Enhanced Nighttime Fog and Low Stratus Occurrence Over the Landes Forest, France

Abstract

Understanding the drivers of fog and low stratus (FLS) cloud occurrence is important for traffic, ecosystems, and climate models, but it is challenging to analyze due to the complex interactions between meteorological factors and land cover. Here, we use active and passive satellite data, as well as reanalysis data to investigate nighttime FLS occurrence over the expansive Landes forest in France from 2006 to 2015. We find significant FLS enhancement over the forest compared to surrounding areas, especially in summer and fall. Lower wind speed and lower temperatures are found over the forest at night, which can enhance FLS development over the forest. Still, other drivers, such as biovolatile organic compounds acting as cloud condensation nuclei, are most likely important as well. The results show that the influence of forests on boundary layer clouds is not limited to convective daytime conditions.

A satellite‐based climatology of fog and low stratus formation and dissipation times in central Europe

“…In addition, the potential influence of multilayer cloud situations and misclassifications on the presented FLS formation and dissipation time have to be considered for the interpretation of the results. In a regional study, the FLS data set by Egli et al (2017) used as a basis for this study has been shown to agree well with active sensor data (Pauli et al, 2022). The formation and dissipation time patterns shown with this novel FLS formation and dissipation data set are also in line with LES and modelling studies over Europe (cf.…”

“…Close to the French Atlantic coast over the Landes forest south of 45 • N, FLS forms earlier over the forest (around midnight) than over its surroundings (FLS formation in the morning), particularly in spring and summer. This is in line with enhanced nighttime FLS occurrence over this forest compared to its surroundings shown in Pauli et al (2022), potentially due to enhanced emissions of biogenic volatile organic compounds over the forest, which can serve as cloud condensation nuclei (Spracklen et al, 2008;Kammer et al, 2018).…”

“…This is in line with enhanced nighttime FLS occurrence over this forest compared to its surroundings shown in Pauli et al . (2022), potentially due to enhanced emissions of biogenic volatile organic compounds over the forest, which can serve as cloud condensation nuclei (Spracklen et al ., 2008; Kammer et al ., 2018).…”

“…In a regional study, the FLS data set by Egli et al . (2017) used as a basis for this study has been shown to agree well with active sensor data (Pauli et al ., 2022). The formation and dissipation time patterns shown with this novel FLS formation and dissipation data set are also in line with LES and modelling studies over Europe (cf.…”

Simulating the Effects of Regional Forest Cover and Windthrow‐Induced Cover Changes on Mid‐Latitude Boundary‐Layer Clouds

“…This confirms and extends the recent results of Pauli et al. (2022) who found enhanced night fog and low stratus occurrence in the same forest, using nighttime satellite data (0–6 UTC) over a 10‐year period. Moreover, an opposite effect appears between 6 and 8 UTC in our simulations, with a positive cloud anomaly after Klaus, as the lower subgrid TKE delays the dissipation of low clouds once they have formed.…”