Validating ENVI-met for Relative Humidity (RH) in high-density temporary encroachment spaces in the streets of tropical Indian megacities

Informal settlements in developing countries have distinct socio-ecological, ethnocultural, and economic patterns. People spend a significant amount of time in these outdoor spaces and modify them with lightweight shade materials (encroachments) according to their needs. We seek to investigate how accurately the 3 Dimensional Computational Fluid Dynamics (CFD) software ENVI-met models Relative Humidity (RH) in the streets of such heterogeneous urban forms in tropical Mumbai and Kolkata in India. Three neighborhoods with similar forms and functions were chosen in each city after (Banerjee et al., 2021), (Banerjee et al., 2020) to perform 12 microclimate simulations (12 hours) in summer and winter. Partial encroachments were modeled using the single z-wall feature of ENVI-met. This is the first study to validate ENVI-met seasonal RH simulations in complex neighborhoods geometries, i.e. an elevated vehicular corridor, a large riverbank, and temporary encroachments.

The research concludes that few studies have validated RH so far. Our validation study reports ENVI-met thoroughly overestimates RH in most cases. In Mumbai, Fashion Street has significant greenery and a Gymkhana nearby, attributed to high RH during the morning hours, especially in hot-humid summers. Naturally, RH decreases with an increase in Air Temperature (T_a). For Dadar, in summer, the deep canyon has the highest RH. This pattern is opposite to the observed summer T_a and Mean Radiant Temperature (T_mrt) pattern in Dadar in both seasons. For Mallickghat, RH decreases with increasing T_a. For both seasons, the deep canyon shows the highest RH profile due to the lack of wind flow in the canyon caused by the blockage of river wind by built structures. In Kumartuli, the deep canyon has the highest RH for both seasons, due to the lack of adequate wind flow from encroachment imparted roughness and trapped moisture in the canyon. This agrees with existing studies that show vegetation or other elements of roughness can block the wind flow or ventilation within a canyon. This deviation may be attributed to boundary conditions assumptions such as a neutrally stratified atmosphere, which is not always valid in cities with strong radiative input such as Kolkata and Mumbai. For Mallickghat, our result shows ENVI-met can predict RH well for a shallow canyon (R sq. = 0.77), although for the deep canyon, the RH prediction ability of ENVI-met is lower (R sq. = 0.59). Similar RH patterns between deep and shallow canyons in both neighborhoods may be due to anthropogenic heat-related discrepancies in deep canyons that can completely change the pattern of ambient RH. Overall, the study concludes that ENVI-met predicts RH well as the correlation between the measured data and simulation demonstrates consistency.

Banerjee, S., Middel, A., & Chattopadhyay, S. (2020). Outdoor thermal comfort in various microentrepreneurial settings in hot humid tropical Kolkata : Human biometeorological assessment of objective and subjective parameters. Science of the Total Environment, 721, 137741. https://doi.org/10.1016/j.scitotenv.2020.137741

Banerjee, S., Middel, A., & Chattopadhyay, S. (2021). A regression-based three-phase approach to assess outdoor thermal comfort in informal micro-entrepreneurial settings in tropical Mumbai. International Journal of Biometeorology. https://doi.org/https://doi.org/10.1007/s00484-021-02136-7