Estimation of a Humidity Profile Using Turbulence Echo Characteristics

Abstract

This paper is concerned with the development of a new stratosphere?troposphere radar technique for determining a height profile of specific humidity (q) by using characteristics of clear-air echoes in the troposphere. Turbulence echoes have been observed with the middle and upper atmosphere (MU) radar in Shigaraki, Japan (34.8°N, 136.1°E), on 2?6 August 1995 and 29?30 July 1999. Applying the radio acoustic sounding system (RASS) technique to the MU radar, the detailed time?height structure of the virtual temperature was simultaneously measured as well. The volume reflectivity (?) for turbulence echoes mainly depends on the refractive index gradient (M), the Brunt?Väisälä frequency squared (N2), and the turbulence energy dissipation rate (?). Because N2 and ? can be estimated from the RASS temperature profiles and the spectral width of a Doppler spectrum of turbulence echoes, respectively, a linear relation between ? and M2 can be derived where a proportion constant should be calibrated by referring to a simultaneous radiosonde measurement. On the other hand, radiosonde results indicate that M below about 10 km is mainly contributed first by dq/dz and second by the atmospheric temperature (T) and pressure. Then, a first-order differential equation between q and M can be derived. The sign of |M|, which is delineated from the MU radar observations of ?, is inferred considering a correlation between dq/dz and N2, where N2 is measured with RASS. A q profile is analyzed from the MU radar?RASS observations in August 1995 and July 1999, assuming an upper boundary condition at 10 km. The inferred q profile agrees reasonably well with a simultaneous radiosonde result at 1.5?10 km.