Evaluation of cloud and precipitation simulations in CAM6 and AM4 using
observations over the Southern Ocean
This study uses cloud and radiative properties collected from in-situ
and remote sensing instruments during two coordinated campaigns over the
Southern Ocean between Tasmania and Antarctica in January-February 2018
to evaluate the simulations of clouds and precipitation in
nudged-meteorology simulations with the CAM6 and AM4 global climate
models sampled at the times and locations of the observations. Fifteen
SOCRATES research flights sampled cloud water content, cloud droplet
number concentration, and particle size distributions in mixed-phase
boundary-layer clouds at temperatures down to -25 C. The six-week
CAPRICORN2 research cruise encountered all cloud regimes across the
region. Data from vertically-pointing 94 GHz radars deployed was
compared with radar-simulator output from both models. Satellite data
was compared with simulated top-of-atmosphere (TOA) radiative fluxes.
Both models simulate observed cloud properties fairly well within the
variability of observations. Cloud base and top in both models are
generally biased low. CAM6 overestimates cloud occurrence and optical
thickness while cloud droplet number concentrations are biased low,
leading to excessive TOA reflected shortwave radiation. In general, low
clouds in CAM6 precipitate at the same frequency but are more
homogeneous compared to observations. Deep clouds are better simulated
but produce snow too frequently.
AM4 underestimates cloud occurrence but overestimates cloud optical
thickness even more than CAM6, causing excessive outgoing longwave
radiation fluxes but comparable reflected shortwave radiation. AM4 cloud
droplet number concentrations match observations better than CAM6.
Precipitating low and deep clouds in AM4 have too little snow. Further
investigation of these microphysical biases is needed for both models.