Mixed-phase clouds over the Southern Ocean as observed from satellite
and surface based lidar and radar
Abstract
This study investigates the occurrence of mixed-phase clouds (MPC) over
the Southern Ocean (SO) using space- and surface-based lidar and radar
observations. The occurrence of supercooled clouds is dominated by
geometrically thin (< 1km) layers that are rarely MPC. We
diagnose layers that are geometrically thicker than 1 km to be MPC
approximately 65%, and 4% of the time from below by surface remote
sensors and from above by orbiting remote sensors, respectively. We
examine the discrepancy in MPC as diagnosed from the below and above.
From above, we find that MPC occurrence has a gradient associated with
the Antarctic Polar Front near 55°S with the rare occurrence of
satellite-derived MPC south of that latitude. In contrast, surface
sensors find MPC in 33% of supercooled layers. We infer that
space-based lidar cannot identify the occurrence of MPC except when
secondary ice-forming processes operate in convection that is
sufficiently strong to loft ice crystals to cloud tops. We conclude that
the CALIPSO phase statistics of MPC have a severe low bias in MPC
occurrence. Based on surface-based statistics, we present a
parameterization of the frequency of MPC as a function of cloud top
temperature that differs substantially from that used in recent climate
model simulations.