Monday, 30 August 2010: 9:45 AM
Alpine Ballroom A (Resort at Squaw Creek)
This wind-profiler based study highlights key characteristics of the barrier jet along the windward slope of California's northern Sierra Nevada. Between 2000 and 2007, roughly 10% of 100,000 hourly wind profiles recorded at two sites contained barrier-jet (BJ) attributes, defined as follows: (1) a relative maximum in terrain-parallel flow (directed from 160°) between 0 and 3 km MSL (Vmax); (2) the V-component decreases by ≥2 m s-1 with increasing height somewhere between the altitude of Vmax and 3 km MSL; and (3) Vmax occurs at the second radar range gate or higher (i.e., >~200 m above ground). The magnitude of Vmax in the BJ core was similar at the valley and windward-slope sites (~17.5 m s-1) and at a comparable altitude 500-1000 m above the surface (i.e., the BJ was terrain-following). The cross-mountain wind speed was weak at the altitude of Vmax, consistent with blocked conditions. The seasonal cycle of BJ occurrences showed a maximum during the cooler months and a minimum in summer. Additionally, BJs were stronger in winter than in summer. An inventory of ~250 BJ cases from the two sites was generated (a case contains >8 consecutive BJ profiles) for the climatologically wet cool season. Up to 60% of the nearby cool-season precipitation fell during BJ cases, and these cases shifted the precipitation down the Sierra's windward slope and enhanced precipitation at the north end of the Central Valley (relative to non-BJ conditions).
The large number of cool-season BJ cases were stratified by the mean strength and altitude of Vmax and by case duration, although for the sake of brevity, we will focus solely on the strength (whereas the follow-on talk by J. Lundquist will focus on BJ altitude). Composite profiles of the along-barrier component for the strongest and weakest 20 ranked cases reveal stark differences in the magnitude and vertical positioning of the BJ and uniquely different regional precipitation characteristics, as will be described in the talk. North American Regional Reanalysis (NARR) plan-view composites were generated to explore the meso- to synoptic-scale conditions responsible for, and to showcase the precipitation distributions associated with, the strongest and weakest 20 ranked BJ cases. The composite analyses yielded large contrasts that could prove useful in forecasting BJs and their precipitation impacts. The BJ cases occurred, on average, in the pre-cold-frontal environment of landfalling winter storms. Although the NARR provided significant insight into the meso-synoptic conditions of strong-vs.-weak BJs, it had difficulty capturing the detailed BJ structures, thus highlighting the importance of observation-based climatology studies.
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