Process-based upgrades to the WRF multi-layer green-roof scheme (WRF-MLGR v2.0) and evaluation against field observations

Abstract. Green roofs can moderate urban heat by increasing latent heat flux and reducing sensible heat flux. However, capturing these effects in models depends on accurate representation of key green-roof processes, including substrate heat and moisture transport, soil-vegetation-atmosphere energy and moisture exchanges, interactions with the underlying roof, and drainage. Here we introduce targeted, process-based updates to the green-roof scheme (hereafter, MLGR) within the multi-layer urban canopy model (BEP-BEM) in the WRF mesoscale model to address key limitations in the original formulation. The updates include a non-linear dependence of soil thermal conductivity on moisture, vegetation-modulated surface thermal conductivity, explicit soil-surface evaporation, multi-layer root water uptake for transpiration, and canopy interception with evaporation and dew formation. We evaluate the original and modified MLGR schemes using hourly observations from an extensive sedum roof in London, Canada, for ‘summer’ (1 July–31 August 2014) and ‘fall’ (1 September–31 October 2014) periods. We also analyze 11–18 October 2025, when green roof modules were placed directly on the roof deck – which corresponds to the model’s lower boundary assumption. Following implementation of the process-based improvements, model–measurement agreement for the conductive heat flux is markedly improved: RMSE is reduced from 105.9 to 24.0 W m⁻² in summer and from 94.2 to 24.0 W m⁻² in fall and the model produces more realistic overall green roof energy partitioning. The modified model better captures post-rain increases in latent heat flux (Q_E) and improves the timing and magnitude of daytime turbulent latent and sensible heat flux peaks (Q_E and Q_H). Drainage is reduced relative to the original scheme; however, it remains slightly underestimated during summer and slightly overestimated during fall, and biases persist in surface temperature (warm during the day and cool at night) and in the magnitude and variability of Q_E. Overall, the revised MLGR physics improves surface-flux realism, and future development should focus on developing a more realistic vegetation canopy submodule.