Urban water bodies offer nature-based heat mitigation solutions, yet the mechanistic contributions of turbulent exchange versus advective transport in redistributing canal-induced cooling remain unquantified. We employed PALM4U large-eddy simulation with realistic urban geometry to investigate canal cooling mechanisms in Amsterdam, Netherlands, a dense historic center whose semi-circular canal ring enables systematic study of varying wind-canal orientations within a single setting. Model evaluation against eddy covariance and urban weather station observations demonstrated strong performance for sensible heat flux, net radiation, and near-surface air temperature. Through controlled experiments, we decomposed cooling into turbulent and advective components, revealing that effectiveness depends fundamentally on wind-canal orientation. When wind flows perpendicular to canals, mechanically-driven canyon vortices redistribute cool air from the water surface to surrounding areas, with advection accounting for 60 to 80% of the total cooling effect over adjacent pavement and above the canopy, extending cooling asymmetrically to 2 to 3 building heights vertically and into adjacent canyons. When wind flows parallel to canals, the absence of a cross-canyon vortex limits redistribution, leaving cooling concentrated near the water surface and driven predominantly by local sensible heat flux reduction. Wind-canal orientation does not markedly alter overall cooling intensity but governs how cooling is spatially redistributed within the canopy, producing pronounced thermal heterogeneity, with temperature differences up to 1.5 °C within individual street canyons. While these magnitudes reflect a single summer day and Amsterdam's specific morphology, the mechanism points to a transferable principle: orienting blue infrastructure relative to prevailing summer winds can steer cooling toward pedestrian spaces.

Temporally compound heatwaves (CHWs), two consecutive heatwaves (HWs) with an intermittent cool break between them, are projected to occur more frequently under a warming globe. However, their spatiotemporal characteristics and interaction with urban heat island (UHI) are unexplored at the continental scale. Using observational data from over 2000 ground-based stations over China, we find that CHWs constitute an increasing portion of HW hazard from 1961 to 2021. The increasing trend is especially evident when using the daily minimum temperature to define hot days, suggesting an aggravated thermal environment at night. Urban-rural contrast of CHW trends illustrates that urbanization contributes substantially to the increased frequency of CHWs in cities, especially in southern China. Results show that mean UHI intensity (UHII) tends to weaken under HW and CHW conditions, which correlates with increased pressure and reduced precipitation. During CHW events, UHII reduces during cool break due to enhanced evaporative cooling in urban areas under precipitation. The interaction between UHI and HW is subject to change with background climate, which is positive for dry regions and negative for wet regions. This study provides insights into CHW evolution over mainland China and demonstrates the need for heat mitigation strategies under climate change.

As global warming and urban overheating continue to intensify, accurate urban microclimate modeling has become critical for sustainable urban planning. While the Single-layer urban canopy model (SLUCM, a reduced-order surface energy balance model) and ENVI-met (a computational fluid dynamic model) are the two most widely used models, a direct comparison of their performance is missing. This study aims to examine potential biases between SLUCM and ENVI-met using Hong Kong as a case study and provide guidance on model selection for different purposes. Evaluated against pedestrian-level observational data, the results show that both SLUCM and ENVI-met simulate air temperatures reasonably well, with mean absolute errors less than 1.5 °C. However, SLUCM outperforms ENVI-met in simulating relative humidity, which is partially caused by the insufficient representation of sea breeze by both models. To extrapolate SLUCM output to different heights, Monin-Obukhov similarity theory is applied. This leads to large gradient of temperature and humidity in the vertical direction, while ENVI-met simulations yield homogeneous profiles due to explicit modeling of the turbulent mixing. Findings suggest that ENVI-met suits heterogeneous neighborhoods where turbulent mixing is largely regulated by urban morphology, but its accuracy on humidity simulation needs special attention. SLUCM performs reasonably well in simulating air temperature, but it tends to yield large bias in the vertical direction. Based on the findings, we recommend development of enhanced turbulence parameterization for SLUCM, and coupling both models with mesoscale models to better account for the effect of land/sea breeze on urban microclimate in coastal cities.