| For cold cirrus clouds (in situ ice formation at temperatures T < 235 K) different competing
formation mechanism could take place within the same environment (homogeneous freezing
of solution droplets/heterogeneous nucleation). Heterogeneous nucleation occurs generally at
lower supersaturations than needed for homogeneous nucleation, thus modifying or
suppressing homogeneous nucleation events (e.g. Spichtinger & Cziczo, 2010). From box
model results (e.g. Kärcher and Ström, 2003) it was claimed that heterogeneous nucleation
can affect homogeneous nucleation events only for warm temperatures and/or low vertical
velocities. The impact of heterogeneous ice nuclei (IN) on stratiform cirrus clouds was
demonstrated in earlier high-resolution 2D studies (e.g. Spichtinger & Gierens,
2009b).
In this study we reinvestigate this paradigm for dynamically more complex
situations, namely shallow cirrus cloud convection. A potentially unstable layer
is lifted by a constant large-scale updraft. Latent heat release of the growing ice
crystals leads to the formation of convective cells and high vertical updrafts up
to w - 1.5ms-1. The impact of heterogeneous ice nucleation with prescribed
IN concentrations (0-5-10-50L-1) is investigated in this high vertical velocity
regime. |