1. T. Connor Nelson 09 December 2016 ATM: 509 Precipitation Processes
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Orographic Effects During a Extreme Precipitation Event in Western South Dakota
T. Connor Nelson
09 December 2016
ATM: 509 Precipitation Processes

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Motivation
The flow regime associated with any appreciable sized barrier is important to understanding orographic effects in extreme precipitation events
(e.g., Parsons and Hobbs 1983; Buzzi et al. 1998; Chu and Lin 2000; Chen and Lin 2005; Lean et al. 2009)
Froude number (Fr) (Stull 1988; Muccilli 2015)
Fr > 1: supercritical  air can flow over mountain ranges, highest precipitation would occur on the lee side
Fr < 0.5: subcritical  blocked flow, precipitation would most likely fall on the windward side

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Methods D1
Two regions of analysis:
Synoptic: D1
Local: D2 D2

4. 05 – 06 August 2014 Black Hills South Dakota
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05 – 06 August 2014 Black Hills South Dakota
Did the overall flow pattern and static stability in conjunction with synoptic conditions aid in long lasting heavy precipitation?
4 km gridded Stage IV Data precipitation data in D2
North American Regional Reanalysis (NARR) synoptic and moisture data in D1
NARR topography, 1000-hPa winds, and 600-hPa omega (Pa s-1) in D2
Compute Froude numbers from 00 UTC 04 August – 12 UTC 07 August 2014
Radar data for key time frames in D2

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Decreased
Stability

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Conclusions
Understanding the effects of local orography serves a crucial role in understanding EPEs in high terrain environments
05–06 August 2014 Black Hills, SD EPE
Prior to the EPE, western South Dakota was dominated by a strong upper level ridge
A short-wave tough on the western side of the ridge caused strong moisture transport of moisture into the region, surface easterly flow, and decreased the local stability
Fr > 1
Leads to strong upslope flow, convection, and lee side precipitation
The strong upslope flow would lift parcels above the shallow LFC

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Conclusions
Understanding the effects of local orography serves a crucial role in understanding EPEs in high terrain environments
05–06 August 2014 Black Hills, SD EPE
Prior to the EPE, western South Dakota was dominated by a strong upper level ridge
A short-wave tough on the western side of the ridge caused strong moisture transport of moisture into the region, surface easterly flow, and decreased the local stability
Fr > 1
Leads to strong upslope flow, convection, and lee side precipitation
The strong upslope flow would lift parcels above the shallow LFC

9. References
Bunkers, M. J., M. Smith, D. Driscoll, and G. Hoogestraat, 2015: Hydrologic response for a high-elevation storm in the South Dakota Black Hills. Internal Report , 21 pp., URL
Buzzi, A., N. Tartaglione, and P. Malguzzi, 1998: Numerical simulations of the 1994 Piedmont flood: role of orography and moist processes. Mon. Wea. Rev., 2369–2383.
Chen, S., and Y. Lin, 2005: Effects of moist Froude number and CAPE on a conditionally unstable flow over a mesoscale mountain range. J. Atmos. Sci., 331–350.
Chu, C., and Y. Lin, 2000: Effects of orography on the generation and propagation of mesoscale convective systems in a two-dimensional conditionally unstable flow. J. Atmos. Sci., 3817–3837.
Gerber, B. M., 2015: Verification and assessment of operational local area weather forecasting models. M.S. thesis, Atmospheric and Environmental Sciences Program, South Dakota School of Mines and Technology, 142 pp., Rapid City, SD.
Lean, H. W., N. M. Roberts, P. A. Clark, and C. Morcrette, 2009: The surprising role of orography in the initiation of an isolated thunderstorm in southern England. Mon. Wea. Rev., 3026–3046, doi: /2009MWR
Mesinger, F., and Coauthors, 2006: North American Regional Reanalysis. Bull. Amer. Meteor., 87 (3), 343–360, doi: /BAMS
Muccilli, M., 2015: Using the Froude number to improve orographic snow forecasts in the Green Mountains of Vermont. Eastern Region Technical Attachment , 43 pp., URL
Parsons, D. B., and P. V. Hobbs, 1983: The mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones. IX: some effects of orography on rainbands. J. Atmos. Sci., 1930–1949.
Schumann, U., 1987: Influences of mesoscale orography on idealized cold fronts. J. Atmos. Sci., 3423–3441.
Stull, R. B., 1988: An introduction to boundary layer meteorology. Kluwer Academic Publishers, Dordrecht, Netherlands.