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

 

Authors

Sally A. McFarlane — U.S. Department of Energy
Jennifer M. Comstock — Pacific Northwest National Laboratory
Laura Dian Riihimaki — CIRES | NOAA ESRL GML

Category

Cloud Properties

Description

Comparison of thin mid-level cloud top temperature frequency distributions (left) and frequency of stable layers (right) for active and break monsoon periods at Darwin, Australia. During the active monsoon periods (top), a double peak in cloud-top temperature does not directly correspond to occurrence of stable layers. During the break period (bottom), however, there is a maximum in cloud tops that occur at the temperatures of a peak in stability.
Thin tropical mid-level layer clouds may be formed by different mechanisms, including detrainment from convective clouds at stable layers, or by the cooling caused by melting of ice and snow from precipitating stratiform clouds. The preconditioning of the atmosphere by mid-level clouds is thought to be necessary for organized convection like the Madden-Julian Oscillation to occur, yet we underestimate the frequency of occurrence of mid-level clouds in GCM simulations. We find that mid-level cloud climatologies determined from combined radar and lidar observations differ during the active and break monsoon periods at Darwin, Australia. During the break monsoon period, a single wide peak is seen in cloud-top temperature distributions slightly above the melting layer, at approximately -5 degrees C. This cloud-top temperature peak corresponds to a peak in the frequency of stability as measured by radiosondes. In the active monsoon period, however, cloud-top temperature distributions of mid-level clouds have two distinct peaks at -2 and -12 degrees C that do not directly correspond to stability peaks. We discuss the possibility that different formation mechanisms cause the differences seen during the active and break monsoon periods. To contribute to the discussion, we use cloud-phase determinations from lidar and cloud radar and comparisons to convective and stratiform precipitation from the scanning precipitation radar.