Millimeter wavelength radars in Meteorology

In the early eighties, a very short wavelength Doppler radar (l=3.2 mm or 94 GHz , W-band) was introduced in radar meteorology by Lhermitte in 1981, as a new tool for cloud physics research, primary for the observation of low radar reflectivity clouds. Similar equipments were later developed and used in various meteorological projects. Starting with the operation of the first cloud radar, a large number of research papers have been published on the performance and advatanges of millimeter wave radars and the results acquired through their use in various research projects. After a brief discussion of the 94 GHz radar characteristics, this article will summarize the main contributions made to cloud and precipitation physics using the radar. The radar sensitivity and performance will be compared to that of other short wavelength radars, namely the K-wand radars presently used in cloud and precipitation physics, including the newly developed ARM radar. A survey of radar improvements based on the addition of new techniques and the use of a new high power transmitter tube, the "gyrotron", will also be considered. New observational methods will be explored such as the implementation of vertical angle scanning techniques yielding observations of air vertical motion velocity and radar reflectivity three dimensional fields.