Une étude des trainées (Virgas) de neige /

The precipitations on meteorological scanning radar may comes from different altitudes and different process. The challenge for operational meteorology is to assess the part of this precipitation which will reach the ground and at what place. Many factors influence the difference between radar data and ground data: partial beam filling, attenuation and beam blocking, bright band enhancement, wind transport of the precipitation, growth or decay of the drops/flakes below the lowest elevation angle of the radar. An important case for operational meteorology is that of light snow aloft whose base has an horizontal slope toward the ground: "snow virgas". I will use the output of two vertical pointing radar in this thesis to find what happens in those trails and try to explain the influencing mechanisms. I will also describe an algorithm that attempts to predict the place where the snow will reach the ground using McGill University scanning radar. My study shows that the slope of the snow virgas is essentially due to the transport by winds in saturated airmasses and evaporation of flakes in unsaturated ones. Finally, finding the slope of the virgas toward the ground, by an automatic algorithm, is extremely difficult on a scanning meteorological radar due to its coarse resolution