Atmospheric blockage
In an atmospheric blocking ( English blocking action ) is, by blocking the west wind drift by a nearly stationary until reaching great heights, hot high-pressure area serving as blocking high is designated.
A blocking high arises in the middle latitudes of the northern hemisphere from a wedge of the subtropical high-pressure belt that is advanced to the north . As a result of the blockage, the zonal flow branches out into several branches with meridional components . The surrounding low pressure areas are no longer shifted to the east. Their track now leads around the high altitude, if the west wind drift does not come to a complete standstill. They are pushed north or disintegrate on the western edge of the high.
Because of their long lifespan, blocking highs often determine the overall weather situation of an area for one to two weeks or longer. Preferred areas for the formation of such blockages are off the west coasts of Europe and North America. Most often, blockages form here in late winter and spring, and such weather conditions are least likely in late summer.
The omegalage is a special form of atmospheric blocking. A high is flanked to the west and east by lows, which are also blocked. The course of the vertical flow resembles the Greek letter Ω .
A measure of blocking in the northern hemisphere is, for example, the blocking index according to Lejenas and Okland (1983), varied by Tibaldi and Molteni (1990). For this purpose, the 500 hPa geopotential height gradient (GHG) is measured as a normalized difference to 80 ° N and 40 ° N in a zone around 60 ° N geographical latitude . The index speaks for a blockage if the gradient to the north is positive, which is less than −10 m pressure area deviation per degree to the south. This is how the north shift of a high in the westerly wind zone is represented. There are also several other approaches to quantifying the blockage.
 The blocking index according to Tibaldi / Molteni - here as a Hovmöller diagram of the moving 5-day mean, plotted over the time axis (from top to bottom) and the geographical length (from left to right), color-coded the intensity of the values - shows typical blockade episodes. It is characteristic of staying at the same length for several days, which represents the broken westerly wind drift, sometimes even a slight drift to the west, an overlap of the jet stream oscillation with northeast currents (graphic from the NOAA-NWS Climate Prediction Center).
This diagram shows several blocking highs in four months (December 1, 2009 - March 1, 2010) over the North Atlantic and the North Pacific, with ingress of polar cold air on their east side (as well as several cold waves in Europe). |
literature
- ↑ H. Lejenas, H. Okland: Characteristics of Northern Hemisphere blocking as determined from a long time series of observational data. In: Tellus 35A (1983), pp. 350-362.
- ^ S. Tibaldi, F. Molteni: On the operational predictability of blocking. In: Tellus 42A (1990), pp. 343-365.
Web links
- Blocking. NOAA-NCEP-Climate Prediction Center: cpc.ncep.noaa.gov > Monitoring and Data> Monitoring Weather & Climate - current data
Individual evidence
- ↑ a b c d Lexicon editor of the FA Brockhaus publishing house (ed.): Der Brockhaus. Weather and climate . Brockhaus, Leipzig / Mannheim 2009, ISBN 978-3-7653-3381-1 , p. 48.
- ↑ blocking action at Spektrum.de, accessed on January 11, 2019
- ↑ Web link blocking. NOAA-NCEP-CPC, Composites Section - For Seasonal Distribution.
- ↑ Omegawetterlage in the weather dictionary at wetteronline.de, accessed on January 11, 2019
- ↑ a b Blocking Index. NOAA-NCEP-Climate Prediction Center: cpc.ncep.noaa.gov (accessed January 23, 2019).