A new ground-based optical instrument to measure snowflakes

The properties of snowflakes such as their size distribution, fall velocity, axial ratio or their orientation when falling are important in modelling precipitation processes or in interpreting returned radar power, especially of polarized radar. Except for some in situ techniques, where snowflakes are captured and examined, optical techniques are the choice when snowflakes or ice crystals are to be investigated.

The ice phase of precipitation can be measured with aircrafts. The optical instruments used for airborne measurements are usually PMS probes like the 2DC or 2DP probe for cloud (i.e. small) or precipitation (i.e. large) particles respectively. The probes exist in different variations such as simply black and white, grey-scaled or with a holographic principle. All these probes have in common that the precipitation particles are sampled while the instrument is passing with the aircraft at the speed of the aircraft through the precipitation. While airborne methods have undeniable advantages such as the availability of data at any time (e.g. summer) or at any lattitude (e.g. tropics), some properties of the snowflakes such as their fall velocity, orientation of fall or their axis ration cannot be investigated at all or only with difficulties as a consequence of the measuring principle.

This disadvantages of airborne measurements can be overcome by ground-based measurements. The Institute of Atmospheric Science at the ETH has developed an optical instrument which is capable to measure reliably, with high accuracy and automatically the properties of each single snowflake falling through a 77 square cm area. The instrument works with two light beams which are slightly offset. Thus, two images are recorded of each particle which can be used to calculate, in addition to the other properties, the fall velocity of the snowflakes. A large data set of two field campaigns from two winter seasons are available, one of which was the Special Observing Period of MAP in the autumn of 1999. One hour of moderate precipitation can yield data of about 30’000 particles in a size range of 0.15 mm to 70 mm. This includes also the very large snowflakes falling within or close to the melting layer without truncating their image. Detailed studies show the dependency of the fall velocity, the axial ratio or other parameters on the size of the snowflake, but also on the type of snow (crystal type, riming degree (determined with Formvar probes)). This new instrument can also be used to study the process of melting of snowflakes with field data since the fall velocity relates to the melted fraction of each single snowflake.