Although clouds play a crucial role for the Earth’s water cycle and its energy budget, large uncertainties in their occurrence and properties limit the validity of current climate predictions. In fact, CMIP5 models show a spread of 2 K to 4.5 K warming partially caused by different cloud parameterisations. This spread is enhanced for recent CMIP6 models in part caused by different model sensitivities to mixed phase and ice clouds. Thus understanding the sensitivity of climate variability to clouds is a grand challenge for climate research.
Ice clouds and aerosol particles in the UTLS exert a substantial radiative forcing onto surface temperatures, which is on the order of 5 Wm−2. Cirrus clouds play an important role for the water distribution in the UTLS. They transport trace species such as aerosol, water and acids, affect UTLS dynamics and modify the Earth’s radiation budget. Cirrus clouds cover large areas of the tropopause region. Since they have a substantial effect on the radiation budget of long- and shortwave radiative fluxes with unknown sign for the net effect, they introduce large uncertainties to net heating rate estimates. The balance between the shortwave cooling and longwave warming effects depends on macro- and micro-physical cloud properties. Ice particle formation is highly sensitive to the local temperature, humidity, and cooling rates as well as dynamics and aerosol abundance.
The aerosol of the lower stratosphere (LS) largely consists of sulfuric acid and water with minor inclusions of meteoric smoke. Despite the low average content, carbonaceous material is present in some particles above the tropopause. During volcanically quiescent times the remaining background stratospheric sulfuric acid stems mainly from carbonylsulfide and sulfur dioxide, transported via the tropical tropopause layer into the ascending branch of the Brewer-Dobson circulation and via quasi-isentropic transport into the extratropical LS. It is not entirely clear where the aerosol forms on which the gaseous sulfuric acid condenses in the stratosphere.
While it is well known that sulfuric acid is a major component both, of the aerosol in the UTLS, much less is known about the role of organics. While the complex chemistry of formation of Highly Oxidised organic Molecules (HOMs) in the gas phase and the contribution of HOMs to aerosol nucleation and growth recently became a research topic of high interest for the chemistry of the boundary layer and the lower troposphere, not much is known about the organics that contribute to the aerosol of the upper troposphere (UT).
While the aerosol number concentration, size distribution and chemical composition of aerosol particles >100 nm are reasonably constrained for the UTLS, much less is known about the ultrafine particles in this region. Ultrafine particles are hardly accessible to remote sensing and in-situ data are still very limited. Large discrepancies exist for tropical convective regions between recent comprehensive observations of size-resolved aerosol concentrations over oceans within the ATOM project and the simulations from several global models. Missing organics in the tropical UT, missing nucleation mechanisms and an incorrect sub-grid aqueous aerosol processing and removal are the most likely reasons for the mismatch in between models and between observations and models. This also affects estimates of the aerosol-cloud radiation effects, as the radiative effect of new particle formation in the tropical UT could be of the order of 0.1 Wm−2 globally.
Even aircraft measurements usually do not provide the simultaneous measurement of aerosol number, size and precursor gas concentrations that would be required in order to identify the nucleation mechanism and to quantify the early growth of the aerosol. Hardly any measurements exist on the composition of particles <50 nm. Furthermore, comprehensive laboratory investigations as well as modelling studies are needed on the uptake and retention of organic and inorganic species in ice particles to understand the transport of these species by deep convective clouds.
Research questions (A)
RQ-A1: Is transport of boundary layer aerosol in warm conveyor belts or convection a dominant source of upper tropospheric aerosol and ice particles and how does this transport affect their radiative properties?
RQ-A2: What is the importance of non-sulfur aerosol components (e.g. meteoric material, organic matter, pollution) for the aerosol properties and their effects in the UTLS?
RQ-A3: Can organic particles be freshly formed in the upper troposphere? Is this a main source region for cloud condensation nuclei in the tropical lower troposphere? Does the aerosol in the extratropical lower stratosphere originate in the tropical upper troposphere?