Stratocumulus (StCu)-topped boundary layers exhibit complex mesoscale cellular convection that remains a primary source of uncertainty in climate radiative forcing and a persistent challenge for climate 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.</p> <p>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 = Tb<sub>12.3 <em>µ</em>m</sub> – Tb<sub>10.3 <em>µ</em>m</sub>), we reveal a robust universal four-phase diurnal cycle: morning growth, early-afternoon plateau, rapid late-afternoon downscaling, and a stable nocturnal regime.</p> <p>We demonstrate a striking decoupling between metrics: while λ varies significantly across years, the AR curves collapse into a nearly identical diurnal signal across the 2020–2025 period, effectively filtering out interannual variability. However, this invariance is modulated by cloud-fraction regimes, which control the amplitude of the cycle and the timing of its growth and decay phases for AR.</p> <p>This allows us to establish a nocturnal AR of 25 ± 2, with a transient daytime maximum of 31 ± 1.5. 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 does mesoscale organization maintain this characteristic scale ratio throughout the diurnal cycle?