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Climatology of Precipitation and Precipitation Extremes in the United States Greg Johnson Applied Climatologist USDA-NRCS National Water and Climate Center.

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Presentation on theme: "Climatology of Precipitation and Precipitation Extremes in the United States Greg Johnson Applied Climatologist USDA-NRCS National Water and Climate Center."— Presentation transcript:

1 Climatology of Precipitation and Precipitation Extremes in the United States Greg Johnson Applied Climatologist USDA-NRCS National Water and Climate Center Portland, Oregon RFC Hydromet November 2000

2 Characteristics of the Mean Precipitation Climate n The principal controlling factors are the availability of atmospheric moisture and lifting mechanisms n Moisture controlled by flow from or proximity to large water bodies n Propensity for lifting influenced by topography, convergence zones (seabreeze, etc.), preferred storm tracks (jet dynamics)

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4 Orographic Precipitation Enhancement Factors n Wind Direction (relative to topography) n Wind Speed n Atmos. Moisture (precipitable water) n Elevation Rise n Slope Angle

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6 Issues of Scale (Spatial and Temporal) n Over long averaging times (say, the 30 year normal maps), only the most important and consistent meteorological factors are evident n Progressively shorter time spans reveal ever-increasing nuances of the atmospheric system

7 A Snapshot in Time

8 24 Hour Time Integration

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10 July normal Precipitation (top) versus July 1993 Precipitation (bottom)

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12 Statistical Properties of Precipitation n Persistence, or lack thereof n Average amount of precipitation n Variability in precipitation amount, and theoretical maximum n Frequency of precipitation n Duration of precipitation

13 Annual Mean of Wet-Dry Day Prob. (x1000), Southwest Idaho

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15 A Spatial Climate Modeling System n PRISM (Parameter-elevation Regressions on Independent Slopes Model) n Statistical/Dynamical/Topographic approach n Funded primarily by the NRCS-NWCC since 1993 for development of spatial climate products for the U.S. n Developed by Dr. Chris Daly of the Spatial Climate Analysis Service, Oregon State University

16 What is PRISM?

17 PRISM n Grid based model, approx. 4 km horiz. resolution n Any given grid cell value is determined by a linear regression of station values against elevation n Stations assigned weights n Combined weight of a station is a function of many factors

18 Form of Grid Cell Prediction n Y = 1 X + 0, where Y is the predicted climate element and X is the DEM elevation at the target cell. 0 and 1 are regression slope and intercept, and are determined by x,y pairs of elevation and climate observations from nearby climate stations where Y is the predicted climate element and X is the DEM elevation at the target cell. 0 and 1 are regression slope and intercept, and are determined by x,y pairs of elevation and climate observations from nearby climate stations

19 Station Weighting n Combined weight of a station is: W = f {W d, W z, W c, W l, W f, W p, W e }, where W d, W z, W c, W l, W f, W p, W e are the distance, elevation, cluster, vertical layer, topographic facet, coastal proximity and effective terrain height weights. where W d, W z, W c, W l, W f, W p, W e are the distance, elevation, cluster, vertical layer, topographic facet, coastal proximity and effective terrain height weights.

20 Vermont Annual Precipitation

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24 Olympic Mtns Terrain

25 Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation On Inches

26 Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation Off Inches

27 Olympic Mtns Annual Precipitation Facets Off + Vertical Extrapolation Off Inches

28 Olympic Mtns Annual Precipitation No elevation Inches

29 Olympic Mtns Annual Precipitation Facets On + Vertical Extrapolation On Inches

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31 Hawaii Annual Precipitation No Layer Weighting

32 Hawaii Annual Precipitation Two Layers

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36 SNOTEL Coverage

37 PRISM-derived Products n Mean Mon. and Ann. Precipitation n Mean Mon. and Ann. Temps (mx/mn) n Frost dates and freeze-free season n Extreme winter min. temps & probs. n Growing, heating, cooling degree days n Snow-water equivalent

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40 Oregon Annual Precipitation

41 Annual Precipitation Map of Elmore County, Idaho Produced by the NRCS NCGC “Cut-out” of State Map

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44 PRISM - SWE Map - Oregon

45 PRISM Product Dissemination n Web Sites: OSU: (Raster and polygon coverages of practically everything produced to date (Arc, GRASS); documentation; metadata; DEM’s) NRCS: (U.S., Regional and State mean annual precipitation cartographic products)

46 Cartographic state mean annual precipitation maps  Cartographic state mean annual precipitation maps  Full repository of “official” NRCS PRISM layers

47 PRISM Product Dissemination n Compact Discs: All precipitation layers for all of the U.S. 3 CD’s (East, Central, West) of the lower 48 states. Includes Arc Explorer viewing software, and all documentation. Available from the NRCS-NCGC:

48 PRISM Product Dissemination n Hardcopy maps: Cartographic-quality, walls-size maps of mean annual precipitation for each state Available from the NRCS Climate Data Liaison in each state

49 Extreme Precipitation Climatology

50 Spatial Considerations n Means and extremes are not always correlated, especially over large spatial domains, or even in small regions with significant climate variations: Portland OR MAP 37.39” Portland OR MAP 37.39” Max ” Max ” Washington DC MAP 39.00” Washington DC MAP 39.00” Max ” Max ”

51 Elevational Effect: Reynolds Creek Watershed Idaho n Valley bottom: MAP: 11.14” 50 yr 1-hour value: 1.18” 50 yr 1-hour value: 1.18” n Mountain top location: MAP: 43.58” 50 yr 1-hour value: 1.26” 50 yr 1-hour value: 1.26” Distance between stations: 10 miles Elevation difference: 3500 feet Elevation difference: 3500 feet

52 Example of Variations over Small Regions: Reynolds Creek, Idaho (values in mm precipitation): n 10 Year Return n 3500 feet Atlas feet Atlas 2 n 30 min n 1 hour n 24 hours n n 50 Year Return n 30 min n 1 hour n 24 hours

53 Geographic Effect on Extreme vs. Mean Precipitation

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59 Precipitation Frequency Definition n Year Event: “An event which has a 1 percent chance of occurrence IN ANY YEAR; NOT ONE event every 100 years.”

60 Hydrometeorological Design Studies Center (HDSC) of the NWS: html

61 TP Yr 24-hour pcpn. (in.) from SCS TR-55, 1986

62 100 Year, 24-hour NOAA Atlas 2 Values (inches) Northwest Colorado

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64 Probable Maximum Precipitation (PMP) n “Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year.”

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67 Storm Characteristics

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