ИСТИНА

Urbanization leads to distinct meteorological anomalies, and one the best-known is the urban heat island (UHI) effect. For megacities, these anomalies become mesoscale phenomena (scale ≥ 10km) that extends to atmospheric boundary layer, are amplified by the tropospheric feedbacks and have substantial implications on human well-being. However, the number of studies devoted to urban-caused climate phenomena that takes the lower troposphere into consideration is much smaller than those focusing only on the near-surface climate anomalies, firstly because of the difficulties of making continuous observations above the roof level. The most part of the such studies is based only on episodic observations (case-studies), which have the insufficient timespan for making conclusions on urban-climate statistics. Moreover, various features of the urban-caused anomalies in the lower troposphere are studied by different methods and for cities located in different climatic zones and terrain types, which makes the understanding of the overall picture especially difficult. This leads to high uncertainties in the estimates of qualitative feature of the urban-induced mesoclimatic anomalies, e.g. the vertical extent of the UHI or magnitude of the urban effect on precipitation and cloudiness. Recent developments in field of numerical weather modelling have opened new opportunities to study these urban-induced mesoscale features in a systematic way towards the climatological timescale. In this study, for the first time, a three-dimensional statistical description the megacity-induced meteorological anomalies extending towards the lower troposphere is acquired on a quasi-climatological timescale (for multiple summer seasons) based on high-resolution (1 km) simulations for Moscow megacity with the COSMO-CLM model (Böhm et al. 2006) with and without its urban canopy model TERRA_URB (Wouters et al., 2016). The details of such simulations are described in (Varentsov et al., 2017). Our results confirm the features from previous observational and modelling studies, including the “boundary layer” UHI itself, the cold anomaly above established at night by the so-called cross-over effect, the urban dry/moist islands (UDI/UMI) and the urban breeze mesoscale circulation. Particularly, the UHI shows a strong diurnal variation in terms of intensity/vertical extent between daytime (≈0.5K/≈1.5 km), evening (≈1.5K/≈1.5 km) and nighttime (>3K/≈150 m), see Figure 1 (a,b). Such estimates agree with the measurements of microwave temperature profilers existing in Moscow region and satellite-based remote sensing data (Kislov et al., 2017). Moreover, we have discovered a systematic veering of the UHI spatial pattern established by the Coriolis effect, and an enhanced stable stratification of the rural surroundings established by the urban heat plumes further downwind (Figure 1c). It is important to highlight that all these features were found and analyzed not for certain cases with favorable conditions, but in terms of the means over summer seasons or over selections of the cases grouped according some simple criteria (wind speed or direction). Finally, a substantial increase was summer precipitation (up to +25%) and daytime cloud cover is found over the city center and its leeward side. Our results highlight that these urban-caused mesoclimatic anomalies need to be taken into account in weather and climate services, including weather and air quality forecasts, biometeorological applications and the design of future megacities. Acknowledgements: The bulk of the research (supercomputer modeling, data management and analysis) performed by M.I. Varentsov and P.I. Konstantinov was funded by Russian Science Foundation (project №17-77-20070 "An initial assessment and projection of the bioclimatic comfort in Russian cities in XXI century against the context of climate change"). Analysis of experimental measurements with microwave temperature profilers was funded by Russian Foundation for Basic Research, project №16-05-00704. References: Böhm, U. et al., 2006. CLM—the climate version of LM: brief description and long-term applications. COSMO newsletter, 6, pp.225–235. Kislov, A. V. et al., 2017. “Heat island” of the Moscow agglomeration and the urban-induced amplification of global warming [in Russian]. Moscow University Vestnik. Series 5. Geography, 4, pp.12–19. Varentsov, M.I., Konstantinov, P.I. & Samsonov, T.E., 2017. Mesoscale modelling of the summer climate response of Moscow metropolitan area to urban expansion. IOP Conference Series: Earth and Environmental Science, 96, p.12009. Wouters, H. et al., 2016. Efficient urban canopy parametrization for atmospheric modelling: description and application with the COSMO-CLM model for a Belgian Summer. Geoscientific Model Development, 9, pp.3027–3054