title: Sensitivity of liquid clouds to homogenous freezing parameterizations.
abstract: Water droplets in some clouds can supercool to temperatures where homogeneous ice nucleation becomes the dominant freezing mechanism. In many cloud resolving and mesoscale models, it is assumed that homogeneous ice nucleation in water droplets only occurs below some threshold temperature typically set at -40°C. However, laboratory measurements show that there is a finite rate of nucleation at warmer temperatures. In this study we use a parcel model with detailed microphysics to show that cloud properties can be sensitive to homogeneous ice nucleation as warm as -30°C. Thus, homogeneous ice nucleation may be more important for cloud development, precipitation rates, and key cloud radiative parameters than is often assumed. Furthermore, we show that cloud development is particularly sensitive to the temperature dependence of the nucleation rate. In order to better constrain the parameterization of homogeneous ice nucleation laboratory measurements are needed at both high (>-35°C) and low (<-38°C) temperatures.
  
  
  CONCLUSIONS
  Homogeneous freezing may be significant as warm as -30°CHomogeneous freezing should not be represented by a threshold approximationThere is a need for an improved parameterization of homogeneous ice nucleation.
authors.affiliation: School of Earth and Environment, University of Leeds Leeds, UK.
authors.forename: Ross J
authors.initials: RJ
authors.lastname: Herbert
authors.affiliation: School of Earth and Environment, University of Leeds Leeds, UK.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Murray
authors.affiliation: School of Earth and Environment, University of Leeds Leeds, UK.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Dobbie
authors.affiliation: Faculty of Chemistry, Bielefeld University Bielefeld, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Koop
keywords: cloud glaciation; cloud ice; droplet freezing; homogeneous nucleation; ice nucleation; mixed-phase clouds