Abstract
The increase in built surfaces constitutes the main reason for the formation of the Urban Heat Island (UHI), that is a metropolitan area significantly warmer than its surrounding rural areas. The urban heat islands and other urban-induced climate feedbacks may amplify heat stress and urban flooding under climate change and therefore to predict them correctly has become essential. Currently in the COSMO model, cities are represented by natural land surfaces with an increased surface roughness length and a reduced vegetation cover, but this approach is unable to correctly reproduce the UHI effect. By increasing the model resolution, a representation of the main physical processes that characterize the urban local meteorology should be addressed, in order to better forecast temperature, moisture and precipitation in urban environments. Within the COSMO Consortium a bulk parameterization scheme (TERRA_URB or TU) has been developed. It parametrizes the effects of buildings, streets and other man-made impervious surfaces on energy, moist and momentum exchanges between the surface and atmosphere, and additionally accounts for the anthropogenic heat flux as a heat source from the surface to the atmosphere. TU implements an impervious water-storage parameterization, and the Semi-empirical Urban canopy parametrization (SURY) that translates 3D urban canopy into bulk parameters. This paper presents evaluation results of the TU scheme in high-resolution simulations with a recent COSMO model version for selected European cities, namely Turin, Naples and Moscow. The key conclusion of the work is that the TU scheme in the COSMO model reasonably reproduces UHI effect and improves air temperature forecasts for all the investigated urban areas, despite each city has very different morphological characteristics. Our results highlight potential benefits of a new turbulence scheme and the representation of skin-layer temperature (for vegetation) in the model performance. Our model framework provides perspectives for enhancing urban climate modelling, although further investigations in improving model parametrizations, calibration and the use of more realistic urban canopy parameters are needed.
Highlights
Urban areas have a significant impact on atmospheric flow and meteorological processes and this field of study is growing within the scientific community
It should be noted that model sensitivity to the change of physical schemes is smaller than for Turin and Naples, which may be associated with weather conditions or with flat terrain in the Moscow region, as well as with a larger number of rural weather stations used in analysis
This paper presents evaluation results of the TU scheme in high-resolution simulations with a recent COSMO model version for selected European cities, namely Turin, Naples and Moscow
Summary
Urban areas have a significant impact on atmospheric flow and meteorological processes and this field of study is growing within the scientific community. The impact of the UHI phenomenon is becoming more relevant and, according to IPCC’s climate projections [3], this situation will aggravate in the 80 years since a continued increase in number, intensity and duration of heat waves during the 21st century is expected [4]. Many studies have already documented that UHIs tend to intensify during the heat waves (e.g., [5,6]) leading to additional deterioration of the human thermal comfort [7,8], amplification of heat-stress hazards, and even increase of heat-related mortality in the cities [9,10]. Higher rates of climate warming and heat stress increase in the 21st century are expected for urban areas in comparison to rural background, according to regional [11,12] or global [13] urban climate projections. The correct representation of UHI in numerical models is a crucial aspect for Numerical Weather Prediction (NWP) applications and regional climate simulations
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