HAPpy Hour Seminar - Urban effects on cloud patterns across CONUS

Oct. 29, 2021

3:00 – 4:00 pm MDT

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Clouds, the important feature of the atmospheric layer, have a tremendous impact on Earth’s radiative budget, climate, and weather systems. Different from the natural ecosystem, the urban surfaces can influence atmospheric processes that are directly linked to the enhancement or suppression of localized clouds in the boundary layer (BL). A handful of case studies showed enhanced local clouds as evidence of the urban effects on the local climate and weather system. However, there is a lack of systematic assessment on urban influences on cloud patterns. Here we investigate the spatial and temporal organization of cloud patterns over and around 500 cities across CONUS to understand why and how cities reshape the regional cloud patterns, using sub-daily MODIS cloud fraction maps in the past two decades (2002 - 2020).

At the first glance, we observed clear observational evidence of urban effects on cloud variances and frequencies across CONUS. We found that differences in urban-rural cloud patterns show strong seasonal variations, in which the warm season features more prevalence of enhanced cloud coverage and more frequent localized clouds over cities during both daytime and nighttime. In winter, fewer clouds are observed over cities and less frequent, typically featured by an asymmetric diurnal pattern: cities remain more cloudy during daytime but less cloudy at night. We also statistically evaluate the effects of urban properties (e.g., city size, surface heating), climate background, and geographical settings (e.g., sea-breeze and mountain-breeze) on the cloud. Preliminary results show that larger cities tend to have greater impacts on the cloud signals in the growing season. However, an opposite effect is found for the cold season, suggesting cloud suppression over large urban areas. Summer cloud enhancement driven by increased surface heating is also unveiled during both day and nighttime. We also unveiled clear evidence that sea-breeze and mountain-breeze circulations interact with urban effects which strengthen the urban signals on clouds.

By observing and analyzing urban clouds over a variety of urban agglomerations, our study is anticipated to provide an improved understanding of the urban effects on the local and regional climate, as cloud frequency and patterns play a critical role in urban hydroclimatology, directly influencing surface temperature and precipitation.


Trang Thuy Vo - Department of Atmospheric Science, The University of Alabama in Huntsville