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Dale R. Durran
and
Leonard W. Snellman

Abstract

The physical reason for quasi-geostrophic vertical motion is reviewed. Various techniques for estimating synoptic-scale vertical motion are examined, and their utility (or lack thereof) is illustrated by a case study. The Q-vector approach appears to provide the best means of calculating vertical motions numerically. The vertical motion can be estimated by eye with reasonable accuracy by examining the advection of vorticity by the thermal wind or by examining the relative wind and the isobar field on an isentropic chart. The traditional form of the omega equation is not well suited for practical calculation.

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Dale R. Durran
and
Leonard W. Snellman

Abstract

No abstract available

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Jeffrey D. Kelley
,
David M. Schultz
,
Russ S. Schumacher
, and
Dale R. Durran

Abstract

On 25 December 2016, a 984-hPa cyclone departed Colorado and moved onto the northern plains, drawing a nearby Arctic front into the circulation and wrapping it cyclonically around the equatorward side of the cyclone. A 130-km-wide and 850-km-long swath of surface winds exceeding 25 m s−1 originated underneath the comma head of the lee cyclone and followed the track of the Arctic front from Colorado to Minnesota. These strong winds formed in association with a downslope windstorm and mountain wave over Colorado and Wyoming, producing an elevated jet of strong winds. Central to the distribution of winds in this case is the Arctic air mass, which both shielded the elevated winds from surface friction behind the front and facilitated the mixing of the elevated jet down to the surface just behind the Arctic front, due to steep lapse rates associated with cold-air advection. The intense circulation south of the cyclone center transported the Arctic front and the elevated jet away from the mountains and out across Great Plains. This case is compared to an otherwise similar cyclone that occurred on 28–29 February 2012 in which a downslope windstorm occurred, but no surface mesoscale wind maximum formed due to the absence of a well-defined Arctic front and postfrontal stable layer. Despite the superficial similarities of this surface wind maximum to a sting jet (e.g., origin in the midtroposphere within the comma head of the cyclone, descent evaporating the comma head, acceleration to the top of the boundary layer, and an existence separate from the cold conveyor belt), this swath of winds was not caused by a sting jet.

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