THREE-DIMENSIONAL AIR MOTION MEASURED IN SNOW

In this letter, we present the results of a method that uses two Doppler radars to observe the three-dimensional wind field during light snowfall. These radars are designed to scan in a series of tilted planes, passing through the two radar positions. From the mean Doppler shifts in the frequency spectra we can determine average radial velocities within the pulse volumes. Since the two radars do not observe the same volume increments at the same time, spatial interpolation is needed before the radial velocities can be combined into a two-dimensional velocity field at a set of common coordinates within each plane. The estimated terminal fall velocity of the snow is subtracted from the particle velocity to obtain the true air motion in the plane. The air motion component normal to the plane is obtained by integrating the equation of continuity in. cylindrical coordinates starting with the surface boundary condition at the plane of zero elevation. The velocity field in the common planes scanned by the radars is then transformed into a rectangular coordinate system with one axis oriented parallel to the average wind direction. After the mean wind field is removed from the results, a well organized "eddy" wind field is evident.