Saturday, February 28, 2015

Fascinating Micrometeorology in Little Cottonwood Today

My son and I spent the day at Alta today and enjoyed some really great lessons in mountain microclimates.

The crest-level flow today was out of the southeast.  You can see this southeasterly flow in the 1700 UTC (10 AM MST) plot below, which shows southeasterly flow at the top of the Snowbird Tram, Mt. Baldy, and Top of Collins.

Source: Mesowest
As this flow ascended from American Fork Canyon, it formed a pronounced orographic cap cloud over Snowbird and Alta.  The flow then descended into upper Little Cottonwood Canyon, confining the cap cloud to over the upper reaches of Alta and Snowbird.  I have sketched out my guestimate of the flow looking up Little Cottonwood Canyon at about 10 am.  I suspect that the flow is crossing the barrier both within and above the cap cloud, but only the lower level airstream is moist enough to form a cloud.  Warming of the airstream as it descends and compressionally warms over Little Cottonwood Canyon results in the cloud evaporating.

The influence of the orographic cap cloud was readily apparent while skiing in the morning and afternoon.  Prior to about 11 am, snowfall was very limited and one could see the cap cloud hanging over Mt. Baldy and upper Collins Gulch from the Wildcat chair.  In upper Collins Gulch, the light was quite flat and it was very windy.  In contrast, on the lower mountain, the light was considerably better.

Late in the morning, snowfall began to increase as a larger-scale precipitation band moved over Little Cottonwood from the south.  Snowfall was light, but clearly increased from the lower to upper mountain, at least through our departure at 3 PM.  There can be many causes of such contrasts in snowfall, but I suspect crystal growth in the cap cloud that was embedded in the larger-scale cloud shield was a contributor.

The weather geek highlight of the day was provided by the ice crystals that were falling during the late morning, many of which were beautiful stellar dendrites.  Oh it's great to see my stellar friends again!

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