Creation of Cold Downvalley Flow Due to Diabatic Cooling leading to Isothermal
Layer and Snow February 14th, 2010
by rasmus
Creation of Cold Downvalley Flow Due to Diabatic Cooling leading to Isothermal
Layer and Snow
At 2200 UTC on February 13, the temperature at Creekside and and Timing Flats
started to cool rapidly. This was in association with the increase of precipitation
at each of these sites. VOL, VOA, and RND showed an increase of precipitation
at the same time. By 0000 UTC the temperature had dropped by by over 2 C at
each site. Examination of the temperature drop during the same period at VOA
and RND indicated only a slight drop in temperature. Since the temperature at
these two sites was below 0 C, the temperature drop at Creekside the Timing
Flats was attributed to diabatic cooling associate with the melting snow. The
precipitation rates were on the order of 1-2 mm/hr from 2300 to 00 UTC. At 2300
UTC the flow was still upvalley on the Whistler radar. Examination of the low
level Doppler radar flow on the Whistler radar indicated the formation of a down
valley flow around 2350 UTC to 1.1 km MSL. The flow continued to deepen
through the next hour. The down valley flow showed up at the Squamish Profiler
at 00 UTC as well up to 600 m MSL. We attributed the formation of this flow to
the formation of a density current associated with the cooling of the air by the
melting snow. The main impact of this is the cooling the temperature profile to
near 0 C, allowing snow to continue. Since over 30 mm of liquid was forecast for
Whistler Creekside and Callaghan area, the change of phase to snow would
result in significant snow accumulations in the area and an impact on venue
operations. Venue forecasters, in fact, were concerned about this possibility, and
asked the POD and RSD forecasters to keep an eye this. The venue forecasters
were informed that we thought the snow was going to continue for at least
another 4-5 hours past 00 UTC based on the forecast of continued high
precipitation rates during this period. Beyond 4-5 hours warm air advection
associated with the warm front may dominate the diabatic cooling and convert
the snow back to rain. The cessation of precipitation would also shut off the
cooling.
Plots from Creekside, Timing Flats, and the Profiler illustrating this effect are
shown below.