In this talk I will present work that I have done on aerosols and clouds in UKCA-based models: the UK Earth System climate model and the regional high resolution UKCA model. The work spans a large range of time and spatial scales ranging from historical climate modelling of the 1850-2014 period to a case study of volcanic event at 4km resolution.
In the historical study we determined what caused the long-term trends in the upwelling short-wave (SW) top-of-the-atmosphere (TOA) fluxes over the North Atlantic region. The UKESM showed a positive trend between 1850 and 1970 (increasing SW reflection) and a negative trend between 1970 and 2014. We found that the 1850–1970 positive trend is mainly driven by an increase in cloud droplet number concentration due to increases in aerosol, while the 1970–2014 trend was mainly driven by a decrease in cloud fraction, which we attributed mainly to cloud feedbacks caused by greenhouse gas-induced warming.
In the high resolution modelling work we use an eruption of sulphur dioxide from the Kilauea volcano in Hawaii as a natural laboratory to evaluate aerosol-cloud interaction processes against satellite observations. The model produces an increase in aerosol optical depth due to the volcano that is too large compared to observations, but the increase in cloud droplet number concentration compares well. The cloud liquid water path (a measure of cloud thickness) and cloud fraction responses are in the opposite direction what is observed, although with a large observational uncertainty. This suggests some issues with the model, but also underscores the difficulty in observing aerosol-cloud interactions.
I'm now working in the UKESM development team at the Met Office. Therefore I will also look forwards towards the next iterations of the UKESM model with respect to aerosols and clouds, where we hope to bring together the things learnt from the research across various time and spatial scales that has been performed by scientists across the UK.