Tuesday, October 25, 2011

SCHUSS IOP1: A Freaky Front

Disclaimer: This post is long, technical, and for the weeniest of Wasatch Weather Weenies, but you should read it anyway if you want to see just how complicated the atmosphere can be over and around the Great Salt Lake.

For the first Intensive Observing Period (IOP1) of the Storm CHasing Utah Style Study (SCHUSS), we spent yesterday evening near Lake Point on the southern shore of the Great Salt Lake examining the dry cold front as it moved southward into northern Utah.   I'm sure there were heads turning along I-80 as they drove past the rest stop.

As we deployed in the late afternoon (~2300 UTC, 1700 MDT), the front passed Locomotive Springs at the northern tip of the Great Salt Lake.  Temperatures over and near the lake were much cooler than over the surrounding landmass.  For example, it was only 62F at Hat Island, but 70F at the Salt Lake airport and in the mid 70s over Dugway Proving Grounds.

A shallow layer of haze was also evident over the Great Salt Lake.

These observations suggest that the cold front would be moving into a cooler, shallower, and possibly more stable low-level airmass over the lake.  Sometimes this leads to the development of what is known as an undular bore, a gravity wave that can develop as a cold front moves into a stable layer.

The front passed the northern tip of Antelope Island just before 0230 UTC (2030 PM MDT).  At that time, the prefrontal surface flow was transitioning from the daytime upvalley/upslope regime to the nighttime downvalley/downslope regime.  Most stations in the Tooele Valley had reversed to southerly (downvalley).  Along I-80 near the Oquirrhs in the Salt Lake Valley, most stations were southerly, southeasterly, or easterly.  In other words, near our observing site on the south shore of the Great Salt Lake, the surface flow was offshore as the front approached.

A vertical scan from the DOW radar from our site near Lake Point northward toward Promontory Point showed both the offshore flow and the front remarkably well.  In the Doppler velocity image below, warm colors indicate flow toward the radar, cool colors flow away from the radar.  The front was approximately 14 km offshore (blue line, range rings below every 2 km), as indicated by an abrupt transition to strong flow toward the radar.  Ahead of it, a shallow layer of offshore flow was surmounted by a layer with strong flow toward the radar.  In other words, the flow at this level had a northerly component even ahead of the front.

This pre-frontal northerly flow aloft existed for some time as the front approached and extended to the surface earlier during the day.  It is unclear if it is the last gasp of the lake-breeze circulation from the prior day, or if other processes were contributing.  The bulge in the layer of northerly flow roughly 10-12 km from the radar is also interesting.  It could be a gravity wave initiated as the front pushed into the stable layer, or perhaps reflects convergence near the land-breeze front (note how the offshore flow weakens just below it).

The front and the pre-frontal bulge move closer to the radar by 0243 UTC (2043 MDT).   During this period, and the time leading up to the frontal passage, we experienced intermittent bursts of northerly flow with nasty lake stink.    We hypothesize that these bursts were driven by shear near the top of the shallow offshore flow, which may have been enhanced as the northerlies intensified aloft.

A curious aspect of this case is the abrupt nature of the wind transition at the front, wheras the temperature transition was modest and less abrupt.  At Hat Island, for example, the pre-frontal flow was northerly (as mentioned above), but the speed increased dramatically with the frontal passage.  The temperature, however, only dropped about 6F in 30 min.  In part, this reflects the cool pre-frontal airmass over the lake.

Hat Island Fropa at ~0120 UTC (1920 MDT)
We didn't expect much as we ventured out into the field, but as usual, mother nature delivered.  For Atmospheric Sciences undergraduates, investigating this case would make for a great senior capstone project.

1 comment:

  1. I wonder if the (pre-frontal) northerly flow above the surface was air that was actually post-frontal to begin with, and was lifted over a shallow cool layer it encountered? Or if it was just part of the pre-frontal lake air mass and was being pushed along ahead of the approaching front. The brief warm/dry spike at Hat Island is interesting too. There must have been some deeper mixing right at the nose of the front.

    By the way, you should figure out how to clone that truck while you have it.