Joint Seminar: Modeling condensation in cloud-scale models

Condensation of water vapor to form and grow cloud droplets is the most fundamental process of cloud and precipitation formation. It drives cloud dynamics through the release of latent heat and determines the strength of convective updrafts. Cloud-scale models simulate condensation by applying two drastically different methods. The first one is the bulk condensation, where cloud water is assumed to exist only in saturated conditions and evaporates instantaneously when the air becomes sub-saturated. The second approach involves prediction of the in-cloud super- or sub-saturation and can be used in models that predict not only condensate mass but also relevant features of the droplet size distribution (e.g., models with the 2-moment microphysics or with the bin microphysics). This presentation will address the question whether the difference between the two approaches has a noticeable impact on convective dynamics. Microphysical piggybacking is used to assess the impact with confidence. The piggybacking is about applying two sets of thermodynamic variables, one driving the flow and the other one following (piggybacking) the simulated flow without affecting it. Piggybacking simulations with the bin microphysics for shallow non-precipitating convection and with the double-moment bulk microphysics for deep convection will be discussed. For the shallow convection, the differences in cloud field simulated with bulk and bin schemes come not from small differences in the condensation, but from more significant differences in the evaporation of cloud water near cloud edges as a result of entrainment and mixing. For the deep convection, piggybacking results show a significant dynamical impact of finite supersaturations and a strong microphysical effect associated with upper-tropospheric anvils. Implications of these results for modeling convective dynamics will be discussed and a possible intermediate modeling methodology will be suggested.




13:30 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Wojciech W. Grabowski, Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado, USA


Bjorn Stevens

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