Diurnal cycle of stratocumulus mesoscale convective cells in the South-East Pacific

Abstract. Stratocumulus (StCu)-topped boundary layers exhibit complex mesoscale cellular convection that remains a primary source of climate radiative forcing uncertainty and a persistent challenge for models. While daytime snapshots have established a characteristic aspect ratio (AR) – the ratio of cell size λ to boundary-layer depth – of 30–40, its evolution over the full diurnal cycle remains poorly constrained. Here we use high-resolution infrared observations from GOES-East (2020–2025) over the South-East Pacific during August–September to provide a continuous day-to-night characterization of StCu spatial metrics. Using a brightness temperature difference framework (ΔTb=Tb12.3µm-Tb10.3µm), we reveal a robust, universal four-phase diurnal cycle: morning growth, early-afternoon plateau, rapid late-afternoon downscaling, and a stable nocturnal regime. We demonstrate a striking decoupling between metrics: while λ varies significantly across years, the AR curves collapse into a nearly identical diurnal signal across 2020–2025, filtering out interannual variability. However, this invariance is modulated by cloud-fraction regimes, controlling the cycle's amplitude and the timing of its growth and decay phases. This allows us to establish a nocturnal AR of about 25, with a transient daytime maximum of about 31. These typical values can be slightly sensitive to methodological choices, but in any case, the diurnal tendency remains the same. These results suggest a fundamental compensation between horizontal and vertical scales, with AR acting as a dynamical attractor of stratocumulus organization. Its persistence raises a key question: why is the mesoscale organization of stratocumulus so constrained?