Wednesday, February 2, 2011
Yesterday's occurrence of thundersnow in Chicago had me looking for explanations. Wiki gave a lightweight summary, with just enough technical jargon to make it hard for a typical reader. Subsequent searching lead me to a bunch of good peer-reviewed data on the electrification of thunderstorms, but little of use understanding thundersnow.   
I finally found a very recent survey by David M. Schultz and R. James Vavrek, which while somewhat technical, gave me the insight I was looking for.
Summarizing everything: Thundersnow occurs when the conditions for thunderstorm-type convection are present at the same time as for general snow (in large amounts). This includes the presence of humid air above the freezing point while general temperatures, especially at the ground, are below freezing. A high lapse-rate is also necessary, in order to drive the rapid updraft which creates hail and/or graupel. Substantial electrification requires this.
We don't yet know for sure how this electrification is caused,      but the best guess involves collisions between growing graupel/hail particles and small ice particles. 
Some Personal Observation:
My difficulty finding explanations is explained: we don't even know precisely what mechanisms lead to lightning even in thunderstorms, much less thundersnow (which has been much, much less studied). I had always assumed (and you know what that does) that electrification resulted from friction of ice particles with dry air, somehow I had never previously noticed the absence of this mechanism from those considered. Since both simulations and direct experimental measurements of this process would have been easy even in the 19th century, we can presumably rule this mechanism out.
None of the articles intended for general consumption (that I read) explicitly mentioned that we don't know the mechanism for electrification, which would have saved me considerable time trying to find it. (Although the descriptions of the theories and research were well worth the reading.) This points up a general defect in science reporting: the fact that the public is being kept pretty much in the dark regarding how much isn't really known for sure in science.
Schultz, D., & Vavrek, R. (2009). An overview of thundersnow Weather, 64 (10), 274-277 DOI: 10.1002/wea.376
1 An overview of thundersnow
2 A Climatology of Thundersnow Events over the Contiguous United States Open Access
3 Thunderstorm Electrification (Semi) Open Access
4 The 29 June 2000 Supercell Observed during STEPS. Part II: Lightning and Charge Structure Open Access
5 Relationships between Convective Storm Kinematics, Precipitation, and Lightning Open Access
6 The Electrical Structure of Thunderstorms(Semi) Open Access
7 The Ice Crystal–Graupel Collision Charging Mechanism of Thunderstorm Electrification Open Access
Posted by AK at 3:20 PM