RefDoc
Haut

Faire une nouvelle recherche
Make a new search
Lancer la recherche


Titre du document / Document title

Observations of the evolution of orogenic blocking

Auteur(s) / Author(s)

MAYR G. J. (1) ; MCKEE T. B. ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

(1) Univ. Innsbruck, Inst. Meteorologie Geophysik, 6020 Innsbruch, AUTRICHE

Résumé / Abstract

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 high. Therefore unblocking happens most frequently when the trough axis of a short wave is immediately upstream of the barrier, but synoptic pressure gradients caused by contrasts in vorticity and differential temperature advection are sometimes also strong enough. The flow returns to its blocked state when the ridge behind the trough approaches the barrier so that the synoptic cross-barrier pressure gradient reinforces the mesoscale high. For a lower barrier or stronger solar insolation, a well-mixed boundary layer can grow almost to the height of the barrier by afternoon and reconnect the blocked layer with the higher cross-barrier winds above the mountain. After sunset the thermal forcing changes sign as the radiative cooling stabilizes the lower atmosphere again and the transition back to the blocked state occurs

Revue / Journal Title

Monthly weather review    ISSN  0027-0644   CODEN MWREAB 

Source / Source

1995, vol. 123, no5, pp. 1447-1464 (28 ref.)

Langue / Language

Anglais

Editeur / Publisher

American Meteorological Society, Boston, MA, ETATS-UNIS  (1872) (Revue)

Mots-clés anglais / English Keywords

Atmospheric blocking

;

Mountain

;

Wind field

;

Winter

;

Rocky Mountains

;

Colorado

;

North America

;

America

;

United States

;

Mots-clés français / French Keywords

Blocage atmosphérique

;

Montagne

;

Distribution vent

;

Hiver

;

Rocheuses

;

Colorado

;

Amérique du Nord

;

Amérique

;

Etats Unis

;

Mots-clés espagnols / Spanish Keywords

Bloqueo atmosférico

;

Montaña

;

Distribución viento

;

Invierno

;

Rocosas

;

Colorado

;

America del norte

;

America

;

Estados Unidos

;

Localisation / Location

INIST-CNRS, Cote INIST : 141, 35400005689634.0120

Nº notice refdoc (ud4) : 3508999



Faire une nouvelle recherche
Make a new search
Lancer la recherche
Bas