Sunday, November 28, 2010

Where is the orographic precipitation enhancement?

This (Sunday) morning was interesting with regards to the lack of orographic precipitation enhancement.  Let's have a look:
  • Snow-water equivalent at CLN from 0000-1400 MST: 0.31 inches
  • Snow-water equivalent at KSLC from 0000-1400 MST: 0.27 inches
Thus, the ratio of mountain to lowland precipitation, known as the orographic ratio, was only 1.1 (Actually, it may have been <1 for the period during which precipitation was falling at KSLC).  In contrast, the climatological Alta/KSLC orographic ratio is ~3.5.  Further, driving down Big Cottonwood Canyon this afternoon, I couldn't help but notice that more snow fell in the lower canyon and along the east bench than in the upper canyon.  How deflating for skiers!  What gives?

The orographic ratio varies considerably from storm to storm and in some cases during storms.  Steenburgh (2003, 2004) shows that the orographic ratio can be low in northern Utah when the precipitation dynamics are governed by strong synoptic forcing, such as during the passage of a cold front.  Blocking, which occurs when the static stability is high, can also can enhance lowland precipitation rates (and hence reduce the orographic ratio) by shifting the vertical motion upstream of the initial slope of the mountain barrier.  Neiman et al. (2002) present a nice conceptual model showing this process (below), and Cox et al. (2005) describe a similar effect upstream of the northern Wasatch Front near Ogden.  

Neiman et al. (2002) conceptual model of orographic rainfall distribution in California's coastal mountains, including the impact of terrain-blocked flow.  
I think this morning's situation was a bit more complex.  Large-scale forcing did contribute as radar reflectivities were relatively uniform and widespread

KMTX 1500 UTC radar reflectivity
and the inverted-S profile in the radar velocity analysis is consistent with low-level warm advection.  

KMTX 1500 UTC Doppler Velocity Analysis
The morning sounding also illustrates that we were dealing with a situation characterized by high static stability below about 775 mb.  Even above this level, the lapse rate was absolutely stable.   

KSLC 1200 UTC Sounding
Although MesoWest data suggests were weren't dealing with a blocking-front scenario similar to that illustrated above by Neiman et al. (2002), the strong static stability and lack of a substantial cross-barrier flow component in the morning sounding is consistent with limited orographic lift.  Often in these situations, the valley is filled with a dry airmass and sub-cloud evaporation helps create a positive orographic ratio, but that is not the case today as we were saturated throughout the valley atmosphere.  Further, anyone skiing in the upper cottonwoods this morning likely noticed that the storm was quite shallow as the sun was visible through the snow for long stretches.

Solitude ski area ~1000 MST (1700 UTC)
All in all, it appears that low-level warm advection, a lack of substantial orographic lift, and the shallow nature of the storm may have conspired to give us the unusually low orographic ratio this morning.  Perhaps others have other hypotheses worth sharing.  Those wishing to see the mountains get back in the action will be pleased to know that the orographic ratio should be increasing now that the upper-level trough has passed, the northwesterly flow is increasing, and the stability is decreasing.  


  1. I agree, it looked like an occluded system... strong low level stability at least initially so that the terrain did not create much lift. I suspect that the large area of snow cover over the Great Basin, keeping low level temps cold, may contribute to more of these types of events. By the way, I have measured 40" of snow so far this season at my house (Taylorsville). Currently 24" depth on the ground.

  2. That is really impressive that you've had 40" so far in Taylorsville. Let it snow!

  3. The west side of the valley has been hit hard by every single storm so far this season, more than the east side has. Plus a good lake effect storm in October.