Abstract
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.
Highlights
The Mediterranean and Europe are frequently affected by outbreaks of mineral dust, as specific atmospheric circulation patterns over northern Africa and the Mediterranean cause wind-driven dust emissions over the Sahara and consecutive transport to the north (e.g., Barkan et al, 2005; Salvador et al, 2014)
A first model run using the operational single-moment bulk microphysics scheme and without dust–radiation and dust–cloud interactions was performed for Germany on a grid with 2.8 km horizontal spacing over the period 3 April at 12:00 UTC to 5 April at 12:00 UTC
Simulated cloud-top temperatures of this model run were compared to Meteosat Second Generation (MSG) infrared imagery, and modeled ice water content (IWC) to cloud radar retrievals from CloudSat satellite and measured at Tropospheric Research (TROPOS) site in Leipzig
Summary
The Mediterranean and Europe are frequently affected by outbreaks of mineral dust, as specific atmospheric circulation patterns over northern Africa and the Mediterranean cause wind-driven dust emissions over the Sahara and consecutive transport to the north (e.g., Barkan et al, 2005; Salvador et al, 2014). Mineral dust is an important aerosol constituent (Carslaw et al, 2010), which influences atmospheric processes. Mineral dust particles directly participate in cloud microphysical processes by acting potentially as cloud condensation nuclei (CCN) (Bégue et al, 2015; Karydis et al, 2011) and icenucleating particles (INPs) (DeMott et al, 2003, 2010; Boose et al, 2016). The impact of dust particles on cloud microphysical and macrophysical properties cannot be generalized as it depends on the cloud type considered, the background aerosol composition and meteorological conditions. In mixed-phase clouds, midtropospheric aerosol entrainment is important to consider (Fridlind et al, 2004), and additional INPs likely accelerate cloud glaciation and precipitation formation and shorten cloud lifetime (DeMott et al, 2010)
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