The Seasonal Cycle of Low Stratiform Clouds

TLDR

Sky‑obscuring fog over midlatitude oceans contributes substantially to summertime stratus, likely driven by warm advection across sharp sea‑surface‑temperature gradients. The study investigates the seasonal cycle of low stratiform clouds using surface‑based cloud climatologies. By comparing Earth Radiation Budget Experiment data with surface cloud climatologies, the authors assess how low clouds influence the radiation budget. Ten active stratocumulus convection regions were identified and grouped into subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus; except for the Chinese region, all high‑stratus areas are oceanic, and in all but the Arctic the peak stratus season coincides with maximum lower‑troposphere static stability, with interannual stratus variations linked to stability changes and a 6 % increase in stratus coverage per 1 °C rise in static stability.

Abstract

The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans. In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6% increase in stratus fractional area coverage is associated with each 1°C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summertime stratus amount. The amount of fog appears to be related to warm advection across sharp gradients of SST.