The present study applies the range imaging (RIM) technique to radio acoustic sounding system (RASS) measurements in an attempt to improve the vertical resolution of temperature profiles obtained by RASS measurements.When an FM-chirped acoustic wave is used for RASS observations, the transmitted radio wave is backscattered from the section of the FM-chirped acoustic pulse where the Bragg condition between the radar and acoustic wavelengths is satisfied. The Bragg resonance region propagates upward with the upward movement of the acoustic pulse. To improve the height resolution of the RIM results, complex time series are extracted for an adequate period corresponding to the effective scattering length of the RASS echo.In the RIM analysis for RASS echo, a model temperature profile is required to compensate the Doppler shift bias due to the shape of range-gate weighting. To remove this bias, an iteration algorithm was developed so that the temperature profiles of the previous RIM results could be used for the bias correction of the next RIM step.The RIM technique was applied to RASS imaging measurements performed using the middle and upper atmosphere (MU) radar on 29-31 October 2006 in an attempt to improve the height resolution to approximately 60 m from the nominal range gate width of 150 m. The temperature profiles inside the radar range volume with a temporal resolution of 26 min were successfully retrieved. The detailed temperature structure, which cannot be revealed by conventional RASS observations, was clarified in the RIM results. In particular, the small-scale inversion layers inside the radar range gate were clearly revealed by the MU radar-RASS imaging data. These detailed temperature variations within the radar range gate agreed well with the simultaneous rawinsonde results.

No abstract