Observations of Tropospheric Temperature Fluctuations with the MU Radar-RASS

Abstract

Applying the RASS (radio acoustic sounding system) technique to the MU (middle and upper atmosphere) radar, profiles of both temperature and wind velocity were observed every 90 s in the height range of about 1.5?7.0 km, with a height resolution of 300 m, for about 40 h on 6?8 August 1990. The temperature profiles obtained with RASS agreed well with the virtual temperature derived from radiosonde sounding, where the mean difference between the temperature values was approximately 0.3°C. The observed frequency spectra above about 2.5-km altitude, having an asymptotic slope of ?5/3 and approximately 0 for temperature and vertical wind velocity fluctuations, respectively, were reasonably consistent with a model spectrum of gravity waves. But, below 2.5 km, low-frequency components were conspicuously enhanced, especially for vertical wind velocity, presumably affected by convection. Wavelike temperature fluctuations with a dominant period of 6?8 h clearly showed downward phase progression and a π/2 phase lag between temperature and vertical wind velocity. In addition, short-period components were also recognizable for both temperature and vertical wind velocity fluctuations. However, for low-frequency components, which were sometimes enhanced at the lowest altitudes of the observation range, the time variations of temperature and vertical wind velocity were in phase. The covariance between temperature and vertical wind velocity was also determined, and heat flux profiles were further estimated. Although a major part of the fluctuations above 2.5 km could be explained by gravity waves, those below 2.5-km altitude seemed to be due to effects of convective motions in the planetary boundary layer.