# Presentation: 916361

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

**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)

**(Average Ratio is 1.76)**

## Why should we care?

**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

## Basinwide

**Snowpack Changes with Imposed Orograpic Gradients**
** **
- North Fork American River, Sierra Nevada 1983

- 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)*

## L

- 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, 

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

**L**

## Slide 11

## 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. 

- 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.

## 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.

## Slide 17

## Transport paths associated with North Pacific decadal variations

## 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*

## CONCLUSIONS

- 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**

- Orographic!