Radiatively driven in-cloud turbulence and cloud-surface coupling in Arctic Stratocumulus

Accurate simulation of Arctic cloud properties is vital as clouds are one of the most significant controlling factors of the Arctic surface energy budget (Curry et al., 1996). The ability of models to successfully reproduce Arctic clouds is particularly poor (e.g. Birch et al., 2012) primarily due to the scarcity of observations and subsequent poor understanding of the complexity of the process occurring within Arctic stratocumulus.

In order to improve the representation of the Arctic boundary-layer turbulent structure and stratocumulus, observational and model analyses are being carried out into the controls on cloud-surface turbulent coupling in the Arctic. The final aim being to derive a simple parameterisation describing the turbulent coupling between the surface and cloud layers.

Tjernström et al. (2013) hypothesised that, as the surface generated turbulence over the sea ice during the Arctic Summer Cloud Ocean Study (ASCOS) was weak, one of the primary factors in determining whether the surface and cloud layer were coupled was the depth and strength of the cloud driven turbulence.

Observations of the turbulent structure of the atmosphere during ASCOS, in terms of the turbulent dissipation rate, were evaluated from MilliMetre Cloud Radar (MMCR) retrievals. The depth and peak of cloud driven turbulence were then derived and related to variations in the clouds geometry, liquid and ice water content as well as the surface LW forcing.

There was a weak relationship observed between peak turbulent strength and LWP and a clearer correlation between LWP and the depth of cloud driven turbulence. The depth of cloud driven turbulence being most responsive to changes in LWP under values of ~30 g m^-2. Comparison of the observed surface down-welling LW with the theoretical LW radiation, based on the temperature at cloud base, showed that the LWP values to which the turbulence was most sensitive are within the region where the cloud no-longer radiates as a blackbody.

The radiative properties of sub 30 g m^-2 clouds, and subsequently their impact upon the surface energy budget, are sensitive to small changes in liquid water. Additionally once no longer behaving as a blackbody the clouds particles effective radius becomes increasingly significant to the clouds radiative properties. It is therefore important to correctly simulate the transport of material that could alter the cloud properties in order to minimise errors in the surface budget due to alterations in the clouds radiative properties.

In addition to the observational analysis 1D radiative transfer and Met Office Large Eddy Model studies are currently being carried out and the preliminary results will be presented.

References

Birch CE; Brooks IM; Tjernstrom M; Shupe MD; Mauritsen T; Sedlar J; Lock AP; Earnshaw P; Persson POG; Milton SF; Leck C (2012) Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies, ATMOSPHERIC CHEMISTRY AND PHYSICS, 12, pp.3419-3435. doi: 10.5194/acp-12-3419-2012

Curry, Judith A., Julie L. Schramm, William B. Rossow, David Randall, (1996) Overview of Arctic Cloud and Radiation Characteristics. J. Climate, 9, 1731–1764. doi: http://dx.doi.org/10.1175/1520-0442(1996)009<1731:OOACAR>2.0.CO;2

Tjernström M; Leck C; Birch CE; Brooks IM; Shupe MD; Persson POG; Wheeler CR; Sedlar J; Mauritsen T; Paatero J; Szczodrak M (2012) Meteorological conditions in the central Arctic summer during the Arctic Summer Cloud Ocean Study (ASCOS), Atmospheric Chemistry and Physics, 12, pp.6863-6889. doi: 10.5194/acp-12-6863-2012