Near-surface meteorology changes driven by aerosol effects during the April 2020 wildfires in the Chornobyl Exclusion Zone, Ukraine

Wildfires, while disastrous for ecosystems, also contribute significantly to pollution impacting human health. They also affect meteorological conditions at both local and regional scales by emitting aerosols into the atmosphere, which have both direct and indirect effects. In Ukraine, forest fires are common in the spring, coinciding with the season of agricultural open burning.

The wildfire episode in April 2020 was among the most severe and especially difficult to extinguish because of its origin and spreading in the abandoned Chornobyl Exclusion Zone (CEZ). Applying the seamless online-integrated Enviro-HIRLAM modeling system, we aimed to study aerosol contamination and how its elevated levels affected regional near-surface meteorology. To achieve this, the model was run in 4 modes: reference (REF) run and runs to simulate direct (DAE), indirect (IDAE) and combined (COMB) aerosol effects. These runs were performed at 15 km horizontal resolution (covering large European territory to consider atmospheric circulation) with downscaling to 5 (focusing on Ukraine) and 2 km (focusing on the CEZ).

Elevated black and organic carbon content accounted for 80% of all aerosol species with the prevailing mass concentration in the accumulation mode in the CEZ. The observed meteorology, driven by aerosol effects, was more intensified in these synoptic conditions, and especially at the edges of the fronts. When aerosol effects were included, the wind speed changed up to ±4 m/s and caused spatial shifts in patterns of cloudiness and precipitation. Moreover, verification showed better results for modelling with IDAE, whereas DAE effects can overestimate changes in near-surface meteorology. This aerosol composition led to noticeable cooling and drying effects. The 2-m air temperature decreased by 3℃ and a specific humidity dropped by 1 g/kg at a local scale. However, these effects varied with atmospheric conditions. In particular, when fronts, especially cold fronts, passed through the CEZ, stronger changes in meteorological parameters were observed as expected. As presence of aerosols influences a humidity regime in the boundary layer (including formation and development of cloudiness and precipitation), the spatial positioning of modeled fronts may be shifted leading to changes of opposite signs.

This study was supported by the grant of HPC-Europa3 Transnational Access Programme for projects “Integrated modelling for assessment of potential pollution regional atmospheric transport as result of accidental wildfires” (IMA-WFires, HPC17TRLGW) & “Research and development for integrated meteorology – atmospheric composition multi-scales and – processes modelling” (Enviro-PEEX(Plus) on ECMWF; SPFIMAHU-2021). The CSC – IT Center for Science Computing (Finland) is acknowledged for computational resources.