Continuous Observations of Humidity Profiles with the MU Radar–RASS Combined with GPS and Radiosonde Measurements

Abstract

In this paper a radar remote-sensing technique of a humidity profile by Tsuda et al. is improved so as to determine continuous time–height variations of humidity. The volume reflectivity (η) and turbulence energy dissipation rate (ɛ) have been determined from Doppler spectra measured with the middle and upper atmosphere (MU) radar located at Shigaraki, Japan, from 29 July to 4 August 1999. By combining the RASS (radio acoustic sounding system) technique with the MU radar, virtual temperature profiles were simultaneously estimated. Profiles of the water vapor mixing ratio (q) were then derived by solving a first-order differential equation of q versus height (z). A constraint for integrated water vapor referring to simultaneous GPS measurement has been incorporated in the retrieval algorithm. Time-interpolated radiosonde results taken every 6 h to obtain the initial guess of q profiles were also used. The inferred q profiles agreed well with simultaneous radiosonde results between 1.5–7.5 km AGL. The radar-derived humidity fluctuated with a typical timescale of a few hours, which cannot be resolved with radiosondes. The cloud-top height estimated from satellite irradiance measurements corresponded well to the region where the radar-derived relative humidity exceeded about 70%. The time–height structure of the radar-derived humidity was investigated in detail by referring to meteorological radar results. Six rain clouds passed over or near to the MU radar during the observations, and their passing coincided well with rising radar-derived humidity.