How Well Do We Know the Supersaturation in Clouds?

The supersaturation field within clouds is critical to understanding aerosol activation and condensational growth of cloud droplets. Aerosols can activate depending on both the mean and relative magnitude of the fluctuations about the mean, which in turn controls the behavior of the cloud. To describe the importance of the fluctuations on activation, three regimes have been identified; mean-dominated, fluctuation influenced, and fluctuation dominated. All three regimes produce unique cloud droplet size distributions. The mean and fluctuations of the supersaturation have been shown to have a large influence on cloud droplet size distribution.

Despite its importance, the supersaturation field in clouds is still not well understood. There are two reasons for this uncertainty. First, it is not fully known how the feedback of water vapor and temperature during condensational growth changes the variance of supersaturation in a turbulent environment. The release of latent heat and loss of water vapor can change the variances and covariances between water vapor and temperature, which changes the variance in supersaturation. Further, it is still not well known how the supersaturation behaves with changing aerosol loadings and cloud droplet concentrations, which affect the condensation rate. Secondly, such a measurement requires collocated measurements of water vapor and temperature, each with a fast and comparable response time. Because of this, direct measurements of the supersaturation in cloudy conditions are rare. Guided by laboratory measurements, we will discuss what is currently known about the supersaturation field and the limitations on measuring it accurately.