WINTER OROGRAPHIC- PRECIPITATION PATTERNS IN THE SIERRA NEVADA— CLIMATIC UNDERPINNINGS & HYDROLOGIC CONSEQUENCES Mike Dettinger 1, Kelly Redmond 2, & Dan Cayan 1 1 U.S. Geological Survey Scripps Institution of Oceanography La Jolla, CA 2 Western Regional Climate Center, DRI, Reno

(Average Ratio is 1.76)

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

Simulated Snowpack 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, , 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 L

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

L 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.

Transport paths associated with North Pacific decadal variationsL

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!