Last night delivered in spades with Alta-Collins picking up 17" of snow with 1.04" of water (6% mean water content). The Utah Avalanche Center reports 14–17" of snow in the upper Cottonwoods, 8-10" of snow on the Park City Ridgeline, and 15" of snow in the Ogden area mountains. Good news for everyone. Total snow depth at Alta-Collins topped out last night at a remarkable 78 inches. Periods of low-density snow will continue today and add another 2-4" to the totals at Alta.
Over the past few weeks, members of my research team (Peter Veals, Michael Wasserstein, and Ashley Evans) have been working to install some new toys such as profiling radars and other precipitation measurement systems at Highland High in Sugarhouse and Alta in order to better observed and understand how the Wasatch affect winter storms. We are especially interested in how storms evolve from the valley to the mountains in northwesterly flow and introducing high-school students to meteorology, so Highland High was a very good location for us to site our equipment. We're also grateful to UDOT for their assistance.
Last night the equipment and comms worked flawlessly and we got some great data. I'll focus here on the radar from Highland High as we got some great data on the transition from rain to snow.
A profiling radar points vertically through the storm. Instead of scanning the storm horizontally like many weather radars that you see on TV, it profiles the storm vertically. One can take these profiles and create a time-height section, like those we create from the forecast models. In the time-height section below, we are plotting three variables (note that time increases to the right, in contrast to the model time heights we produce): reflectivity (top, roughly a measure of precipitation rate), doppler velocity (resulting from vertical air motions and the fall speed of the snow, graupel, or rain), and spectral width (a measure of turbulence and other factors that cause variability).
The top plot above shows the pulse-like nature of the precipitation overnight with fluctuations from weak to strong echoes and in the depth of echoes. The middle plot shows a transition from positive (upward) velocities at upper levels (red) to downward at low levels (blue), consistent with the fallout of snow. We'll skip spectral width for this post.
We run the radar in two modes. The first is a low-resolution deep mode, which is presented above. The second is a high-resolution shallow mode, covering the red box in the plot above. This allows us to look in detail of what is happening near the surface.
Below is a plot of the high-resolution data. In it, I've identified the melt layer, which descends to near the surface from about 0000– 0400 UTC (5–9 PM MST). Above this layer, the storm is mostly snow. The radar reflectivities are relatively low (15-20 dBZ) and the doppler velocities are 0.5–1 m/s. Those velocities are consistent with the fall speed of snow.
As precipitation falls through the melt layer, the snow turns into rain. Reflectivities increase to 25–40 dBZ) and fall speeds increase to 7–8 m/s. This is because wet snow and rain scatter more radar energy back to the radar than dry snow and they also fall faster. Eventually, we'll work up some code to automatically identify the melt layer for weather monitoring and forecasting purposes (I've identified the melt layer above by eye).
In the future, we may talk about some of the other observations we are collecting. We hope to eventually put this data on a public-facing web page.
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