Talk by Olga Henneberg

(ETH Zurich)

Title: Orographic mixed-phase clouds - What determines their occurrence?

Mixed-phase clouds constitute clouds containing all three water phases - vapour, supercooled liquid and ice. The radiative budget and the hydrological cycle are particularly sensitive to the scale of phase partitioning within these clouds. The coexistence of the three phases at temperatures between 0 and -38  C is thermodynamically unstable because of the lower saturation vapour pressure over ice than over liquid. Nevertheless mixed-phase clouds are observed from tropical to polar regions. Persistent mixed-phase clouds are frequently observed in orographic terrain, as in the Swiss Alps.
Orographic induced updraft velocities play a key role in the longevity of orographic mixed-phase clouds. In updraft velocities cloud droplets are easily activated to grow on aerosol particles when water saturation is reached. Moreover, aerosols can act as ice nucleating particles (INPs). In the presence of INPs, cloud droplets can freeze and form ice crystals or ice crystals may form directly from the vapour phase. However, these particles are rare in the atmosphere. With a lack of INPs clouds will remain as supercooled liquid clouds. The cloud glaciates by undergoing the mixed-phase regime given a sufficient number of INPs is available. High updraft velocities in turn can keep the cloud in the mixed-phase.
In a series of model experiments using the numerical weather prediction model COSMO with a two-moment microphysics scheme on a 1km horizontal resolution we examined the sensitivity of orographic mixed-phase clouds to changes in the aerosol concentration and the dynamic conditions. In an artificially reduced topography some of the simulated mixed-phase clouds can no longer persist. The resulting higher ambient temperatures inhibit the formation of ice crystals and the clouds remain in the liquid phase. Under different synoptic conditions the reduced barrier height in turn enables the formation of mixed-phase clouds. The reduced barrier height reduces the blocking of air masses. Lifting and higher ambient temperatures let cloud droplets grow. The liquid phase can not be sustained in the transition from updrafts to downdrafts and the evaporating droplets favour the growth of ice crystals. This mechanism shifts the cloud and resulting precipitation leeward, known as spillover effect.
The comparison of simulations with modified updraft velocities, concentration of cloud condensation nuclei (CCNs) and INP concentration confirm the importance of dynamics. INP concentration need to be modified to the upper limit of the feasible range to cause changes comparable to changes caused by moderately adjusted updraft velocities. Changes in the CCN concentration are hardly detectable compared to the changes due to INP and dynamics.