Measurements of blowing snow, Part I: Particle shape, size distribution, velocity, and number flux at Churchill, Manitoba, Canada

Abstract

Blowing snow is a frequent and significant winter weather event, and there is currently a need for more observations and measurements of blowing snow, especially in arctic and subarctic environments. This is the first paper in a two part series studying blowing snow in Churchill, Manitoba, and Franklin Bay, NWT. In this part, the development and use of a camera system to measure the shape, size and velocity of blowing snow particles is described. This system was used, along with standard meteorological instruments and optical particle counters, during field campaigns at Churchill, MB, which took place in March, 2005 and December, 2006. Measurements of blowing snow particle shape demonstrate that blowing snow particles are generally non-spherical, with an average ratio of the longest particle profile length to the perpendicular width of 1.41. Measurement of particle velocity with the camera system is shown to be inaccurate. However, measurements of particle number correlate well (r2 = 0.85) with simultaneous measurements made with a particle counter at the same height as the camera. The particle size distributions generally follow a Gamma distribution, with an average shape parameter of α = 1.9. The shape parameter, α, shows no trend with height. The blowing snow particle number density decreases with height as approximately N ∝ z− 1.4. The blowing snow particle number densities, interpolated to a height of 0.1 m, are in the range of 1.4 × 105 < N < 4.7 × 107 m− 3. Particle number densities are shown to generally increase over a limited range of wind speeds, corresponding to 10-m wind speeds in the range of 13 < U10 < 17 m s− 1. Over this range of wind speeds, the average particle size at a given height does not change significantly with wind speed. However, at a lower wind speed of U10 ≈ 9 m s− 1, the average particle size is generally smaller. Particle sizes generally decrease with height between 0.06 and 1 m. Above 0.2 m, the average particle diameters found in this study (120 µm < d― < 154 µm) are similar to or generally larger than the range of sizes found in previous studies. Below 0.15 m, the average particle diameters in this study (103 µm < d― < 172 µm) are similar to or generally smaller than the sizes found in previous studies. There is a large variation in the mean particle sizes of this and other studies, suggesting that particle sizes may be dependent on a number of other factors, such as upwind snow conditions and surface characteristics.