The impact of mineral dust on cloud formation during the Saharan dust event in April 2014 over Europe

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.

The role of aerosol–radiation–cloud interactions in linking anthropogenic pollution over southern west Africa and dust emission over the Sahara

“…These interactions are complex and not completely known, and many studies are currently investigating this relationship to quantify how aerosols can affect meteorology and radiation. These studies cover many different scientific questions from hourly air quality (Yu et al, 2014;Forkel et al, 2015;Zhao et al, 2017) to long-term climate impacts (Mahowald et al, 2003;Luo et al, 2003).…”

“…Other recent studies show a variable impact on clouds and precipitation depending on the dust composition, size of the particles or altitude of the plume. In the upper troposphere, Hande et al (2015), Nickovic et al (2016) and Weger et al (2018) showed the importance of mineral dust to create ice clouds.…”

Aircraft-based observation of meteoric material in lower-stratospheric aerosol particles between 15 and 68° N

“…More recently, aircraft-based in situ aerosol mass spectrometry in the tropical and mid-latitude lower stratosphere at altitudes up to 19 km showed a significant fraction of particles containing meteoric material and sulfuric acid (Murphy et al, 1998(Murphy et al, , 2014Cziczo et al, 2001;Froyd et al, 2009). Indirect evidence for the existence of meteoric aerosol particle material in the Arctic lower stratosphere up to 20 km altitude was reported by Curtius et al (2005) and Weigel et al (2014). They measured non-volatile particles that were thermally stable on exposure to 250 • C and had diameters of 10 nm to a few micrometres.…”

“…Alpers et al, 2001;Gumbel and Megner, 2009;Megner and Gumbel, 2009;Rapp et al, 2010), and therefore, it is suggested that MSPs have an impact on polar mesospheric summer echoes (Rapp and Lübken, 2004;Megner et al, 2006). As MSPs are too small to sediment gravitationally, it is widely assumed that MSPs are drained from the mesosphere into the stratosphere most efficiently due to the air mass subsidence within the polar winter vortex, on a timescale of months (Plumb et al, 2002;Curtius et al, 2005;Megner et al, 2008;Saunders et al, 2012;Weigel et al, 2014;Plane et al, 2015;Kremser et al, 2016). The aerosol particles in the stratospheric aerosol layer (Junge et al, 1961;Junge and Manson, 1961;Kremser et al, 2016) consist mainly of sulfuric-acidwater (H 2 SO 4 -H 2 O) droplets (Lazrus et al, 1971;Rosen, 1971;Gandrud, 1974, 1977;Sedlacek et al, 1983;Gandrud et al, 1989;Arnold et al, 1998), but a significant fraction of none-pure sulfate particles has been observed by balloon-borne measurements throughout the stratosphere (Renard et al, 2020).…”

“…It is thought that MSPs partially dissolve in the droplets (Murphy et al, 1998(Murphy et al, , 2014Cziczo et al, 2001;Saunders et al, 2012) such that a dilute solution of highly soluble ferrous or ferric sulfate and hydrated magnesium sulfate and silicic acid is formed (Saunders et al, 2012). It was further suggested that silicon and aluminium are present as undissolved granular cores within the droplets (Murphy et al, 2014), which would explain the observations of refractory particles in the Arctic lower stratosphere (Curtius et al, 2005;Weigel et al, 2014).…”