Abstract
Abstract. Low clouds persist in the summer Arctic with important consequences for the radiation budget. In this study, we simulate the linear relationship between liquid water content (LWC) and cloud droplet number concentration (CDNC) observed during an aircraft campaign based out of Resolute Bay, Canada, conducted as part of the Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments study in July 2014. Using a single-column model, we find that autoconversion can explain the observed linear relationship between LWC and CDNC. Of the three autoconversion schemes we examined, the scheme using continuous drizzle (Khairoutdinov and Kogan, 2000) appears to best reproduce the observed linearity in the tenuous cloud regime (Mauritsen et al., 2011), while a scheme with a threshold for rain (Liu and Daum, 2004) best reproduces the linearity at higher CDNC. An offline version of the radiative transfer model used in the Canadian Atmospheric Model version 4.3 is used to compare the radiative effects of the modelled and observed clouds. We find that there is no significant difference in the upward longwave cloud radiative effect at the top of the atmosphere from the three autoconversion schemes (p=0.05) but that all three schemes differ at p=0.05 from the calculations based on observations. In contrast, the downward longwave and shortwave cloud radiative effect at the surface for the Wood (2005b) and Khairoutdinov and Kogan (2000) schemes do not differ significantly (p=0.05) from the observation-based radiative calculations, while the Liu and Daum (2004) scheme differs significantly from the observation-based calculation for the downward shortwave but not the downward longwave fluxes.
Highlights
Observations show a warming trend in the Arctic that is 2.5 times greater than the rest of the world (ACIA, 2005)
Our model simulations show that the linear relationship between liquid water content (LWC) and cloud droplet number concentration (CDNC) observed by Leaitch et al (2016a) in summer Arctic low clouds is consistent with parameterizations of autoconversion, other processes, such as variability in meteorological conditions, entrainment of dry air without mixing and increased condensation rates, may contribute to the observed relationship
The choice of autoconversion scheme in the SCM-ABLC changes the simulated relationships between LWC and CDNC, with the best simulated linear relationship obtained from a combination of the K–K scheme at CDNC below 20 cm−3 and the L–D scheme at higher concentrations. These results are consistent with a regime change between very low and higher CDNC corresponding to the Mauritsen limit
Summary
Observations show a warming trend in the Arctic that is 2.5 times greater than the rest of the world (ACIA, 2005). One known uncertainty in our understanding of climate change is the effect of clouds on the radiation budget (Lohmann and Hoose, 2009), with important consequences for Arctic climate. Microphysical properties of Arctic clouds are sensitive to changes in cloud condensation nuclei (CCN) concentrations (Coopman et al, 2018) as is cloud radiative effect (Rosenfeld et al, 2019). As observed at other latitudes for comparable liquid water content (LWC), smaller cloud droplets in the Arctic are associated with less shortwave radiation at the surface than larger droplets due to an increased reflectivity (Peng et al, 2002). The net radiative effect of cloud droplet size and number concentration can vary in sign in the Arctic due to the interplay between longwave and shortwave radiative effects when there are high surface albedo and large solar zenith angle (Curry et al, 1996). The radiative forcing from shortwave radiation due to cloud is controlled by cloud microphysical properties such
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