The vertical velocity of hydrometeors is one of the most important parameters to get right in weather and climate models. The problem is particularly difficult when it comes to determining the vertical speed of frozen hydrometeors, since their shapes are infinitely variable and their fallspeeds correspondingly difficult to predict.
Many parameterizations for fallspeed have been developed as a function of hydrometeor habit. Yet, even though snow clearly swirls, measurements or theory have contrived or assumed conditions of still air. Also, even though the density of accreted deposits on graupel and hail is known theoretically to be a function of temperature, the most widely used formulae for graupel fallspeed are a function only of particle size.
The role of environmental conditions on precipitation fallspeeds is being tested using recent measurements from the Multi-Angle Snowflake Camera, a new instrument that takes high resolution photographs of hydrometeors in freefall while simultaneously measuring their fallspeed. Millions of snowflake images were collected at Alta Ski Area over a four month period, coincident with vertically pointing radar and a 750 m deep profile of temperatures.
Overall, the MASC data show remarkably weak correlation between particle size or shape and fallspeed. While the measured range of fallspeeds spans more than two orders of magnitude, there appears to be a nearly equal preference for particles to fall at about 1 m/s whether the particles are compact graupel or aggregate flakes, or whether the particles are small or large. The spread in measured velocities does grow however in proportion to the degree of ambient turbulence. Also, a ten degree drop in temperatures is associated with generally slower graupel fallspeeds, by about a factor of two. Both results suggest possible revisions to commonly used parameterizations for precipitation in weather and climate models.
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