Abstract
Abstract. In situ airborne observations of cloud microphysics, aerosol properties, and thermodynamic structure over the transition from sea ice to ocean are presented from the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign. A case study from 23 March 2013 provides a unique view of the cloud microphysical changes over this transition under cold-air outbreak conditions. Cloud base lifted and cloud depth increased over the transition from sea ice to ocean. Mean droplet number concentrations, Ndrop, also increased from 110 ± 36 cm−3 over the sea ice to 145 ± 54 cm−3 over the marginal ice zone (MIZ). Downstream over the ocean, Ndrop decreased to 63 ± 30 cm−3. This reduction was attributed to enhanced collision-coalescence of droplets within the deep ocean cloud layer. The liquid water content increased almost four fold over the transition and this, in conjunction with the deeper cloud layer, allowed rimed snowflakes to develop and precipitate out of cloud base downstream over the ocean. The ice properties of the cloud remained approximately constant over the transition. Observed ice crystal number concentrations averaged approximately 0.5–1.5 L−1, suggesting only primary ice nucleation was active; however, there was evidence of crystal fragmentation at cloud base over the ocean. Little variation in aerosol particle number concentrations was observed between the different surface conditions; however, some variability with altitude was observed, with notably greater concentrations measured at higher altitudes ( > 800 m) over the sea ice. Near-surface boundary layer temperatures increased by 13 °C from sea ice to ocean, with corresponding increases in surface heat fluxes and turbulent kinetic energy. These significant thermodynamic changes were concluded to be the primary driver of the microphysical evolution of the cloud. This study represents the first investigation, using in situ airborne observations, of cloud microphysical changes with changing sea ice cover and addresses the question of how the microphysics of Arctic stratiform clouds may change as the region warms and sea ice extent reduces.
Highlights
Projected increases in mean temperature due to climate change are greater in the Arctic than the midlatitudes (ACIA, 2005)
Turbulent kinetic energy (TKE), sensible heat fluxes, and latent heat fluxes all increase and become more variable over the marginal ice zone (MIZ) and ocean compared to the sea ice, with the greatest values typically observed at low altitude over the ocean
In situ aircraft observations of cloud microphysics, aerosol properties, and boundary layer structure have been presented from the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign
Summary
Projected increases in mean temperature due to climate change are greater in the Arctic than the midlatitudes (ACIA, 2005). Singlelayer MPS are prevalent in the transition seasons (Shupe et al, 2006; Morrison et al, 2012), whereas multilayered MPS are more common during the summer (Curry et al, 1988, 1996) It is uncertain how Arctic cloud fractions will evolve with increased global temperatures (Curry et al, 1996) and comprehending their relationship with sea ice extent is key to improving the representation of radiative interactions in numerical models. Jackson et al (2012) found a greater mean liquid water content in clouds over the ocean (during M-PACE) compared with those over the sea ice (during ISDAC), and substantial microphysical differences have been previously identified between cloud observations at three permanent measurement stations in the Canadian Arctic, based on meteorological differences (Shupe, 2011) Given such heterogeneity, studies of other Arctic regions are necessary. In this study, detailed observations from one case study are presented to illustrate the changing microphysical structure of clouds with sea ice extent
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