DATE2022-06-22 09:11:06
TITLEProjecting the evolution of the urban climate of coastal Mediterranean cities using a convection-permitting model
AUTHORSYohanna Michau (1) ,Aude Lemonsu (1) ,Philippe Lucas-picher (2) ,Cécile Caillaud (1)
  1. 1) Centre National De Recherches Météorologiques, Université De Toulouse, Météo-france, Cnrs, Toulouse (France) ,2) Département Des Sciences De La Terre Et De L’atmosphère, Université Du Québec à Montréal, Montréal (Canada)
ABSTRACTThe Mediterranean region is projected to be one of the most responsive region to climate change and future climate extreme conditions (Giorgi, 2006 ; Diffenbaugh & Giorgi, 2012). According to Lionello et al. (2012), the climate over this region may become warmer and drier by the end of the century. Changes of extreme meteorological events are also expected (Sanchez et al., 2004): flood-producing events will become more frequent (Gao et al., 2006), and the number of summer days (Tmax > 25°C) and heatwave days will increase (Giannakopoulos et al., 2009). Such changes already caused severe environmental issues, economics damages, and many casualties, especially in urban areas where most activities and populations are concentrated. Understanding and evaluating the impacts of future extreme events on urban areas and population requires addressing the complex coupling between the local urban climate and regional-scale climate changes. The urban climate arises from the modification of various dynamical and physical processes of cities. The complex 3-D shape of the city disturbs air flows and turbulent exchanges, and combined to the radiative and thermal properties of artificial materials, enhances radiation trapping and heat storage (Oke et al., 2017). The modification of radiation and energy exchanges leads to higher near-surface air temperatures in the urban areas than in the surrounding rural areas, especially at night (so-called Urban Heat Island – UHI – phenomenon). The intensity of UHI is firstly dependent on the city characteristics (size, urban density, architectural features, materials, population, etc.), but also on the geographic and topographic contexts, and local atmospheric conditions (sunshine, cloudiness, and wind). Cities are very complex systems that require specific modeling tools. Most of current climate models do not consider explicitly urban areas with a dedicated surface model, and therefore the urban climate evolution by the end of the century is poorly simulated yet. The latest advances in regional climate modeling allow simulations to be performed over longer time periods with finer horizontal resolutions of up to few kilometer (Kendon et al., 2021). The scientific community emphasizes the considerable improvements in using Convection-Permitting Models (CPM), especially for the representation of small-scale phenomena (Shu et al., 2021), as well as extreme weather events such as extreme storms, floodings, and heat waves (Termonia et al., 2018). Also, CPMs offer a very interesting modeling framework for studying UHI effects, and also some urban impacts (Weverberg et al., 2008) through the explicit coupling of the atmospheric climate model with an Urban Canopy Model (UCM). In this study, the French numerical weather prediction model AROME adapted to long-term climate simulations (then called CNRM-AROME) and coupled with the Town Energy Balance (TEB—Masson, 2000) is used at 2.5-km horizontal resolution. Climate simulations were performed on an extended France domain (northwestern Europe) as part of the EUropean Climate Prediction system (EUCP) project (Lucas-Picher et al., 2022) over an historical period (1986-2005) and two future periods (mid-term, 2041-2050 and long-term, 2080-2099) using the RCP8.5 emission scenario. Here, scientific objectives are (1) to evaluate the urban climate evolution for some French Mediterranean cities, including the UHI, and (2) to quantify the evolution of specific meteorological hazards on cities and population. With this aim, we selected indicators related to heatwaves and Heavy Precipitation Event (HPE), considered as some of the most relevant extreme meteorological events on the Mediterranean coast. Heatwaves will be studied using the methodology of Ouzeau et al. (2016). Based on the precipitating system detection and tracking algorithm developed in Caillaud et al. (2021), we will represent the main characteristics of HPE according to their frequency, intensity, area, and duration. This original approach will be applied to 5 medium-size cities located over the Mediterranean coast, such as Marseille, Montpellier or Nîmes.