The Effect of Regional Sources on Cloud Properties During an Extreme Warm-Air Advection in the Arctic

Abstract:

Anomalously large heat and moisture transport in the Arctic can result in enhanced surface warming and sea-ice melting. An extremely anomalous warm-air episode was observed during the Arctic Clouds in Summer Experiment (ACSE) in 2014, resulting in a rapid decrease in sea-ice concentrations from 90% to 50-60% (Tjernström et al, 2015). During that period varying cloud characteristics were observed (elevated clouds, optically-thick and optically-thin fog). This episode was mainly driven by changes in the large-scale circulation however the properties of the transported aerosols (number and mass concentrations, chemical composition) can also affect cloud properties and their radiative impact on the Arctic surface. In this study we investigate the impact of regional sources on CCN-activation and cloud droplet formation during this extreme episode.

Numerical simulations are performed with the Weather Research and Forecasting model, fully coupled with chemistry (WRF-Chem), which explicitly solves cloud-aerosol interactions. This model has successfully reproduced the thermodynamic aspects of this episode and also the cloud characteristics (Sotiropoulou et al, 2019). Through a number of sensitivity simulations we further investigate the role of regional emission sources (anthropogenic, biomass-burning and their synergy) on the aerosols’ vertical distribution and on CCN activation and cloud droplet formation.

During the cloudy periods, the omission of the anthropogenic emissions decreases accumulation and cloud droplet number concentrations up to 16% and 23%, respectively. For the biomass burning source, the decreases are up to 37% and 30% respectively. The omission of both sources decreases the accumulation number concentrations by 92-96%, and offers an insight to the aerosol background conditions of the pristine Arctic environment.