PNNL

Abstract

Knowledge of cloud and precipitation formation processes remains incomplete, yet clouds containing the ice phase yield much of the global precipitation. Ice first forms in clouds warmer than -36ºC via the action of particles termed ice nuclei. We describe simultaneous measurements of ice nuclei and aerosol particles made at various boundary layer and free tropospheric locations indicating that number concentrations of ice nuclei are strongly related to number concentrations of large aerosol particles and temperature. We formulate a parameterization to quantify these dependencies and implement it in a global climate model to demonstrate improvement in simulating Arctic clouds, and important radiative impacts including a sensitivity of global cloud radiative forcing of ~1 W m-2 for an order of magnitude increase in ice nuclei.