Cold Smoke

What an incredible night of low-density snow at Alta.  From the 4 PM board wipe yesterday afternoon to 7 AM this morning they've had 10 inches of snow with .31" of water for an average water content of 3.1%.  There is a tendency for precipitation gauges to undercollect snowfall, but still, this is very much cold smoke. 

Let's take a look at why the snow is so low density.  Two important clues are in the morning upper-air sounding from the Salt Lake City International Airport.  First, the temperatures between about 750 mb (2500 m/8000 ft) and 650 mb (3500 m/11,500 ft) are between -12 and -18˚C.  This is in the heart of the temperature range for growing dendrites, those six-armed snowflakes that we all love and can create low-density snow.  I've identified this region with the green box. 

The second is that the winds in this and the surrounding layers are very light: 15 knots or less.  Thus, Mother Nature isn't bashing these flakes and they are less damaged when and after they reach the ground.  Ridge top winds have picked up some this morning, with ridge-top gusts reaching 30 mph, so there could be some localized wind transport going on, but for the most part, this is fantastically dry snow.  

We can also have a look at the profiling radar system we are operating at Alta this winter.  This radar provides vertical profiles through the storm, so we can create time-height sections like the one below, which are a high-resolution profiles focused on the lowest 900 meters or so above Alta (up to about 3600 m/11,800 ft).  Radar reflectivity in the top panel indicates that echo tops were very shallow, sometimes reaching only 700 m above the instrument.  At other times the max out just beyond the high-resolution range (lower resolution data shows spikes to just over 1000 meters).  Basically, this is a very shallow storm and most of it is in the dendritic growth zone.  

Notice also that the radar reflectivity generally increases toward the ground.  This is consistent with the growth of snowflakes as they are falling, maximizing near the surface.  This is all happening in an incredibly shallow layer.

The middle plot presents the Doppler velocity, which in this case represents the air motion plus the fall speed of the snowflakes.  These velocities are generally weak, and fluctuate from weakly positive to weakly negative, with a bias toward weak negative vertical velocities.  This is consistent with shallow, weak convection in which there are fluctuations from weak ascent to weak descent.  The slight negative bias is a result of the fall speed of the dendrites, which is around 0.5 to 1 meter per second and always toward the ground.  Basically, the storm is like a shallow pot of water on simmer.  

We also have an instrument at Alta that measures the size and vertical velocity of falling particles.  This allows us to get some idea of the composition of the precipitation.  In the plot below, we present a summary of the observations over a 1-hour period ending at 0900 UTC (2 AM MST).  The fall speed of particles varies depending on the composition and size, so we have added lines of what we might expect from pure rain, graupel, and dendrites.  Many of the particles during this period lie near or along the dendrite line, which is what we would expect and consistent with the cold smoke (the spike on the left side of the graph to the lower part of the rain line is an artifact that we haven't quite teased out yet).

There are some variations in what this instrument saw overnight, so if you were to take a close look at the snow crystals, perhaps you would see some very small graupel particles or other ice crystals at times.  Storms are almost always a stew of particles.  

Enjoy the skiing if you are up today.