Evaluation of Southern Ocean cloud in the HadGEM3 general circulation model and MERRA-2 reanalysis using ship-based observations

Abstract

Southern Ocean (SO) shortwave (SW) radiation biases are a common problem in contemporary general circulation models (GCMs), with most models exhibiting a tendency to absorb too much incoming SW radiation. These biases have been attributed to deficiencies in the representation of clouds during the austral summer months, either due to cloud cover or cloud optical thickness being too low. The problem has been the focus of many studies, most of which utilised satellite datasets for model evaluation. We use multi-year ship based observations and the CERES spaceborne radiation budget measurements to 5 contrast cloud representation and SW radiation in the atmospheric component Global Atmosphere (GA) version 7.0 and 7.1 of the HadGEM3 GCM and the MERRA-2 reanalysis. We find that MERRA-2 is biased in the opposite direction to GA (reflects too much SW radiation). In addition, MERRA-2 performs better in terms of absolute SW bias than nudged runs of GA7.0 and GA7.1 in the 60-70 • S latitude band. GA7.1 reduces the SO SW radiation biases relative to GA7.0, but significant errors remain at up to 20 Wm −2 between 60 and 70 • S in the austral summer months. Using ship-based ceilometer observations, we find low 10 cloud below 2 km to be predominant in the Ross Sea and the Indian Ocean sector of the SO. Utilising a novel surface lidar simulator developed for this study, derived from an existing COSP-ACTSIM spaceborne lidar simulator, we find that GA7.0 and MERRA-2 both underestimate low cloud occurrence relative to the ship observations by 18-25% on average, though the cloud cover in MERRA-2 is closer to observations by about 7%. Based on radiosonde observations, we also find the low cloud to be strongly linked to boundary-layer atmospheric stability and the sea surface temperature. GA7.0 and MERRA-2 agree 15 well with observations in terms of boundary-layer stability, suggesting that subgrid-scale parametrisations do not generate enough cloud in response to the thermodynamic profile of the atmosphere and the surface temperature. Our analysis shows that MERRA-2 has a much greater proportion of cloud liquid water in the SO in January than GA7.0, a likely key contributor to the difference in SW radiation. We show that boundary-layer stability and relative humidity fields are very similar in GA7.0 and Atmos. Chem. Phys. Discuss., https://doi.

Constraining the Surface Flux of Sea Spray Particles From the Southern Ocean

“…It has been well established that there is currently a large shortwave radiation bias over the Southern Ocean (Bodas‐Salcedo et al, ). Observations of cloud base height from radiosondes and ceilometer measurements throughout this same voyage showed that the shortwave radiation bias is related to the lack of low‐level cloud and fog predicted within atmospheric models (Kuma et al, ). Therefore, it is reasonable to expect that even with an improved in‐cloud activation scheme (e.g., Grythe et al, ), FLEXPART‐WRF still would have underestimated the frequency of droplet activation events.…”

“…It is useful, however, to recall that clouds over the Southern Ocean are not very well represented within modern atmospheric models (Schuddeboom et al, 2019;Trenberth & Fasullo, 2010). Current era atmospheric models systematically underpredict the amount of low-lying cloud and fog relative to the true cloud observed over the Southern Ocean (Kuma et al, 2019). Hence, even if a state-of-the-art microphysical parameterization of in-cloud scavenging had been present in FLEXPART-WRF, it is likely that the scavenging of sea spray still would have been underestimated.…”

The sensitivity of Southern Ocean aerosols and cloud microphysics to sea spray and sulfate aerosol production in the HadGEM3-GA7.1 chemistry–climate model

“…propane, formaldehyde, acetaldehyde, acetone, NH 3 , black carbon and organic carbon were prescribed based on the year 2000 (Lamarque et al, 2010). A further eight sensitivity simulations were performed, each of 10-year duration, from 1989 to 1998.…”

The sensitivity of Southern Ocean aerosols and cloud microphysics to sea spray and sulfate aerosol production in the HadGEM3-GA7.1 chemistry-climate model

“…Turnock et al (2019) show that the effect of pH on particles larger than 50 nm in diameter (which may activate to CCN) over the Southern Ocean is not negligible. Aqueous-phase chemistry may also be affected in the model due to the lack of persistent low-lying cloud over the Southern Ocean (Kuma et al, 2019). Aqueous-phase chemistry is more efficient at processing sulfur-containing gases than gas-phase chemistry, but cloud droplets are needed to allow in-cloud droplet chemistry to occur.…”