Formation and climatology of tropical mid-level clouds at Darwin, Australia

We use a new merged lidar and radar dataset to present a climatology of tropical mid-level clouds at the Atmospheric Radiation Measurement (ARM) site at Darwin, Australia. This combined instrumentation allows a comprehensive picture of the frequency and macrophysical characteristics of mid-level clouds. Thin tropical mid-level layer clouds are thought to be formed primarily either by detrainment from convective clouds at stable layers, or by the cooling caused by melting of ice and snow from precipitating stratiform clouds. We compare the variability in the frequency of mid-level layer clouds to the frequency of convective clouds in order to test the hypothesis that mid-level clouds are formed primarily from convection or its associated stratiform precipitation. Mid-level clouds occur most frequently (~20% of measurements) at Darwin during the convectively active monsoon season, however, we also find 5% occurrence of mid-level clouds in the dry season when very little convection exists. Cloud height base and top distributions show two mid-level peaks around 0° and 15° C as seen in other studies. We use lidar observations to look for the frequency of super-cooled liquid water layers at these two temperatures to examine whether the cloud properties and formation may be related to microphysical ice growth.