The Community Land Model - Urban (CLMU) is a module within the land component of the Community Earth System Model (CESM; Danabasoglu et al. 2020, Lawrence et al. 2019) that parametrizes urban surface and hydrological processes (Oleson et al. 2008a, Oleson and Feddema 2020). The CLMU model is built based on the urban canyon concept and includes roof and canyon components, the latter of which further include walls of buildings (sunlit and shadow), the impervious and pervious ground. This model has been validated using observational data and has been widely used to study urban climate within the context of global environment change (Oleson et al. 2010, 2011; Oleson 2012).

Recently, we add new capabilities to CLMU to simulate cool (white) and green roofs, which are common heat mitigation strategies (Wang et al. 2020). For green roofs, we further add an irrigation module (Wang et al. 2021) which allows us to explore the tradeoffs between heat mitigation and water conservation for cities in dry regions. In short, we separate the roof component in the default CLMU into three types: the regular roof, the cool roof, and the green roof. The regular roof is treated in the same way as in the default CLMU. The main difference between the regular roof and the cool roof is the roof albedo: the regular roof albedo comes from a global dataset (Jackson et al., 2010), which also provides the urban extent and other urban properties needed by CLMU like morphological and thermal properties, while the cool roof albedo is a global constant that can be set and changed by the user. In contrast, the green roof is a more complex system that includes three parts: vegetation, soil, and roof deck. The treatment of the roof deck is identical to that of the regular roof, and the treatment of soil follows the treatment of pervious ground in the urban canyon. The momentum, energy, as well as water fluxes between the roof and the air are weighted averages of those from the regular roof, the cool roof, and the green roof.

The improved CLMU model simulates the temperatures and heat fluxes (e.g., the sensible heat flux, latent heat flux, and ground heat flux) of different surfaces (e.g., the roofs and the walls), and the near-surface air temperature. These outputs are used to investigate the impact of cool and green roofs on the local urban climate. The improved CLMU model will also be employed to investigate the impacts of heat extremes on building energy consumption under the compound influences of future climate/heat waves, population dynamics, and urbanization.