11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Thursday, 6 June 2002: 11:15 AM
Fall velocity and axial ratio of snowflakes
Raphael Schefold, ETH, Zurich, Switzerland; and B. Baschek, M. Wueest, and E. Barthazy
Poster PDF (1.1 MB)
The properties of snowflakes such as their size distribution, fall velocity or axial ratio are important for several purposes. The knowledge of accurate fall velocities, e.g., can be used to calculate aggregation rates in modelling precipitation processes. The size distribution and especially the axial ratios provide information for the interpretation of returned radar power, in particular of polarized radar.

Except for some in situ techniques, where snowflakes are captured and examined, optical techniques are the choice when snowflakes are to be investigated. However, accurate experimental observations of size distributions, fall velocities and axial ratios with description of the true spread of each property (e.g. within a size class) are scarce due to either the limited number of observed snowflakes, or to the limitations of the observations techinques.

To overcome these limitations, an optical disdrometer was developed at the ETH Zurich. The instrument is capable to measure several properties like the real time-dependend size distribution in the range of 0.15 mm - 70 mm or axis ratios and fall velocities of single hydrometeors. The optical disdrometer works with two vertically slightly offset sheets of light, each of them illuminating an array of 508 photodiodes. The geometrical information of a particle falling through the two sheets of light is extracted from the form of the shadow on the arrays, whereas the fall velocity is obtained from the time difference between the two shadow patterns on the two arrays. A matching algorithm is capable to measure the properties, including the fall velocity, of each single hydrometeor even if several particles are falling trough the two sheets of light at the same time.

This optical disdrometer is used in ongoing field campaigns (Storm Peak Laboratory on Mt. Werner, Colorado, Desert Research Institute, and Mt. Rigi and Mt. Uetli, Switzerland) to collect snowflake data of different crystal types. One snowfall event of a few hours can yield data of several tens of thousands of snowflakes. In addition, ice crystals are replicated (Formvar) regularly with time intervals of 5 to 10 minutes to determine their habbit and riming degree.

Detailed studies show on a large statistical base the dependency of the fall velocity and the axial ratio on the size of the snowflake, but also on the type of snow (crystal type, riming degree).

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