Background
The authors inform us that "low-level clouds frequently occur in the Arctic and exert a large influence on Arctic surface radiative fluxes and Arctic climate feedbacks," noting that during winter, in particular, surface net longwave radiation (FLW,NET) has a bi-modal distribution, with extremes that have been termed "radiatively clear" and "radiatively opaque." And in further discussing these clouds, they say that Arctic ice clouds "tend to have small optical depths and a weak influence on FLW,NET," which explains the "radiatively clear" condition, while they add that Arctic liquid-containing clouds "generally have large optical depths and a dominant influence on FLW,NET (Shupe and Intrieri, 2004)," which explains the "radiatively opaque" condition, as discussed by Doyle et al. (2011).

What was done
Cesana et al. employed real-world Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data to document cloud phases over the Arctic basin (60-82°N) during the five-year period 2006-2011, after which they used the results they obtained "to evaluate the influence of Arctic cloud phase on Arctic cloud radiative flux biases in climate models."

What was learned
The five researchers report that their evaluation of climate models participating in the most recent Coupled Model Inter-comparison Project (Taylor et al., 2012) revealed that "most climate models are not accurately representing the bimodality of FLW,NET in non-summer seasons." In fact, they indicate that even when advanced microphysical schemes that predict cloud phase have been used, such as those currently employed in the fifth version of the Community Atmosphere Model (CAM5, Neale et al., 2010), "insufficient liquid water was predicted."

What it means
Cesana et al. conclude from what they learned that "the simple prescribed relationships between cloud phase and temperature that have historically been used in climate models are incapable of reproducing the Arctic cloud phase observations described here," which must inevitably lead to similarly inaccurate values of "Arctic surface radiative fluxes and Arctic climate feedbacks," when employed in current state-of-the-art climate models.

References
Doyle, J.G., Lesins, G., Thakray, C.P., Perro, C., Nott, G.J., Duck, T.J., Damoah, R. and Drummond, J.R. 2011. Water vapor intrusions into the High Arctic during winter. Geophysical Research Letters 38: 10.1029/2011GL047493.

Neale, R.B., et al. 2010. Description of the NCAR Community Atmosphere Model (CAM 5.0). Technical Note 486+STR. National Center for Atmospheric Research, Boulder, Colorado, USA.

Shupe, M.D. and Intrieri, J.M. 2004. Cloud radiative forcing of the Arctic surface: The influence of cloud properties, surface albedo, and solar zenith angle. Journal of Climate 17: 616-628.

Taylor, K.E., Stouffer, R.J. and Meehl, G.A. 2012. An overview of CMIP5 and the experiment design. Bulletin of the American Meteorological Society 93: 485-498.