Sierra Rotors: A Comparative Study of Two Sierra Rotors Events
Brian J. Billings and Vanda Grubišić Desert Research Institute Reno, NV
Acknowledgments
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All the members of the Sierra Rotors Project, including
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Dr. Steven Cohn (NCAR) Dr. Bill Brown (NCAR) Mr. Harold Klieforth (DRI) Dr. James Doyle (NRL) Dr. Dale Durran (UW)
Dr. Qingfang Jiang (UCAR/NRL)
Ms. Ming Xiao (DRI)
Observed Cloud Formations
IOP 8 IOP 14
~01 UTC (~17 PST) 25 March
~01 UTC (~18 PDT) 20 April
W of Independence, looking SE
W of Lone Pine, looking N
Photo by Vanda Grubišić
Photo by Alex Reinecke
700 hPa Analyses
IOP 8 IOP 14
00 UTC 26 March (16 PST 25 March)
00 UTC 21 April (17 PST 20 April)
SW flow aloft, Passage of strong cold front
NW flow aloft, Weak cold front remains offshore
Common Characteristics ●light, variable winds during the early morning hours ●thermally forced circulations in the valley throughout the morning ●a sharp transition to strong westerlies between 13-14 PST ●a stronger northerly component during the evening IOP 14 IOP 8 No reversed surface flow Reversed, easterly flow during the height (rotor remains elevated) of the event (rotor circulation).
Hour (PST)
12
18 00 06 12 12 18
Hour (PST)
00 06 12
Hour (UTC)
Hour (UTC)
Common Characteristics ●area of well-mixed, reversed flow underneath the main wave crest (rotor) IOP 8 Long wavelength, wave crest and rotor over center of valley IOP 14 Shorter wavelength, wave crest and rotor further up the slopes
IOP 8 ●weaker pressure gradients ●equally strong winds on slopes and valley floor ●reversed flow at 3 UTC (19 PST)
IOP 14 ●stronger pressure gradients ●stronger winds on valley floor than on slopes ●no reversed flow
L
H
L
H
Common Characteristics
strong downslope winds along the eastern slopes of the Sierra Nevada ●gap jets (Kearsarge, Sawmill) which extend further into the valley
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IOP 8
Downslope winds extend further into the valley, gap jets are narrow
IOP 14
Downslope winds do not reach valley floor, gap jets merge into an area of broad flow
N2 1 2U l 2 2 2 U U z
2
k l coth h k l
2 2
2 1
2 1
k l2 0
2
2
Scorer (1949)
IOP 8 2 l1 longer, more variable ●Smaller trapped lee wavelengths 2 ● l profile changes from a sharp decrease with height to a constant profile after frontal passage.
10000 10000
IOP 14 2 ●Larger l1 shorter, less variable trapped lee wavelength 2 l profile does not ●Decreasing change through the IOP.
8000 Height (m) Height (m)
8000
6000
6000
4000
4000
2000 -2e-07
0
2e-07 Scorer parameter (m-2)
4e-07
6e-07
2000
0
2e-06
4e-06
6e-06
Scorer parameter (m-2)
IOP 8 After frontal passage: ●wave breaking aloft ●energy propagates vertically
IOP 14 No frontal passage: ●energy remains trapped at low levels
Summary
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Common Characteristics:
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visual evidence of mountain wave and rotor activity strong cross-mountain flow in a pre-frontal environment a similar diurnally varying sequence of wind regimes strong downslope winds and gap jets trapped lee waves through the major part of both events
horizontal wavelength and its temporal variability touchdown of rotors on valley floor
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Fundamental Differences:
– –
Summary
IOP 8
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SW flow w/ frontal passage reversed, easterly surface flow weaker pressure gradients, more penetrating downslope flow, and narrower gap jets
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longer wave, leading edge over valley center
wave trapping aided by unstable layer aloft w/ vertically propagating wave at end of event
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Summary
IOP 14
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NW flow w/o frontal passage no reversed surface flow stronger pressure gradients, less penetrating downslope flow, and broad, merged gap jets
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shorter wave, leading edge further upslope
wave trapping aided by temperature inversion aloft w/ wave trapping throughout event
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Implications for future research
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Strong rotor events form in similar synoptic environments with similar diurnal variations. A strong diurnal component points to the importance of surface heating, location and scale need to be explored. Variable positions of the wave leading edge and rotor (wavelengths), downslope penetrations, and gap jet widths indicate 3D variability that require varying observational strategies (mobile equipment, whole valley scanning with remote sensors). How do upstream flow properties determine specific event characteristics (wind direction, inversions, moisture distribution, boundary layer)?
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Common Characteristics ●favorable trapped lee wave profiles at the start of the event - decreasing stability and sharply increasing wind speed IOP 8 Wave trapping due to unstable layer at 500 hPa IOP 14 Wave trapping due to inversion layer at 500 hPa
IOP 8 After frontal passage: ●more uniform stability and wind profiles
IOP 14 No frontal passage: ●temperature inversion and strong wind shear remain in place