Aerosol–cloud–radiation interaction during Saharan dust episodes: the dusty cirrus puzzle

Abstract

Abstract. Dusty cirrus clouds are extended optically thick cirrocumulus decks that occur during strong mineral dust events. So far they have mostly been documented over Europe associated with dust-infused baroclinic storms. Since today's global numerical weather prediction models neither predict mineral dust distributions nor consider the interaction of dust with cloud microphysics, they cannot simulate this phenomenon. We postulate that the dusty cirrus forms through a mixing instability of moist clean air with drier dusty air. A corresponding sub-grid parameterization is suggested and tested in the ICOsahedral Nonhydrostatic model with Aerosol and Reactive Trace gases (ICON-ART). Only with the help of this parameterization is ICON-ART able to simulate the formation of the dusty cirrus, which leads to substantial improvements in cloud cover and radiative fluxes compared to simulations without this parameterization. A statistical evaluation over six Saharan dust events with and without observed dusty cirrus shows robust improvements in cloud and radiation scores. The ability to simulate dusty cirrus formation removes the linear dependency on mineral dust aerosol optical depth from the bias of the radiative fluxes. For the six Saharan dust episodes investigated in this study, the formation of dusty cirrus clouds is the dominant aerosol–cloud–radiation effect of mineral dust over Europe.

“Seeing” Beneath the Clouds—Machine‐Learning‐Based Reconstruction of North African Dust Plumes

“…During mid‐March 2022 high loads of Saharan dust were transported to Central Europe via the Iberian Peninsula (cf. A. Seifert et al., 2023). This second case study concerns 12:00 UTC on 15 March 2022 (Figure 8).…”

Impact of Saharan dust outbreaks on short‐range weather forecast errors in Europe

“…However, also in ICON‐ART the cirrus cloud shield in the area of elevated DOD is absent (not shown). Recalling that our ICON‐ART version does not include aerosol–cloud interaction, we take this as a strong indication that dust played an important role for the formation of the high‐level cloud and suggest it being a dusty cirrus cloud (Seifert et al, 2023).…”

“…Aerosol particles are represented by lognormal modes, where the median diameter of the distribution is a diagnostic variable (Rieger et al, 2015). ICON‐ART offers a flexible and interoperable framework that can be configured to include tropospheric and stratospheric chemistry (Schröter et al, 2018), aerosol dynamics (Muser et al, 2020), aerosol–radiation (Hoshyaripour et al, 2019; Rieger et al, 2017), and aerosol–cloud interactions (Seifert et al, 2023). At the time of this study, only aerosol–radiation interaction were implemented in the pre‐operational forecast system.…”

“…Aerosol–cloud interactions, that is, indirect effects, were not yet implemented into the system used here. Recently, Seifert et al (2023) suggested a subgrid parametrisation for dusty cirrus, but the full aerosol–cloud interaction remains the topic of ongoing developments.…”

“…Weger et al (2018) found that cirrus cloud cover and ice content, in particular, increase after including prognostic calculation of dust and its effects. Recently, Seifert et al (2023) suggested a novel subgrid parametrisation which improves the representation of dust‐induced cloud decks and thus of the indirect effect of dust.…”