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Observations of the evolution of orogenic blocking
The evolution of low-level flow upstream of the Continental Divide (Rocky Mountains) and the Wasatch Range from being unable to surmount the mountain range, to becoming unblocked and blocked again is studied observationally. During two months in the winter of 1991/92, a transect of three wind profilers measured the wind field every few minutes with unprecedented temporal detail. The average state of that region during winter is blocked. A total of 47 blocked events were observed. A blocked flow event lasted on the average one and a half days, but the duration varied widely from a few hours to eight days controlled by the synoptic situation. The transition between the two states happened rapidly on the order of 1 h with a minimum of 20 min and a maximum of 4 h. The depth of the blocked layer during one blocking episode fluctuated considerably but reached on the average one-half to two-thirds of the barrier depth (depending on the location). Previous research of idealized equilibrium situations focused on changes of the cross-barrier wind speed and stability as determining variables to build a mesoscale high over the barrier. Since their values were in the blocked range, other mechanisms had to trigger the transitions to an unblocked state. A conceptual model proposes synoptic and radiative forcing to drive the blocking evolution. When the mountain-induced mesoscale high blocks the low-level flow, an opposing synoptic cross-barrier pressure gradient can negate the mesoscale hig
American Meteorological Society
ISSN : 0027-0644 CODEN : MWREAB
Monthly weather review A. 1995, vol. 123, n° 5, pp. 1447-1464 [bibl. : 28 ref.]
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