Observations of snow growth by a vertically pointing radar

A vertically pointing X-band radar provides partial information of the growth of snow from its level of detectability to the melting level or to the ground, as it may be the case. Analysis of a year worth of data show that Doppler velocities have a systematic minimum at approximately -15ºC and -5ºC. This decrease of Doppler velocity may be due to upward air motion, to activation of new particles associated with the updraft, or to the shift in the Doppler spectrum toward slower particles due to change in growth habit. The slowing down at -5ºC is easier to understand by inspecting the Doppler spectra: apparently it is related to activation of new small particles likely created by vertical air motions associated with the melting layer. This leads to a shift in the average of the Doppler spectrum to smaller values. On the other hand, the slower Doppler velocities at -15ºC, where dentritic growth is dominant, is associated with an increase in reflectivity. However, the flux of reflectivity has no particular signature at this level, suggesting that the change in Doppler velocity is due to air vertical motion rather than microphysics. Moreover, it is not possible to discern any changes in fall velocity due to a possible modification of particle shape due shift to dentritic growth of particles falling from above. Computations of air divergence indicate that upward air motion is responsible for the slowing of fall velocity. The regularity with which this happens at -15ºC is very intriguing.