Optimizing C-Band radar settings for mesocyclone and tornado detection

The detection of mesocyclones and tornadoes by radar heavily relies on the accurate measurement of Doppler wind speeds. The associated circulations are characterized by strong tangential gradients of the radial wind. With C-band radar, the type of radar most commonly used in Europe, such measurements are challenging. The main difficulty stems from the fact that the returned signal is not associated with one unique velocity v, but with a velocity v = v +/- N v_N whereby v_N is known as the Nyquist velocity. For C-band radars v_N is often between 5 and 10 m/s as a result of the other chosen settings. Dual- or triple-pulse repetition frequency (PRF) is a technique to mitigate this problem and effectively increase v_N substantially and to around or above 30 m/s, however usually introducing errors whereby velocities are offset from their real values by a multiple of 2N. In response, corrections and filters are applied, however any filtering must be tuned not to throw out genuine signal.  Furthermore, wind speed in strong circulations may still exceed the extended Nyquist velocity of 30 m/s.

Multiple-PRF and other techniques and settings are used differently across Europe. Using ESSL’s radar viewer we have investigated how difficult it is to detect tornadoes and mesocyclones using those settings. In some countries no multiple PRF technique is used, rendering the detection of mesocyclones next to impossible. In others, excessive filtering of the extreme winds in mesocyclones occurs to varying degrees, hindering their confident detection. In another country, the effective beam width and gate length reduce the ability to detect tight circulations. In other countries, scans below 1.0° are missing, or a staggered scanning pattern of different elevations complicates the detection. The settings for detecting circulations appear to be best in Udine (FVG), Finland, and Germany. We present advice on how to optimize C-band radar settings for the detection of strong, tight circulations.