This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Modelling the relationship between liquid water content and cloud
droplet number concentration observed in low clouds in the summer
Arctic and its radiative effects
Received: 26 Mar 2019 – Accepted for review: 05 Apr 2019 – Discussion started: 09 Apr 2019
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 NETCARE 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 schemes we examined, the autoconversion 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 fluxes 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 fluxes at the surface for all three schemes do not differ significantly (p = 0.01) from the observation-based radiative calculations.
Low clouds persist in the summer Arctic with important consequences for the radiation budget. We found that the ability of precipitation parameterizations to reproduce observed cloud properties was more variable than their better representation of radiative effects. Our results show that cloud properties and their parameterizations affect the radiative effects of clouds.
Low clouds persist in the summer Arctic with important consequences for the radiation budget. We...