Research / Research Highlights

Climatology of Cloud-Land-Surface Coupling Across Different ARM Sites

Submitter

Li, Zhanqing — University of Maryland

Area of Research

Cloud Distributions/Characterizations

Journal Reference

Roldán‐Henao N, T Su, Z Li, Y Zheng, and J Yorks. 2026. "Climatology of Cloud‐Land‐Surface Coupling across Different ARM Sites." Journal of Geophysical Research: Atmospheres, 131(1), e2025JD044010, 10.1029/2025JD044010.

Science

Hourly variation of the percentage of coupled and decoupled clouds at the five ARM observatories. The red lines correspond to the coupled cases while the blue lines represent the decoupled cases. Error bars represent the standard deviation.

Average percentage of coupled and decoupled low-level clouds at five ARM observatories. Each bar represents the mean percentage with an error bar indicating the standard deviation. Coupled conditions have the following means (±SD): SGP 66.81 ± 4.65%, MAO 64.98 ± 7.54%, COR 68.44 ± 7.99%, TMP 64.44 ± 17.08%, and FKB 65.23 ± 13.94%. Decoupled conditions show corresponding means of 33.18 ± 4.64%, 35.73 ± 7.54%, 31.55 ± 7.99%, 35.55 ± 17.08%, and 34.77 ± 13.94%.

Land-atmosphere interactions are crucial for the formation and evolution of low-level clouds and planetary boundary layer (PBL). However, characterizing the different coupling states between clouds and the surface is uncertain, particularly in continental regions due to complex thermodynamics. This study examines cloud-surface coupling at five ARM sites across three continents, highlighting both the complexities and commonality of cloud-land-surface interactions.

Impact

This study first attempts to optimize our algorithm of determining the state of cloud-surface (PBL) coupling but found consistent coupling thresholds that were then applied to all five sites to develop the climatology of the coupling. The percentages of clouds that are coupled with surface and PBL are consistent across the diverse climatic and geographical continental regions, ~66%. The consistency may reveal a fundamental relationship between cloud and land surfaces that may help guide future model evaluation and parameterization efforts for understanding cloud-surface interactions.

Summary

This study leveraged ARM ground-based observations at five distinct locations to investigate cloud-surface coupling over continental regions. The findings show similar coupling percentages and thresholds regardless of the location. Overall, 66% of clouds were classified as coupled to the surface, and 34% were classified as decoupled. Coupled clouds are characterized by humid environments, in which vertical motions connect the surface and lower atmosphere to the cloud base. In contrast, decoupled clouds tend to occur in drier, colder environments, with vertical motions within the boundary layer detached from the cloud base. Coupled clouds peak during warmer hours and seasons, and vice versa for decoupled clouds.