WINTER OROGRAPHIC-PRECIPITATION PATTERNS IN THE SIERRA NEVADA—CLIMATIC UNDERPINNINGS & HYDROLOGIC CONSEQUENCES

Mike Dettinger1, Kelly Redmond2, & Dan Cayan1

1 U.S. Geological SurveyScripps Institution of OceanographyLa Jolla, CA

2 Western Regional Climate Center, DRI, Reno

1910 1920 1930 1940 1950 1960

Starting Year

1970 1980 1990 20000.50

1.00

1.50

2.00

2.50

3.00

Tahoe City / Sacramento Ratio

Ratio of June thru July PrecipitationTahoe City / Sacramento. 1909-10 thru 2000-01.Blue: 7-year running mean.

2001-02 :~1.7 thru Feb

(Average Ratio is 1.76)

1910 1920 1930 1940 1950 1960

Starting Year

1970 1980 1990 20000.50

1.00

1.50

2.00

2.50

3.00

Tahoe City / Sacramento Ratio

Ratio of June thru July PrecipitationTahoe City / Sacramento. 1909-10 thru 2000-01.Blue: 7-year running mean.

2001-02 :~1.7 thru Feb

Why should we care?

Most of our water comes from high altitudes, and yet most of our observations of precip come from low elevations.

Also, the strength of orographic gradients influences runoff timing and the sizes of floods.

Simulated Runoff Responses to Imposed Orograpic Gradients North Fork American River, Sierra Nevada 1983

SimulatedSnowpack Changes with Imposed Orograpic Gradients North Fork American River, Sierra Nevada 1983

Basinwide

High vs low

--> Basinwide snowmelt comes earlier

So, how and why does the ratio of precipitation at low- and high-altitude stations on the west slope of the Sierra Nevada vary?

High-altitude site

Low-altitude site

+/- 10%?925 mb

Surface

850 mb

700 mb

300 mb

+

+

+

Water-vapor transport rates and directions were vertically integrated from surface to 300 mbars each day , 1948-2000, in the NCEP/NCAR Reanalysis products to arrive at a daily transport vector through each grid cell

(assuming linear variation of q, u, & v between levels)

+

+

…

q v dp/g =

Transport vectors provide a focused perspective on storm-time thru seasonal-scale circulations & conditions

(New Years 1997)

Differences between the averages of transports during 180 LARGE-STORM DAYS vs 180 SMALL-STORM DAYS, Dec-Feb 1949-99

L

Differences between the averages of transports during 180 STRONGLY OROGRAPHIC STORMS vs 180 WEAKLY OROGRAPHIC STORMS, Dec-Feb 1949-99

L

14 Low Ratio Winters

Composite

October-March

700 mb Departures

11 High Ratio Winters

No obvious “special” layers or reversals to prohibit vertical averaging, at least in the means.

Locally, orographic storms (winds) blow from somewhat more westerly directions than do “large” storms.

MODE OF OROGRAPHIC STORMS

MODE OF LARGE STORMS

IN MAP VIEW:

Thus, orographic storms are NOT always the largest…

Although, almost by definition, they often are large.

LL

Transport paths significantly associated with El Ninos

La Nina storms are strongly inclined towards “orographic” approaches…

So they provide more of the most strongly orographic storms.

Mean 1.76

-2.5 -2.0 -1.5 -1.0

June - Nov SOI

-0.5 0.0 0.5 1.0 1.5 2.0

1.00

1.50

2.00

2.50

3.00

TahoeCity / Sacramento Ratio

Ratio of Tahoe City / Sacramento Precipitation, July thru Juneversus June-November Southern Oscillation Index.1909-1910 thru 2000-2001. N = 68. r = 0.37 (p < .01) Mean 1.76.

7 9

51416

17

Transport paths associated with North Pacific decadal variations

LL

Despite large looking correlations, PDO modifies storm directions and orographic gradients only modestly.

This approach might even be used to project how orographic precipitation would be under global warming scenarios.

e.g., PCM ---> marginally weaker orography

Winds that carries moisture directly across the range yields most precipitation, but orographic influences may require a bit more westerly approaches.

La Ninas (and, perhaps, negative PDOs) may provide slightly more orography as a result.

Other factors being equal, stronger orographic gradients yield later snowmelt & river discharge; weaker gradients threaten larger flood peaks.

CONCLUSIONS

Wet

!

Orographic!