Hydrological Determinants of Temperature Extremes in Cities

Around 70% of the world population will reside in metropolitan regions by 2050, resulting in a doubling of the world’s built area compared to 2015. Urbanization will thus lead to extensive direct and indirect impacts on water use, water quality, and regional hydrometeorology. The impervious cover of cities that limits evaporation is known to be one of the primary causes of the Urban Heat Island effect, but the influence of hydrological processes on the microclimatology and hydrometeorology of cities, especially during extreme events, is far more pervasive and complex. In this talk we present a suite of recent results that elucidate how hydrological determinants modulate the urban climate, and how these findings can guide us in designing future metropoles. Our finding indicate that surface evaporative cooling intensifies during heat waves, leaving cities with limited green cover at a higher risk than their surrounding. On the other hand, during extreme cold, the reduced snow cover in urban areas and their thermal inertia produce a shield that protects urban dwellers. The importance of these influences, along with the well-documented impacts of urban surfaces on extreme precipitation, underline the need for more realistic representation of urban terrain in geophysical models. The talk therefore ends with an overview of the open challenges for urban surface representations in atmospheric and hydrological models.