Tuesday, 11 February 2003
Recent advances in the use of mm-wavelength radars for cloud and precipitation research
The development and use of Doppler mm-wavelength radars during the past decade has advanced our capability for observing clouds and precipitation. In this paper we review some recent applications of 3-mm wavelength radars for studying dynamical and microphysical processes in fair-weather cumuli, marine stratocumulus clouds, cirrus anvils and convective and stratiform precipitation. The key to many of these studies has been the use of high temporal resolution Doppler spectra (and the associated Doppler moments) obtained from the radar operating in an upward-looking mode. This allows for not only a definition of the macroscopic structure of clouds, but also quantitative estimates of the vertical velocities in clouds on horizontal scales as small as 10-20 m at 1 km and 100-200 m at 10 km and the turbulence characteristics at even smaller scales. The Doppler spectra from marine stratocumulus clouds observed along the coast of California during the summer of 1999 are used to examine drizzle characteristics and the turbulence variability these clouds. Observations made over South Florida are used to study the updraft-downdraft circulations and turbulence in fair-weather cumuli. The dynamical and microphysical characteristics of cirrus anvils are being made using radar observations made during the Cirrus Regional Study of Tropical Anvils and Cirrus Levels—Florida Area Cirrus Experiment (CRYSTAL-FACE, 2002). Although the development of 3-mm wavelength radars was motivated by the need for studying cloud properties, radars operating at this wavelength have characteristics that make them unique for studying air-motions and precipitation drop-size-distributions (DSDs) in convective and stratiform rain through the Mie signatures that are observed in the Doppler spectra. While attenuation limits the application of this technique under heavy rain conditions to the lowest 2-3 km, for stratiform rain it is possible to obtain useful observations to the melting level and higher. A review of applications of the Mie technique for characterizing vertical air velocities and DSDs in convective and stratiform rain from a 3 mm wavelength radar will be presented. Recent applications provide new insight into key process associated with the effects of updrafts and downdrafts on DSDs and advance our understanding of processes operating at the melting level in stratiform rain. Future possible applications of 94 GHz radars for studying cloud and precipitation processes will also be discussed.