Radiative consequences of low-temperature infrared refractive indices for supercooled water clouds

Simulations of cloud radiative properties for climate modeling and remote sensing rely on accurate knowledge of the complex refractive index (CRI) of water.  Although conventional algorithms employ a temperature independent assumption, recent infrared measurements of supercooled water have demonstrated that from 460 to 980 cm^-1 the CRI spectrum for liquid water shifts to higher wavenumbers as temperature decreases, becoming more similar to that of ice.  We assess the biases that result from ignoring this temperature dependence in atmospheric radiative transfer calculations.  We show that the downwelling radiative fluxes using the temperature independent assumption are lower than those for the temperature-dependent CRI by as much as 1.7 W m^-2 (in cold regions), while top-of-atmosphere fluxes are larger by as much as 3.4 W m^-2 (in warm regions). Thus, the local greenhouse warming from supercooled clouds is increased when the temperature dependence of the CRI is properly accounted for.  In addition, remote sensing retrieval algorithms that use the temperature independent assumption will introduce spurious ice into pure, supercooled water clouds, or will underestimate the cloud thickness and droplet size.  Because of the importance of the refractive indices of water on atmospheric radiative transfer calculations, the current experimental uncertainty in the CRI of supercooled water at low temperatures (-40 to 0 C) must be reduced to properly account for supercooled clouds in climate models and cloud property retrievals.