A dense rain-gauge observation framework for advection-consistency diagnosis of urban convective rainfall

Abstract. Convective rainfall identification is commonly based on intensity thresholds, radar reflectivity patterns, or supervised classification methods. However, these approaches often lack an explicit connection to the physical constraints governing rainfall-field evolution. This study proposes a physics-informed definition of convective rainfall based on the continuity equation, interpreting convective rainfall as a spatiotemporal deviation from rainfall evolution consistent with horizontal advection under the rainfall continuity equation.

The analysis utilizes data from a high-density rain gauge network in Bangkok. Rainfall fields are reconstructed on a kilometer-scale grid, and the motion field is estimated using the Horn-Schunck optical flow method under no-flux boundary conditions. The rainfall field is then advected forward in time without including local source or sink terms. The residual between the observed field and the purely advected field is interpreted as representing non-advective processes. The framework is designed for dense urban rain-gauge networks, where direct surface rainfall measurements are available and quantitatively calibrated radar rainfall products are not available for the present analysis.

Residual-feature analyses indicate that the detected events exhibit properties consistent with convective rainfall, including spatial localization, rapid development, and a clear association with the diurnal cycle. The separation between convective and non-convective events in the residual feature space provides an empirical evaluation of the diagnostic behavior of the framework.

The results suggest that advection consistency provides an interpretable physics-based foundation for defining and detecting convective rainfall using rain gauge observations alone.