A marvelous example of the Sierra PW wall and downstream cloud and precipitation shadow has developed. Note the local minimum in PW that extends downstream of the High Sierra across nearly all of Nevada, the lack of cloud-cover within this minimum.
|2045 UTC 19 Nov 2010 Visible Sat & RUC PW and 925 mb Wind|
At issue are the processes responsible for the PW minimum. One possibility is airmass transformation in which water vapor is lost as orographic precipitation on the windward side of a mountain barrier, leading to a drier airmass in the lee (Smith et al. 2003). In the present case, radar imagery suggests there is little windward precipitation over the High Sierra, except near Lake Tahoe.
|2045 UTC 19 Nov 2010 Radar Composite|
Perhaps radar coverage over the High Sierra is poor (anyone?), but even if it isn't, there are other mechanisms besides water vapor loss to orographic precipitation that might contribute to the Sierra PW wall including:
- Differences in elevation upwind and downwind of the Sierra. Because the Intermountain West is at least 1500 m higher than the central valley, a portion of the PW decrease is simply the lack of the lower-troposphere. On the otherhand, this doesn't account for the pronounced minimum in the lee of the High Sierra.
- Topographic blocking. This is my personal favorite. The Sierra frequently produce a strong barrier jet that advects large quantities of moisture poleward over their windward slopes (e.g., Marwitz 1987). Essentially, the barrier jet acts like a robber baron. Instead of the windward airmass being forced directly over the barrier and into the lee, blocking results in the low-level flow being diverted poleward until it encounters the lower portion of the barrier north of Lake Tahoe where it turns anticyclonically and penetrates into the Great Basin.
There is some evidence of the topographic blocking effects in the analysis above. Note how the flow upstream of the high Sierra is SW, but the flow north of Lake Tahoe is more westerly.
Heather Reeves did a really nice job of explaining this process and its role in producing an isolated maximum in precipitation in the northern half of the Sierra Nevada (Reeves et al. 2008). At issue is whether or not this process also contributes to the leeward PW minimum. This would be an interesting topic for future work.