Meteorological Physical Background

Dust-infused baroclinic systems are large mid-latitude cyclones that ingest large amounts of mineral dust from desert surfaces and transport them over long distances. Strong ascending motions inside the system lift the mineral dust particles up to the tropopause where they can be involved in the formation of small ice crystals to form so-called dusty cirrus clouds.

Figure 1: Formation of DIBS over northern Africa. Strong magenta colors indicate the rising dust plumes. SEVIRI Dust RGB from 19 - 22 March 2023 (00:00 - 05:45 UTC).

Transport mechanism

For Europe, most DIBS events originate over North Africa, where developing dust plumes can be detected in the Dust RGB. Some of these dust plumes are transported by southerly airstreams towards Europe. A large number of cases occur over Europe from January to March; this is consistent with the general circulation pattern at this time of year, favouring the formation and propagation of mid-latitude cyclones further south than during the rest of the year.

Figure 2: Mineral dust transport (magenta color) over the Mediterranean Sea. SEVIRI Dust RGB from 19 - 20 June 2023 (05:15 - 05:15 UTC).

The part of the cyclone involved in the upward motion of the dust particles is the warm conveyor belt, a stream (relative to the motion of the cyclone) of prefrontal warm and humid air that follows an ascending motion towards the warm front and, at later stages, towards the occlusion cloud band. Consequently, most dusty cirrus events are observed on top of the warm front of a mid-latitude cyclone.

Figure 3: The main relative streams (conveyor belts) involved in a mid-latitude cyclone. The rising orange stream is the so-called Warm Conveyor Belt.

Dusty cirrus formation

The mineral dust particles act as cloud condensation nuclei and at higher levels as ice deposition nuclei. Due to their abundance and the limited availability of water vapour at higher atmospheric levels, these cloud top ice crystals are extremely small. Dusty cirrus clouds are synoptic-scale opaque ice clouds with reduced visible reflectance and a cellular cloud top texture.

The characteristic cellular structure of dust-infused cirrus clouds is a strong indication of ongoing shallow convection at the cloud top layers. Later in the day, the pockmarked structure of the cirrus cloud deck usually vanishes. The influence of desert mineral dust on cloud formation processes is still a critical yet poorly understood element of atmospheric cloud physics. Currently, two partially overlapping explanations can be found in the literature:

Night-time radiative cooling (Nagy and Kollath)

  • Mineral dust is transported with the warm conveyor belt up to the tropopause.
  • The dust-loaded air mass of the warm conveyor belt has high humidity.
  • With mineral dust in the atmosphere, outgoing longwave radiation during the night is stronger for dusty cirrus than for classical (i.e., clean air) cirrus clouds.
  • Strong radiative cooling leads to a destabilization of the topmost cloud layers under night-time conditions.
  • Shortwave absorption processes in daylight conditions lead again to a stabilization of the dusty cirrus layer and the cellular pattern disappears.

Figure 4: Schematic showing the physical processes that lead to the formation of dusty cirrus with a cellular cloud top pattern according to the night-time cooling theory.

Mixing instability theory (Seifert et al.)

Based on the observation that not all DIBS produce a dusty cirrus cloud deck with a cellular pattern, Seifert et al. (2023) formulated the mixing instability theory.

  • As a precondition, the air near the cloud tops must be stratified: moist clean air above and a drier air mass containing the mineral dust particles below.
  • Initially, small amounts of mineral dust are mixed into the moist air above (heterogeneous ice nucleation). This is usually achieved through continuous, large scale lifting of the air layers as typically happens with the warm conveyor belt.
  • Destabilization by increased radiative cooling during the night, followed by turbulent mixing, reinforces ice nucleation (homogeneous ice nucleation).
  • Shortwave absorption processes in daylight conditions lead again to a stabilization of the dusty cirrus layer and the cellular pattern disappears.

Figure 5: Schematic summarizing the physical processes that lead to the formation of dusty cirrus at the interface between a dry Saharan dust layer below and a moist atmospheric layer above. The conceptual model assumes cold environmental conditions typical for the upper troposphere and near-neutral to weakly-stable stratification (from Seifert et al. 2023).