Beyond the Water Surface: Advective and Turbulent Pathways of Canal-Induced Cooling in a Dense Historic City

Urban water bodies are increasingly recognized as nature-based solutions for heat mitigation, yet the relative contributions of turbulent exchange and advective transport to canal-induced cooling remain poorly constrained. This study employs PALM4U large-eddy simulation (LES) with realistic urban geometry to systematically investigate canal cooling mechanisms in Amsterdam, Netherlands, a dense historic city whose semi-circular canal ring provides a unique natural laboratory for examining varying wind-canal orientations within a single urban setting. Model evaluation against eddy covariance flux tower and urban weather station observations showed agreement for sensible heat flux (Index of Agreement = 0.94), net radiation (IoA = 1.00), and near-surface air temperature (IoA = 0.72), confirming LES as a reliable tool for quantifying urban surface-atmosphere energy exchange. Through controlled numerical experiments comparing simulations with and without canals, we decomposed the total cooling effect into its turbulent and advective components. The results reveal that wind-canal orientation is a fundamental determinant of cooling effectiveness. When wind flows perpendicular to canals, mechanically-driven canyon vortices actively redistribute cool air from the water surface into surrounding areas, with horizontal advection accounting for 60–80% of the total cooling effect over adjacent pavement and above the urban canopy. This redistribution extends cooling asymmetrically, propagating vertically up to 2–3 building heights and horizontally into adjacent street canyons lacking water features. When wind flows parallel to canals, the absence of a cross-canyon vortex suppresses lateral and vertical redistribution, confining cooling near the water surface where it is dominated by local sensible heat flux reduction. In this configuration, advection contributes a smaller and spatially inconsistent share of the cooling budget, limiting thermal benefits to the immediate vicinity of the canal. Crucially, wind-canal orientation does not strongly alter the overall intensity of cooling but critically determines its spatial distribution and heterogeneity. Temperature differences of up to 1.5°C arise within individual street canyons depending on position relative to the canal and prevailing wind, with direct implications for the thermal environment at the pedestrian level. These findings challenge simplified representations of water body effects in urban climate models and provide actionable guidance for blue infrastructure planning: canal orientation relative to prevailing summer winds is a key parameter governing the spatial reach and equity of cooling benefits across the urban fabric.