De-risking cirrus management

Cirrus cloud modification (CCM) is a proposed solar radiation modification approach that could reduce the net warming from high-level ice clouds by altering how ice forms and how long cirrus persists. Under some conditions, changing the availability and properties of ice nucleating particles
(INPs) may shift ice formation towards fewer, larger crystals that sediment and sublimate quicker. Although the potential global benefit has been estimated at 2-3 W m^-2, published modelling studies remain inconsistent, and in several cases suggest limited or no benefit. A major reason is
uncertainty in background INP concentrations and aerosol-ice microphysics at cirrus temperatures, which makes it difficult to identify when and where CCM is feasible.

This talk will present the plan and preliminary results from an observation-led programme to de-risk these uncertainties, using aviation as a relevant, existing perturbation and preparing for an observational campaign to assess the effects of aviation soot on cirrus. Phase 1 focuses on “data mining” of historical cases. We will identify events where aircraft traverse clear air that is predicted to become ice supersaturated shortly afterwards and then track the affected air mass downwind using Lagrangian trajectories and coincident satellite observations. Geostationary thermal infrared imagery will be used to assess whether detectable cirrus changes emerge within several hours after passage. Where available, CALIPSO and CloudSat will be used to constrain
cloud vertical structure, and DARDAR-Nice and CALIOP-IIR retrievals will help evaluate changes in ice crystal number concentration. These analyses will quantify detectability limits and prioritise meteorological regimes and target regions for phase 2.

Phase 2 is a UK observational campaign using the FAAM research aircraft, where the only “intervention” is normal engine exhaust, mirroring what occurs globally every day but with dedicated measurement and attribution. We plan approximately 50 flight hours across 10 sorties, timed to occur in forecast ice-supersaturated layers with relatively simple advection and clear satellite viewing. The flight pattern will be designed to create a controlled perturbation region alongside nearby unperturbed control regions, allowing matched comparisons downwind. We will coordinate in situ sampling with new INP  measurements using PINEair (targeting cirrus-relevant temperatures) to constrain background INP levels, complemented with laboratory studies of aviation soot ice nucleation under cirrus conditions, including the role of plausible impurities such as fuel additives and engine metal oxides.

A dedicated high-resolution forecast capability, informed by in situ and satellite observations, via RIKEN/Fugaku, will support go/no-go decisions and flight targeting. Finally, we will translate observed signals into an efficacy estimate using an observation system simulation experiment with km-scale and regional modelling (including ICON). The primary outcome metric is trajectory-integrated outgoing longwave radiation from initial cirrus development to 24 hours afterwards, compared to matched control air masses.