The impact of mineral dust on cloud formation during the Saharan dust event in April 2014 over Europe
2018
Abstract. A regional modeling study on the impact of desert dust on cloud
formation is presented for a major Saharan dust outbreak over Europe from 2 to 5 April 2014. The dust event coincided with an extensive and dense
cirrus cloud layer, suggesting an influence of dust on atmospheric ice
nucleation. Using interactive simulation with the regional dust model
COSMO-MUSCAT, we investigate cloud and precipitation representation in the
model and test the sensitivity of cloud parameters to dust–cloud and
dust–radiation interactions of the simulated dust plume. We evaluate model
results with ground-based and spaceborne remote sensing measurements of aerosol and
cloud properties, as well as the in situ measurements obtained during the
ML-CIRRUS aircraft campaign. A run of the model with single-moment bulk
microphysics without online dust feedback considerably underestimated cirrus
cloud cover over Germany in the comparison with infrared satellite imagery.
This was also reflected in simulated upper-tropospheric ice water content
(IWC), which accounted for only 20 % of the observed values. The
interactive dust simulation with COSMO-MUSCAT, including a two-moment bulk
microphysics scheme and dust–cloud as well as dust–radiation feedback, in
contrast, led to significant improvements. The modeled cirrus cloud cover and
IWC were by at least a factor of 2 higher in the relevant altitudes
compared to the noninteractive model run. We attributed these improvements
mainly to enhanced deposition freezing in response to the high mineral dust
concentrations. This was corroborated further in a significant decrease in
ice particle radii towards more realistic values, compared to in situ
measurements from the ML-CIRRUS aircraft campaign. By testing different
empirical ice nucleation parameterizations, we further demonstrate that
remaining uncertainties in the ice-nucleating properties of mineral dust
affect the model performance at least as significantly as
including the online representation of the mineral dust distribution.
Dust–radiation interactions played a secondary role for cirrus cloud
formation, but contributed to a more realistic representation of
precipitation by suppressing moist convection in southern Germany. In
addition, a too-low specific humidity in the 7 to 10 km altitude range in
the boundary conditions was identified as one of the main reasons for misrepresentation
of cirrus clouds in this model study.
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