There are some recurrent things that happen during storms over northern Utah that I don't understand. Here's one of them.
Early this morning in the pre-frontal southwesterly flow, precipitation developed downstream of Mount Timpanogos and the surrounding Wasatch Range in the area circled in red below.
The precipitation pattern was somewhat disorganized, but exhibited some banded structures at times. Echoes clearly were forming near and downstream of the Wasatch Crest and extended well downstream into the western Uinta Mountains.
I have seen a few examples of leeward precipitation over the years, all typically associated with strong cross-barrier flow. I have some hypotheses for how this happens, but have never been able to explore them. In this case, I think it is especially interesting that the echoes appear downstream of the range, suggesting that this is not a case of precipitation forming on the windward side and being carried into the lee by the prevailing flow. Instead, it appears that something is happening dynamically to spur ascent and precipitation growth downstream of the Wasatch Crest.
Somewhat similar features have been documented in the pre-frontal environment on the northern side of the Alps. Below is a radar image presented by Siedersleben and Gohm (2016) showing a case with southerly pre-frontal flow across the Alps with banding forming on the leeward (north) side and extending downstream over southern Germany.
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| Source: Siedersleben and Gohm (2016) |
The generation of those bands is related to how the flow interacts with small-scale topographic features in the Alpine topography, combined with atmospheric instabilities (for brevity, I'm not going to get into those instabilities). A smooth mountain range doesn't produce these features, and even with a rough one like the Alps, these features don't form in every storm.
The precipitation pattern observed downstream of the Wasatch Range last night featured banding on a somewhat small scale than that pictured above. Further analysis is needed to evaluate this hypothesis, or to refine and come up with a better one.
Check out Kirshbaum and Schultz (2018) where moist instability is responsible for some of these bands. https://journals.ametsoc.org/view/journals/atsc/75/12/jas-d-18-0211.1.xml
ReplyDeleteIn some model simulations, these bands are even formed in smooth terrain. Even smoothed terrain can set off the instabilities.
https://journals.ametsoc.org/view/journals/mwre/143/4/mwr-d-14-00255.1.xml?tab_body=pdf
Ah yes. Thanks for sharing.
DeleteIn a perfect world, a 12Z sounding at Heber City airport would be awfully useful right now. The KSLC sounding showed pretty steep lapse rates below 550 mb, and certainly was NOT a classic inverted V. In fact, there was a bit of elevated CAPE between 590 mb and 450 mb and a dramatic increase in speed shear in the same layer. At that location, the KMTX radar beam is at about 12.5k - 13k feet, which is a tad below the LFC on the sounding. Perhaps the flow over the barrier in a low Reynolds environment was enough to physically push parcels past the LFC.
ReplyDeleteTangential question from a novice: When snow is picked up from ridgelines, does it show up on radar returns?
ReplyDeleteTheoretically possible, but in general there's not enough snow lofted high enough for a detectable signal in National Weather Service radars.
DeleteOther types of radars can be used, for example, for avalanche detection.