High-resolution satellite-based cloud detection for the analysis of land surface effects on boundary layer clouds

In this study, geostationary satellite observations are used to develop and validate two high-resolution cloud-masking approaches for the region of Paris to show and improve applicability for analyses of urban effects on clouds. 

Firstly, the Local Empirical Cloud Detection Approach (LECDA) uses an optimised threshold to separate the distribution of visible reflectances into cloudy and clear sky for each individual pixel accounting for its locally specific brightness. Secondly, the Regional Empirical Cloud Detection Approach (RECDA) uses visible reflectance thresholds that are independent of surface reflection at the observed location.

Results show that

• A decrease of cloud cover during typical fog or low-stratus conditions over the urban area of Paris for the month of November is likely a result of urban effects on cloud dissipation.
• The regional approach, RECDA, is a more appropriate choice for the high-resolution satellite-based analysis of cloud cover modifications over different surface types than LECDA with regional biases of ±5 %.

This approach can provide comprehensive insights into spatiotemporal patterns of land-surface-driven modification of cloud occurrence and locally induced cloud processes, such as the diurnal variation of the occurrence of fog holes and cloud enhancements attributed to the impact of the urban heat island. Further, it is potentially transferable to other regions and temporal scales for analysing long-term natural and anthropogenic impacts of land cover changes on clouds.