Ice and Supercooled Liquid Water Distributions over the Southern Ocean
based on In Situ Observations and Climate Model Simulations
Abstract
An evaluation of three climate models is conducted using in situ
airborne observations from the Southern Ocean Clouds, Radiation, Aerosol
Transport Experimental Study (SOCRATES) campaign. The evaluation targets
cloud phases, microphysical properties, thermodynamic conditions, and
aerosol indirect effects at -40°C – 0°C. For cloud phase frequency
distribution, the Community Atmosphere Model version 6 (CAM6) shows the
most similar result to the observations, which allows more
liquid-containing clouds below -10°C compared with its predecessor –
CAM5. The Energy Exascale Earth System Model (E3SM) underestimates
(overestimates) ice phase frequencies below (above) -20°C. Compared with
580-second averaged observations (i.e., 100 km horizontal scale), CAM6
and E3SM overestimate (underestimate) liquid (ice) water content (i.e.,
LWC and IWC), leading to lower a glaciation ratio when ice and liquid
coexist. Thermodynamic conditions, specifically relative humidity (RH),
is likely a key factor contributing to model cloud occurrence and cloud
phase biases. Simulated in-cloud RH shows higher minimum values than
observations, possibly restricting ice growth during sedimentation. As
number concentrations of larger and smaller aerosols (> 500
nm and > 100 nm) increase, observations show increases in
glaciation ratio, cloud fraction, LWC and liquid number concentration
(Nliq) at -18°C to 0°C, and IWC and ice number concentration (Nice) at
-35°C to 0°C. CAM6 and E3SM show slight increases of LWC and Nliq, and
E3SM shows small increases of Nice. These results indicate that models
underestimate aerosol indirect effects on ice and mixed phase clouds
over the Southern Ocean.