Megacities – Refining Models to Client Environment

By Sergej Zilitinkevich1, Markku Kulmala2, Igor Esau3, and WMO Secretariat4

Fast-growing urbanisation, environmental deterioration and climate change are making individuals, organisations and businesses more vulnerable to meteorological and environmentalhazards. Modern life requires detailed knowledge about our immediate personal environment – the climate and weather as well as the air, water and soil quality – at work, home or play, may we be indoors or out.

WMO recognizes that there are “increasing demands for weather, climate and water products and services, research and monitoring from an escalating number of individuals, institutions and governments to support the challenges presented by a changing climate” and to inform policy developments and decision-making. National Meteorological and Hydrological Services and private sector value-added weather and climate service providers both need to respond to these growing demands for more detailed information. The Global Framework for Climate Services (GFCS) aims to provide a worldwide mechanism for coordinating actions to enhance the quality, quantity and application of climate services.

However, an interdisciplinary and inter-sectoral platform is needed to promote further developments in atmospheric science, to join private and public-sector meteorological and air-quality observations, and to develop standards for information technology and telecommunication companies – all for better environmental management. Such a platform should focus on extreme and dangerous weather events, the high-resolution features of air pollution and local manifestations of climate change.

Air quality can dramatically differ in neighbouring streets: The urban street canyons in Copenhagen demonstrate heterogeneity of air pollution (Courtesy of R. Nuterman et al., EU project MEGAPOLI, 2009-2011)

Refining the models

From the scientific standpoint, it is the lower turbulent atmospheric planetary boundary layer (PBL) directly affected by interaction with underlying land and water surfaces thatforms our immediate environment. Local features of PBL are controlled to a large extent by the properties of the underlying soil, vegetation, buildings and surface waters – giving the PBL its own physical, chemical and electromagnetic weather and climate. Thus, urban, coastal, mountainous, forested and other complex-terrain PBLs are strongly heterogeneous evenat scales of a few hundred metres or less. For example, in cities, air quality can dramatically differ in neighbouring streets.

Shallow boundary layer in Bergen visualized by water haze, winter 2012 / © T. Wolf

The horizontal grid resolution currently achievable in operational weather prediction and climate services is limited to a few kilometres; therefore, interactions between the PBL and the Earth’s surface are characterized by strongly smoothed, grid-averaged vertical turbulent fluxes of energy, momentum and matter as shown in the Paris graphic (below). Furthermore, the resolution of the vertical structure of the atmosphere is still insufficiently fine to detect the PBL upper boundary, especially in the case of shallow, stably stratified PBL as shown in the picture of Bergen (above).

Paris – On 1 x 1 km scale (centre), the NOx emission inventory show main city transport arteries; the boxes around it show regional emissions on a scale of 7 x 7 km (Courtesy of H.A.C. Denier van der Gon, MEGAPOLI, 2009-2011)



This is a severe drawback, as the finer features of our immediate environment caused by local emissions and/or heterogeneities of the landscape remain unresolved. Urban air pollution, resulting from the interplay between emissions, long-range transport, local mixing and deposition, is strongly variable and threatening to life: atmospheric particulate matteralone has been estimated to kill more than three million people per year around the world. Research shows that strong air pollution in megacities is connected to anthropogenic impacts and climate change.5

Over the course of time, spatial resolution in operational atmospheric models has been gradually refined thanks to advancement in global observational network and computational technology. But in recent years, this steady progress has faced an obstacle: traditional representation of PBLs and turbulence in models has been found to be inconsistent with resolutions finer than the PBL depth. The critical point is that the underlying conventional theory of turbulence overlooks essential self-organized motions typical of well-mixed convective PBLs.

This is why, instead of the anticipated improvement, sub-kilometre resolutions quite often make model performance worse by triggering artificial large-scale motions resembling real self-organized motions typical of well-mixed PBLs but much deeper and wider. This artefact (inherent to the so-called “grey zone” of downscaling model resolution) is most probably caused by poor representation of supercritically stratified turbulence in the free atmosphere and confusion between the free atmosphere and PBL. The point is that supercritically stratified turbulence differs from the usual PBL turbulence in that it has a dramatically reduced heat transfer compared to momentum transfer.6 This newly discovered phenomenon is precisely the reason why diurnal variations of temperature are limited to PBL, why pollutants released from ground sources can be held for as long as several days within PBL, and why self-organized structures do not extend beyond PBL. The representation of atmospheric turbulence, aerosol, chemical processes and basic features of PBLs, along with a refinement of horizontal resolution, urgently needs to be updated in a new generation of weather and climate models.

But this is not the only issue. The major challenge is to speed up development of very high resolution monitoring and forecasting of physical and chemical weather and climate, beyond the basic, top-down and, inevitably, slow-progress approach. Refinements in the nature and representation of our personal environment could be gained, in particular, through the integration of high-resolution topical observations with turbulence-resolving simulations such as those now run for municipalities, for example, in Bergen, Norway.

A bottom-up approach

WMO7 has identified the development of “integrated urban weather, environment and climate services” as one of its priorities. A concept based on the integration of information flows from weather services (with inevitably limited resolution dictated by the grid of regular observations) and private personal observations with arequired resolution for a specific place could permit monitoring and forecasting one’s immediate urban environment. In this way, the usual low-resolution forecasts can be corrected for a specific location, accounting for the totality of data from personal observations provided by relevant users.

A practical application for this bottom-up approach is in planning new urban development such as the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS); the Shanghai EXPO-2010 Multi-hazard Early Warning System; or Big Moscow. Trustworthy planning should take into account all essential megacity-climate feedbacks from the megacity scale to the personal-environment scale.

There are strong grounds for believing that in the near future meteorological monitoring by centralized weather services will be ever more supplemented by private bottom-up monitoring by individuals, weather-dependent businesses (such as transport, precise gardening or agriculture, the energy sector, etc.), volunteer organizations (e.g. schools, hospitals or apartment-dwelling communities) and environmental agencies – especially in megacities.

This prospective scenario, combined with modern information, modelling and observation technologies, is challenging, but it will bring opportunities for:

  • improving environmental science and education;
  • industrial development and marketing of instruments for private meteorological and air and water quality observations;
  • personal environmental services that take into account both physical and chemical weather and climate and address human comfort and health problems; and
  • optimal management of the urban environment.

Heavily polluted megacities are prepared to establish massive private environmental monitoring; hence there is rapid growth in the market for instruments for measuring a wide spectrum of parameters. One example, the life-threatening risks associated with air pollution have increased sales of instruments that measure outdoor and indoor air. Bottom-up monitoring is thus already on track, but harmonization is required between atmospheric and other environmental sciences in order to advance standards for the makers of observational instruments and modernize the practices of environmental managers. National Meteorological and Hydrological Services and WMO are taking the lead in coordinating these processes. The time is ripe to start a dialog between the environmental-science community and the companies producing environmental-observation instruments and services with prospective goal being to make observations by private customers consistent with and complementary to the regular monitoring by weather services.

The recently launched Pan-Eurasian Experiment (PEEX), involving the European Union, Russia and China12, may form the basis of the interdisciplinary and inter-sectoral platform that is required to move forward in this area.


1 Division of Atmospheric Sciences, University of Helsinki; Finnish Meteorological Institute; University of Nizhny Novgorod;  Moscow State University; Institute of Geography of Russian Academy of Sciences
2 Division of Atmospheric Sciences, University of Helsinki
3 Nansen Environmental and Remote Sensing Centre / Bjerknes Centre for Climate Research, Bergen, Norway
4 Alexander Baklanov, WMO, Research Department, Atmospheric Research & Environment Branch
5 Arneth A., Unger N., Kulmala M., Andreae M.O., 2009: Clean the air, heat the planet? Science, 326, 672-673. Kulmala, M., et al., 2011: General overview: European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) – integrating aerosol research from nano to global scales. Atmos. Chem. Phys., 11, 13061–13143.
6 Zilitinkevich S.S., Hunt J.C.R., Grachev A.A., Esau I.N., Lalas D.P., Akylas E., Tombrou M., Fairall C.W., Fernando H.J.S., Baklanov A., Joffre, S.M., 2006: The influence of large convective eddies on the surface layer turbulence. Quart. J. Roy. Met. Soc. 132, 1423-1456. Zilitinkevich S.S., Elperin T., Kleeorin N., Rogachevskii I., Esau I.N., 2013: A hierarchy of energy- and flux-budget (EFB) turbulence closure models for stably stratified geophysical flows. Boundary-Layer Meteorol. 146, 341-373.
7 Grimmond S., Xu T., Baklanov A., 2014: Towards integrated urban weather, environment and climate services. WMO Bulletin 63 (1) 10-14.
12 Lappalainen H. et al., 2014: Pan-Eurasian Experiment (PEEX) – A research initiative meeting the grand challenges of the changing environment of the northern Pan-Eurasian Arctic-boreal areas, 2014: Geography, Environment and Sustainability 7 (2), 13-48. Geography, Environment & Sustainability, 7 (2), 13-48.

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