Temperature Effect on Phase Transition Radiation of Water

Abstract

Infrared radiation associated with vaporliquid phase transition of water is investigated using a suspension of cloud droplets and midinfrared (IR) (3–5 خ¼m) radiation absorption measurements. Recent measurements and Monte Carlo (MC) modeling performed at 60 آ°C and 1 atm resulted in an interfacial radiative phasetransition probability of 5 أ— 10−8 and a corresponding surface absorption efficiency of 3–4%, depending on wavelength. In this paper, the measurements and modeling have been extended to 75 آ°C in order to examine the effect of temperature on water's liquidvapor phasechange radiation. It was found that the temperature dependence of the previously proposed phasechange absorption theoretical framework by itself was insufficient to account for observed changes in radiation absorption without a change in cloud droplet number density. Therefore, the results suggest a strong temperature dependence of cloud condensation nuclei (CCN) concentration, i.e., CCN increasing approximately a factor of two from 60 آ°C to 75 آ°C at near saturation conditions. The new radiative phasetransition probability is decreased slightly to 3 أ— 10−8. Theoretical results were also calculated at 50 آ°C in an effort to understand behavior at conditions closer to atmospheric. The results suggest that accounting for multiple interface interactions within a single droplet at wavelengths in atmospheric windows (where anomalous IR radiation is often reported) will be important. Modeling also suggests that phasechange radiation will be most important at wavelengths of low volumetric absorption, i.e., atmospheric windows such as 3–5 خ¼m and 8–10 خ¼m, and for water droplets smaller than stable cloud droplet sizes (<20 خ¼m diameter), where surface effects become relatively more important. This could include unactivated, hygroscopic aerosol particles (not CCN) that have absorbed water and are undergoing dynamic evaporation and condensation. This mechanism may be partly responsible for water vapor's IR continuum absorption in these atmospheric windows.