32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Wednesday, 6 August 2003: 11:45 AM
Influence of Embedded Convection on Microphysics of Snowfall
Bjrn Baschek, ETH, Zrich, Switzerland; and R. Schefold and E. Barthazy
Poster PDF (1.0 MB)
The connection between embedded convection and riming is generally acknowledged. However, experimental observations of the riming degree of ice crystals together with convective parameters are scarce and several questions cannot be answered satisfactorily:

Which dynamic or thermodynamic reasons are causing embedded convection? What are suitable means for a quantification? What are the correlations to the microphysics of riming? The answers to these questions are of fundamental interest to understand the formation of precipitation and thus also of interest for, e.g., high resolution weather modelling.

To answer some of the open questions, a field experiment is in progress in the pre-Alpine region of Switzerland. At the base of a mountain, a vertically pointing X-band Doppler radar with 1s temporal and 50m spatial resolution is situated below the melting layer measuring and saving the full Doppler spectrum of the precipitation particles. On the top of the mountain, above the radar, ice precipitation is observed with an optical disdrometer, measuring the size distribution and fall velocity of the ice particles. In addition, ice crystals and snowflakes are replicated (Formvar) and photos taken to determine their size, type and riming degree.

The measured Doppler velocity is a combination of particle fall velocity and vertical wind speed. The vertical winds can be estimated out of combined optical disdrometer and radar data. Convective indices are used to quantify embedded convection in stratiform precipitation. For a case study, a coupling between the degree of riming and a convective index will be shown.

Supplementary URL: http://www.iac.ethz.ch/staff/bjoern/