Mapping the extent of temperature-sensitive snowcover and the relative frequency of warm winters in the western US

Transcription

1 Mapping the extent of temperature-sensitive snowcover and the relative frequency of warm winters in the western US Anne Nolin Department of Geosciences Oregon State University

2 Acknowledgements Chris Daly, Oregon State University NASA Cooperative Agreement NNG04GC52A

3 Research Goals Map temperature sensitive snowcover in the Western US Quantify the relative frequency of warm winters (recent and potential) for selected areas Consider impacts on hydrology ski industry

4 Focus Areas Background image: PRISM digital elevation

5 Model output is too coarse (10 x 12 km) for watershedscale hydrology Data-driven approach can provide higher resolution Mote et al., 2005

6 Mapping temperature sensitive snowcover Snow classification based on Sturm et al., 1995: Used temperature, precipitation, and wind speed to define snow classes 0.5 x 0.5 degree grid resolution (Data courtesy NSIDC)

7 DATA PRISM temperature and precipitation Historical monthly averages for km x 4 km MODIS Vegetation Cover Fraction (VCF) product (proxy for wind speed) Jan T mean Jan Pre % treecover

8 Precipitation is classified based on a temperature threshold, T snow, above which all precipitation is considered to fall as rain (a) 0 o C 0 o C Colder than 0 o C Colder than 0 o C (b) Because this threshold temperature is somewhat arbitrary, we use a range of temperatures in the snow classification exercise

9 Now Let s assume climate warming over the next years Using the IPCC Climate Model output for the Pacific Northwest, the models are in good general agreement that temperatures will continue to warm at the rate of o C per decade Here, we modify the transition temperature for warm vs. cold snow by 0.5 degree increments for a total warming of 2 o C

10 Decision tree thresholds Snow vs. No Snow: DJF T mean -2.0 to +2.0 o C, in 0.5 o C increments Warm snow vs. cold snow: DJF T mean -2.0 to 0 o C, in 0.5 o C increments High precip vs. low precip: DJF P 2mm/day Low wind vs. high wind: Forest cover density 35%

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12 Western US Snowcover Classification Temperature-sensitive snow

13 Pacific Northwest Snowcover Classification

14 Sensitivity to Rain-Snow Temperature Threshold area of PNW at-risk snow % of PNW snow at risk (6.5 km 3 of water) Percent of PNW Snow Cover At Risk Rain-Snow Temperature Threshold ( o C)

15 Percent of Snow-Covered Area That is At-Risk Pacific Northwest study area <3% 1. Oregon Cascades..22% 2. Washington Cascades..12% 3. Olympic Range...61% 1 2 3

16 Sierra Nevada, CA Total snow area = 24,128 km 2 At-risk snow area = 7872 km 2 At-risk snow percent = 32% m elevation

17 White Mountains, AZ Total snow area = 1600 km 2 At-risk snow area = 640 km 2 At-risk snow percent = 40% m elevation

18 What is the relative frequency of warm winters? First, what is a warm winter? Winter = DJF Warm = When at least one winter month has a mean temperature above the 0 o C If T mean LE 0 o C in December and January and February then it is not a warm winter Relative Frequency: The number of times (N ) an event occurs within a number of N trials Thus, the relative frequency of an event is N /N

19 We use monthly DJF T mean from PRISM data ( ) Evaluate relative frequency of DJF T mean below a threshold temperature Shift threshold temperature upwards by increments of 0.5 o C (going from -2 o C to 0 o C)

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21 Table 2. List of Pacific Northwest ski areas that are projected to experience a significant increase in the relative frequency of warm winters for a range of temperature thresholds. Relative frequency of winters with a mean DJF temperature exceeding: Ski Areas by Region Base Elevation (m) -2.0 o C -1.5 o C -1.0 o C -1.0 o C 0.0 o C Oregon Cascades Timberline Mt. Hood Meadows Mt. Hood Ski Bowl Cooper Spur Hoodoo Mt. Bachelor Willamette Pass Warner Canyon Mt. Ashland Eastern Oregon and Washington Spout Springs Mount Spokane Bluewood Washington Cascades Mt. Baker Mission Ridge Crystal Mountain The Summit at Snoqualmie White Pass Stevens Pass Olympic Range Hurricane Ridge Current In yrs Nolin and Daly, 2006

22 California Ski Areas 1.2 Relative Frequency of Winter Monthly Mean Temperature Future Present-day Alpine Meadows Badger Pass Bear Valley Big Bear Boreal Ridge Heavenly Valley Homewood June Mountain Mammoth Mountain Mt. Shasta Ski Park Squaw Valley SugarBowl Tahoe Donner Temperature (deg C)

23 Hydrologic Implications Temporal centroid of hydrograph will continue to shift to earlier date (Stewart et al., 2005) Snowmelt is a significant contributor to mountainfront groundwater recharge Snowmelt vs. rainfall runoff Occurs during season of low evapotranspiration How will landscape controls (geology, vegetation) interact with climate controls to change the spatial and temporal patterns of streamflow?

24 Monthly discharge for the Clear Lake, OR watershed in two historical periods ( , ) and a predicted future discharge from Jefferson et al., submitted to Hydrological Proc.

25 To summarize: Data-driven approach is useful for sensitivity studies At risk snow represents a proportion of the Oregon and southern Washington Cascades, Olympic range, CA Sierra Nevada, and AZ White Mountains Relative frequency of warm winters will likely influence lower elevation ski areas across the Western US Hydrologic impacts are already evident Mapping efforts such as this can help identify sensitive areas that need to be integrated into climate measurement networks