Underestimation of the greenhouse effect by Polar supercooled water clouds

Simulations of cloud radiative properties for climate modeling and remote sensing in the polar regions rely on accurate knowledge of the complex refractive index (CRI) of water. Although conventional algorithms employ a temperature independent assumption (TIA), recent infrared measurements of supercooled water have demonstrated that the CRI becomes increasingly ice-like at lower temperatures. Here, we assess biases that result from ignoring this temperature dependence. We show that TIA-based cloud retrievals introduce spurious ice into pure, supercooled clouds, or underestimate cloud thickness and droplet size. TIA-based top-of-atmosphere fluxes are higher by as much as 2 W/m2 (for an Arctic summer / Antarctic coastal summer model), while downwelling radiative fluxes are lower than those for the temperature-dependent CRI by as much as 1.7 W/m2 (for an Arctic winter / Antarctic interior summer model). Proper accounting of the temperature dependence of the CRI, therefore, leads to significantly greater local greenhouse warming due to supercooled clouds than previously predicted. The current experimental uncertainty in the CRI at low temperatures must be reduced to properly account for supercooled clouds in both climate models and cloud property retrievals.