Seasonal energy budget and thermal stratification stability in a confined shallow thermal system: A high-resolution case study

Air–water heat-flux partitioning provides an important surface forcing associated with thermal storage and stratification in confined shallow systems. Such geometrically confined basins reduce advective complexity because lateral inflow/outflow is negligible, while remaining open to atmospheric and boundary heat exchange. This case study employs year-round (2024) high-frequency meteorological and vertical temperature-profile measurements from a small enclosed shallow basin in Wuhan, China, to quantify seasonal surface heat-flux components and thermal stratification stability. Net shortwave radiation was the dominant heat source, contributing over 68% of the total absolute heat flux during spring and summer. The principal cooling pathway shifted seasonally from latent heat flux in summer to net longwave radiation in winter. The thermal structure evolved through four regimes: near-isothermal mixing in winter, persistent multi-day stratification in spring, diurnally oscillating stratification in summer, and a progressive autumn transition toward mixing with intermittent daytime restratification. Spring stratification persisted over consecutive days rather than resetting nightly, with annual peaks in the monitored-column temperature difference, Schmidt stability, and buoyancy frequency occurring in late spring. In summer, strong diurnal heating and cooling were associated with pronounced oscillations in mixed-layer depth and stability indices. Thus, the observed seasonal transitions were associated with evolving calculated surface heat fluxes, water-column heat-storage changes, and weak wind forcing, rather than with a fixed wind-mixing threshold alone. This study provides a high-resolution observational baseline linking surface energy exchange to stratification stability. These insights may provide process-level insight for comparable engineered shallow basins, such as solar ponds and cooling reservoirs.