Synoptic Scale Controls and Aerosol Effects on Fog and Low Stratus Life Cycle Processes in the Po Valley, Italy

Fog and low stratus clouds (FLS) form as a result of complex interactions of multiple factors in the atmosphere and at the land surface and impact both the anthropogenic and natural environments. Here, we analyze the role of synoptic conditions and aerosol loading on FLS occurrence and persistence in the Po valley in northern Italy. By applying k‐means clustering to reanalysis data, we find that FLS formation in the Po valley is either based on radiative processes or moisture advection from the Mediterranean sea. Satellite‐based data on FLS persistence shows longer persistence of radiatively formed FLS events, likely due to air mass stagnation and a temperature inversion. Ground‐based aerosol optical depth observations further reveal that FLS event duration is significantly higher under high aerosol loading. The results underline the combined effect of topography, moisture advection and aerosol loading on the FLS life cycle in the Po valley. Fog and low stratus clouds (FLS) are influenced by various drivers in the atmosphere and near the ground. Here, the impact of the large‐scale weather situation and aerosols is analyzed over the Po valley in northern Italy. Using reanalysis and satellite data we find that FLS events driven by nighttime cooling under stable conditions can persist longer than FLS events formed as a result of moisture transport. Investigating ground‐based observations of aerosols, particles on which moisture can condensate and fog and cloud droplets form, we find that FLS event duration is higher when a higher amount of aerosols is present. These results can help when predicting the duration of FLS events, which is particularly important for traffic safety. Fog and low stratus (FLS) cloud formation in the Po valley is primarily controlled by radiative processes or moisture advectionFLS persistence from the satellite perspective is highest for radiatively formed FLS events likely due to a stable boundary layerFLS persistence is significantly higher under high aerosol loading Fog and low stratus (FLS) cloud formation in the Po valley is primarily controlled by radiative processes or moisture advection FLS persistence from the satellite perspective is highest for radiatively formed FLS events likely due to a stable boundary layer FLS persistence is significantly higher under high aerosol loading