Friday, January 23, 2015

Snow Climate Vulnerability and Resiliency

There is much to be learned about future climate change by looking at the present or the past and that certainly is the case for the current water year, which began on October 1st.

So far, it has been an unusually warm water year with an average temperature of 43.1ºF at the Salt Lake City airport through January 20.  Only four similar periods in the instrumented period have been warmer.

One way to isolate the influence of temperature on the mountain snowpack is to examine the fraction of water year precipitation that is retained in the snowpack.  This fraction will be near 100% if most of the precipitation falls as snow and there is little loss of snow on the ground to either melting or evaporation and it will be less than 100% (possibly far less) if some of the precipitation falls as rain and is not retained in the snowpack (for example, rain that falls before snow is on the ground or falls on a snowpack that is water saturated) or if there is melting or evaporation.

We can do this using data collected by USDA Natural Resources Conservation Service SNOTEL stations.  These remote, automated stations measure water year precipitation using a large storage precipitation gauge (liquid precipitation equivalent - i.e., the combined water from rainfall and frozen precipitation after melt) and the amount of water in the snowpack (i.e., snow water equivalent) using a pillow that rests on the ground and measures the weight of the snow.

Source: Secrets of the Greatest Snow on Earth
These observations are not perfect.  Sometimes the precipitation gauge doesn't collect all the snow that is falling due to strong winds.  Sometimes the snow depth on the pillow might be unrepresentative due to wind transport.  Nevertheless, they are quite useful.

We begin with a low-elevation SNOTEL station characteristic of what I'll call a vulnerable snow climate, the Ben Lomond Trail SNOTEL at 5829 ft at the base of Ben Lomond Peak in the North Ogden Valley.  This is the lowest SNOTEL in the Wasatch Mountains.  This site has observed 10.1 inches of precipitation (black line) of which only 6.5 inches (64%, blue line) is retained in the snowpack.

Source: NRCS
Most of the 36% not in the snowpack is precipitation that appears to have fallen as rain and was not retained in the snowpack.  There are only a couple of brief downturns where there is water loss due to melting (note: the snow on the surface can melt without net loss of snowpack if it refreezes as it percolates through the pack).  This site is well shaded from the afternoon sun and thus tends to retain snow well.  More sun exposed areas near this site likely saw greater losses at times.

Next, let's go up the mountain to an upper-elevation station characteristics of what I'll call a resilient snow climate, the Ben Lomond Peak SNOTEL at 8000 feet.  This site has observed 15.5 inches of precipitation (black line) of which 14.1 inches (91%, blue line) is retained in the snowpack.  Thus, despite the warmth of the water year, most of the precipitation that has fallen is still stored in the snowpack.

Source: NRCS
One can find similar results at other upper elevation sites, such as Thaynes Canyon at Park City Mountain Resort.

Source: NRCS
So, this has been a warm year and it has had an impact on the snowpack at lower elevations because a significant fraction of precipitation has fallen as rain.  At the lowest elevations and on aspects that receive afternoon sun, there have also been losses due to melting.  This is why the cross country center at Mountain Dell has sometimes looked less than desirable.

Mountain Dell, January 12, 2015
At upper elevations, however, most of the precipitation that has fallen (about 90% or more) is still retained in the snowpack on northerly aspects and other shady locations.  These areas have some insurance against warming because of their aspect and their altitude.  I emphasize northerly aspects and other shady locations because there clearly have been losses due to melt on other aspects this year (except perhaps at the highest elevations).  

Overall, this provides some glimpse of the future snow climate of the Wasatch Mountains.  It remains unclear how our average precipitation climate will shift in the future (the models have varying projections), but warming is coming.  It is likely that the fraction of precipitation retained in the snowpack at low elevations during winter and at the end of the snow accumulation season will decline.  Upper elevation northerly aspects have some insurance against the initial wave of global warming and their future will depend on just how high greenhouse gas concentrations get and how much the climate warms.  

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