Layer echoes from under a canopy of high cloud

Observations are presented of extensive layer echoes seen during the past summer season on the modified Marconi type 277 10 cm radar at Aspendale, near Melbourne. A detailed study is made of "all-sky" echoes from the middle troposphere persisting for several hours while practically no rain fell in the State. The phenomenon is analysed two-dimensionally as snow trails from a high cloud canopy using the Browne-Marshall model of parabolic trail patterns in constant vertical wind shear and for constant particle fall velocity, and a generalised model of exponential patterns for variable fall velocity. The absence in the radar scans of a melting ("bright") band indicates total evaporation above melting level. An assessment of the evaporation rate for particles falling through desiccated air beneath the high cloud is made to deduce their median mass and the reflectivity. A particle precipitation rate of the order 0.5 mm/hr or 0.5 inch/24 hr is indicated." The study emphasises the importance of the depth and dryness of subsided air as factors inhibiting the Bergeron rain mechanism and their possible relevance to the wider problem of drought in Australia.