MU Radar and Lidar Observations of Clear-Air Turbulence underneath Cirrus

• Published in: Monthly weather review Vol. 138; no. 2; pp. 438 - 452
• Main Authors: LUCE, Hubert, NAKAMURA, Takuji, YAMAMOTO, Masayuki K, YAMAMOTO, Mamoru, FUKAO, Shoichiro
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.02.2010
• Subjects: Air; Atmosphere; Backscatter; Balloons; Cellular convection; Cirrus clouds; Clouds; Convection; Convection heating; Convective instability; Crystals; Doppler sonar; Downdraft; Dynamic stability; Earth, ocean, space; Exact sciences and technology; External geophysics; Free convection; Humidity; Ice; Ice crystals; Instability; Instruments; Kelvin-Helmholtz instability; Lidar; Lidar backscatter; Lidar observations; Meteorological balloons; Meteorology; Numerical simulations; Observatories; Perturbation; Precipitation; Protuberances; Radar; Radar data; Radiosondes; Shear; Stratosphere; Sublimation; Thermal convection; Turbulence; Updraft; Upper atmosphere; Vertical oscillations; Far East; Asia; Japan; Updraft; Convective instability; Cross section (collision); buoyancy; Clear air; Ice cloud; Variance; Ice crystals; Cloud base; Cirrus; internal waves; winds; Radar observation; Lidar; warming; Height; Upper atmosphere; Air motion; Clear air turbulence; Downdraft; sublimation
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/2009mwr2927.1

Abstract Turbulence generation mechanisms prevalent in the atmosphere are mainly shear instabilities, breaking of internal buoyancy waves, and convective instabilities such as thermal convection due to heating of the ground. In the present work, clear-air turbulence underneath a cirrus cloud base is described owing to coincident observations from the VHF (46.5 MHz) middle and upper atmosphere (MU) radar, a Rayleigh–Mie–Raman (RMR) lidar, and a balloon radiosonde on 7–8 June 2006 (at Shigaraki, Japan; 34.85°N, 136.10°E). Time–height cross section of lidar backscatter ratio obtained at 2206 LT 7 June 2006 showed the presence of a cirrus layer between 8.0 and 12.5 km MSL. Downward-penetrating structures of ice crystals with horizontal and vertical extents of 1.0–4.0 km and 200–800 m, respectively, have been detected at the cirrus cloud base for about 35 min. At the same time, the MU radar data revealed clear-air turbulence layers developing downward from the cloud base in the environment of the protuberances detected by the RMR lidar. Their maximum depth was about 2.0 km for about 1.5 h. They were associated with oscillatory vertical wind perturbations of up to ±1.5 m s−1 and variances of Doppler spectrum of 0.2–1.5 m−2 s−2. Analysis of the data suggests that the turbulence and the downward penetration of cloudy air were possibly the consequence of a convective instability (rather than a dynamical shear instability) that was likely due to sublimation of ice crystals in the subcloud region. Downward clear-air motions measured by the MU radar were associated with the descending protuberances, and updrafts were observed between them. These observations suggest that the cloudy air might have been pushed down by the downdrafts of the convective instability and pushed up by the updrafts to form the observed protuberances at the cloud base. These structures may be virga or perhaps more likely mamma as reported by recent observations of cirrus mamma with similar instruments and by numerical simulations.