blocking. A catalogue of'664 blocking cases was prepared but is not included here because of its size; however, copies of the catalogue may be obtained from the authors.RÉSUMÉ A l'aide de critères établis à partir d'études scientifiques, on évalue de manière subjective et on analyse du point de vue statistique, les situations de blocage observées dans l'hémisphère nord, au cours de la période de 1945 à 1977. On confirme de nombreux résultats déjà obtenus et on en présente de nouveaux sur les tendances de blocage saisonnières et séculaires ainsi que sur la probabilité de blocage multiple. Un catalogue de 664 situations de blocage a été préparé mais étant donné son volume, H n'est pas présenté ici; on peut cependant en obtenir des exemplaires en s'adressant aux auteurs.

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Some Remarks on the Climatology of Blocking Action

Cited by 19 publications

References 4 publications

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

Evidence for inertia gravity waves forming polar stratospheric clouds over Scandinavia

Blocking action in the northern hemisphere: A Climatological study

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