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As the hydrological cycle is altered by global warming, society is increasingly being impacted. While some regions have experienced a drying trend, the global level of precipitation is generally increasing. Over the last decades, an increase in extreme precipitation has also been observed. Highly populated regions are vulnerable to short-duration, extreme precipitation, which can overwhelm existing infrastructure and cause urban flooding (Hammond et al., 2015). Changes in the rate of such events can lead to large costs to society. This is, for instance, occurring in northern part of Europe, such as in Copenhagen in 2011 with around 90 000 reported injuries with a cost of around 0.5 billion euros for the insurance companies.

  • However, IPCC AR5 states that there is a lack of adequate understanding of what controls the occurrence of such extreme precipitation events (Boucher et al., 2013).
  • Several factors that influence precipitation are special for urban and highly populated regions. Hourly extreme precipitation may therefore increase at different rates in these regions relative to other regions in the future. In SUPER, sub-daily extreme precipitation in highly populated regions will be investigated, with emphasis on impacts from changes in atmospheric aerosols and urban heat island effect.
  • A warmer atmosphere has capacity for higher water vapour content.
  • Extreme percipitation events are expected to increase considerably faster than global mean precipitation
  • For reliable, sub-daily precipitation information, high spatial and temporal resolution is required.
  • Atmospheric aerosols are a vital determinant for the rate and formation of percipitation processes through acting as cloud condensation nuclei and ice nuclei, but the overall effect of aerosols is uncertain.
  • Aerosols differ in impact on percipitation from greenhouse gases.
  • It is unclear whether aerosols dampen short-duration rainfall as it does annual-mean rainfall.
  • How different climate change drivers impact extreme precipitation is currently highly uncertain.
  • The aerosol abundances is changing significantly over time and in different parts of the world.
  • The urban heat island effect may increase surface temperature by several degrees between urban and rural areas

Main focus areas for SUPER

 

Objectives:

The goal of SUPER is to combine new knowledge of precipitation such as anthropgenic aerosols and the urban heat island effect, with spatial and temporal knowledge from observations and modelling, to study the historical and future evolution of sub-daily precipitation in urban areas.
Sub-objectives include:

  • Quantifying the increase in sub-daily extreme precipitation relative to daily extremes
  • Identifying variations in trends in sub-daily extreme precipitation in observations
  • Exploring the link between anthropogenic aerosols and hourly extreme precipitation
  • Identifying the factors that control changes in short-duration extreme precipitation in large cities, with emphasis on anthropogenic aerosols and urban heat island

 

Methods:

SUPER uses a combination of global and regional modelling to understand past observations of
extreme precipitation, and to simulate future changes over this century. The main models and
datasets to be used are:

  • CMIP5: The Coupled Model Intercomparison Project Phase 5 contains historical and future projections from more than 20 different global climate models run with a range of future scenarios (Taylor et al. 2011).
  • CORDEX: The Coordinated Regional Climate Downscaling Experiment is an initiative where several regional climate models are run with CMIP5 input (Giorgi et al. 2009).
  • NCAR CESM1: The National Center for Atmospheric Research's Community Earth System Model is a state-of-the-art global climate model (Gent et al. 2011). In SUPER it will be run with both CAM4 (Neale et al. 2010a) and CAM5 (Neale et al. 2010b) atmospheriv component
  • WRF: The Weather Research and Forecasting (WRF) model (Skamarock and Klemp 2008) is a flexible modelling tool increasingly used for regional climate studies. WRF will be used to down-scale global data from in-house CMIP5 and CESM simulations. WRF will be used to study impacts on precipitation to higher levels of detail, accounting for urban heat island effects and combined with the chemistry module WRF-CHEM to includ effects of aerosols.
  • UK Unified Model: This climate model uses a consistent modelling framework is on both global and regional scale, and can hence be used to examine the interaction of small- and large-scale processes (Hewitt et al. 2011). In SUPER we plan to examine a cascade of these models from 10-day integration with a 4 or 12 km resolution limited area model through to 100 year integrations of an atmosphere-ocean global climate model.
  • OsloCTM3: The global aerosol and chemistry transport model OsloCTM3 (Myhre et al. 2009, Søvde et al. 2012) will be used to generate updated aerosol historical and future time evolution fields.

Further, SUPER is tightly linked to ongoing international
collaborations, addressing the need for multi-model experiments to explore certainties and
uncertainties as well as understanding of robust physical processes. The array of SUPER models are
well established tools representing the state of the art with regard to extreme precipitation. A
combination of global and regional models is needed to understand the large scale changes
occurring as well as to resolve the very small scale. The results from
regional and global models will be compared for selected regions, and differences will be sought
described from differences in physical processes in the models. This is found to be important for reliable locale scale effects (Prein et al., 2015). To explore the variability and uncertainties in the future climate change among
the global models, the regional modelling will be using boundary conditions from several of the
global models.

Advancement in the availability of super-computing resources allows regional
modelling with high resolution. An innovative aspect of SUPER, which was previously
challenging and hence poorly explored, is the combination of large samples of occurrence of
extreme events with multi-model boundary information for the regional modelling.


Observations of daily precipitation extremes are relatively easily available, and will be a crucial tool
in this study. Short-duration such as hourly data of precipitation is less publicly available and
explored, but hourly data described below will be used and quality checked against the more widely
used daily data. Emphasis will be put on collecting data both in highly populated regions, including
megacities, and in more rural areas to contrast the evolution in extreme precipitation.
 

Main outcomes of the project

Extreme precipitation events are expected to increase substantially with global warming. The key contribution from this proposal is to investigate whether highly populated regions will experience larger changes in extreme precipitation than other regions. Whether factors associated with high population density has influenced local (extreme) precipitation, and may have compensated or dampened historical changes, is poorly investigated. In the past aerosols and urban heat islands may have had a balancing effect on precipitation, but recently they may have started to act in the same direction – a trend which then is likely to continue in the future. If this is the case, sensitive highly populated region may face even higher extreme precipitation event rates in the future relative to other regions. Infrastructure changes in highly populated regions are expensive, and early plans for adaptation are invaluable. Overall, SUPER will give insights into which mid-latitude regions can expect large changes in extreme precipitation.


It is established that atmospheric aerosols generally reduce precipitation, and that this is very likely also the case for extreme precipitation. However, it is unclear whether this has a direct local effect. A novel aspect of SUPER is the association of locally generated pollution to regional variation in extreme precipitation. For large parts of the European population, vulnerable to substantial damages from single extreme precipitation events, this is a highly important issue.

A further novel part of SUPER is the relation of extreme precipitation to new aspects influencing precipitation, such as aerosols and the urban heat island effect. Lack of sufficiently high resolution modelling and understanding of atmospheric aerosols has made earlier studies on this topic challenging. SUPER aims to further improve the scientific understanding of the aerosol-cloud interaction, a research topic which is likely to remain at the forefront of the field for several decades to come. Although uncertainties in some of the physical processes will remain, the multi-model approach can be expected to yield solid and innovative results if signals are sufficiently clear. The risks associated with the proposal are relatively small, as the methodology is well known to the team of SUPER. The main challenges lie in supercomputer resources and availability to observations of hourly precipitation data. However, at current these are quality checked and described below.
The SUPER research findings will be published in scientific journals aimed for the scientific community and the IPCC. Publications will be mainly in open access journals, and our datasets and model results will be made publicly available. In addition results from SUPER will frequently be made available to our business partner IF, the leading Nordic insurance companies.
 

References

Gent, P. R., Danabasoglu, G., Donner, L. J., Holland, M. M., Hunke, E. C. et al., J. Climate, 24(19), 4973-4991, 2011.
Giorgi, F., Jones, C. and Asrar, G., WMO Bulletin, 58(3), 175-183, 2009.
Hammond, M. J., Chen, A. S., Djordjević, S., Butler, D. and Mark, O., Urban Water Journal, 12(1), 14-29, 2015.
Haywood, J. M., Jones, A., Bellouin, N. and Stephenson, D., Nature Clim. Change, 3(7), 660-665, 2013.
Hewitt, H. T., Copsey, D., Culverwell, I. D., Harris, C. M., Hill, R. S. R. et al., Geosci. Model Dev., 4(2), 223-253,
Myhre, G., Berglen, T. F., Johnsrud, M., Hoyle, C. R., Berntsen, T. K. et al., Atmos. Chem. Phys., 9(4), 1365-1392, 2009.
Neale, R. B., et al., NCAR Technical Report, NCAR/TN-486+STR, 2010a.
Neale, R. B., et al., NCAR Technical Report, NCAR/TN-485+STR , 2010b.
Prein, A. F., et al., Rev. Geophys., 53, In press, 2015.
Skamarock, W. C. and Klemp, J. B., J. Comput. Phys., 227(7), 3465-3485, 2008.
Søvde, O. A., et al., Geosci. Model Dev., 5(6), 1441-1469, 2012.
Taylor, K. E., Stouffer, R. J. and Meehl, G. A., Bull. Am. Meteorol. Soc., 93(4), 485-498, 2011.