The role of aerosols in orographic convection

Nocturnal cooling in clear, calm conditions may often lead to the formation of surface inversions within valleys, where reduced turbulence allows air adjacent to the surface to cool significantly.  If the valley is deep, the resulting Cold Air Pool can survive for many days.  This has important health implications in urbanised valleys (for example in the Alps), where pollution becomes trapped.  Subsequent mountain venting can release these built-up aerosols into the free troposphere. If this venting produces a convective cloud, the pollution could modify its development.

Aerosols act as Cloud Condensation Nuclei (CCN) – the seeds on which cloud droplets form as moist air rises above the Lifting Condensation Level and becomes saturated.  Polluted air contains a larger number of CCN, resulting in much smaller droplets than in clean air.  This can have important implications for the development of the convective cloud.  Generally, higher numbers of smaller cloud droplets act to delay precipitation formation, increasing cloud lifetime.  In a convective cloud, this could allow the cloud to become deeper, which might eventually lead to a more intense storm.   However, the effects of pollution on cloud develop will be complicated by other factors.  For example, smaller droplets are more reflective, enhancing the cloud albedo.   Glaciation and subsequent ice microphysics could also be affected.  The effect on in-cloud turbulence would modify the entrainment rate, which plays an important role in convective cloud evolution though dilution of the buoyant updraft.

This project will investigate these processes and how well they are modelled by the Met Office forecast model (the MetUM) using observations from the recent international TEAMx field campaign which took place in summer 2025 over the European Alps. TEAMx was focused on atmospheric transport and exchange processes over mountains. The Inn and Adige Valleys, along with other target areas, were extensively instrumented with ground based in situ and remote sensing instrumentation, supported by aircraft observations, including from the UK FAAM aircraft.  These observations will provide a useful dataset for investigating the processes described above.  Understanding of these observations will be aided using high-resolution MetUM simulations using the new CASIM microphysics scheme, which is able to represent cloud-aerosol interactions.

In addition, coarse-resolution simulations will test the performance of the new CoMorph convection parameterisation scheme.  CoMorph passes precipitation to the microphysics scheme to handle its fall to the surface.  This precipitation can be enhanced by the Seeder-Feeder scheme, which was developed for frontal situations where frontal rain is enhanced by falling through lower-level orographic cloud.  This project will test the scheme in convective situations.

Questions to be addressed in the project will include

• How well do km-scale simulations with the MetUM using CASIM capture the role of aerosol on the generation of convection over the Alps during TEAMx?
• Under what conditions is aerosol venting important in modifying the orographic convection?
• How well do global models using the new CoMorph convection scheme capture these aerosol effects on convection?

  This project is a potential CASE award with the Met Office with Dr Samantha Smith as the CASE supervisor. The project will complement ongoing work as part of the TEAMx-UK programme and give the student the opportunity to collaborate with others in Leeds and elsewhere.