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National audience; Historical in situ sub-daily rainfall observations are essential for the understanding of short-duration rainfall extremes but records are typically not readily accessible and data are often subject to errors and inhomogeneities. Furthermore, these events are poorly quantified in projections of future climate change making adaptation to the risk of flash flooding problematic. Consequently, knowledge of the processes contributing to intense, short-duration rainfall is less complete compared with those on daily timescales. The INTENSE project is addressing this global challenge by undertaking a data collection initiative that is coupled with advances in high-resolution climate modelling to better understand key processes and likely future change. The project has so far acquired data from over 23 000 rain gauges for its global sub-daily rainfall dataset (GSDR) and has provided evidence of an intensification of hourly extremes over the US. Studies of these observations, combined with model simulations, will continue to advance our understanding of the role of local-scale thermodynamics and large-scale atmospheric circulation in the generation of these events and how these might change in the future.

The January 2014 floods paralyzed nearly all of Jakarta, Indonesia. The precipitation events that lead to these floods were not very unusual but show positive trends in the observed record.

Observations show that subdaily precipitation extremes increase with dew point temperature at a rate exceeding the Clausius-Clapeyron (CC) relation. The understanding of this so-called super CC scaling is still incomplete, and observations of convective cell properties could provide important information. Here the size and intensity of rain cells are investigated by using a tracking of rainfall events in high-resolution radar data. Higher intensities are accompanied by larger rainfall areas. However, whereas small rain cells mainly follow CC scaling, larger cells display super CC behavior. Even more, for dew point exceeding 15°C, the rain cell size has to increase in order to sustain super CC scaling and a remarked increase in rain cell area is found. Our results imply that the source area of moisture, the cloud size, and the degree of mesoscale organization play key roles in the context of a warming climate.

[Departement_IRSTEA]Territoires [ADD1_IRSTEA]Adaptation des territoires au changement global; International audience; Extreme precipitation events are widely thought to intensify in a warmer atmosphere through the Clausius-Clapeyron equation. The temperature-extreme precipitation scaling was proposed to analyse the temperature dependency of short-duration extreme precipitation and since then, the concept has been widely used in climatology. Bao et al. (2017) suggest that the apparent scaling reflects not only how surface air properties affect extreme precipitation, but also reflects how synoptic conditions and localized cooling due to the storm itself affect the scaling - implying two-way causality. We address here critical issues of this paper and provide evidence that dew point temperature drives extreme precipitation, with the direction of causality reversed only for the storm's peak intensity. This physical inference may serve as a basis to better quantify scaling rates and to help establish the relationship between extreme precipitation and environmental conditions in the current climate, and thereby provide insights into future changes to precipitation extremes due to climate change.

Research on relations between atmospheric conditions and extreme precipitation is important to understand and model present-day climate extremes and assess how precipitation extremes might evolve in a future climate. Here we present a statistical analysis of the relation between large-scale conditions and hourly precipitation at midlatitudes, by using observations of the Netherlands combined with a regional reanalysis. The aim is to gain a better understanding of the typical large-scale atmospheric conditions and large-scale forcing associated with extreme hourly precipitation and determine the typical differences between cases of extreme precipitation and weaker events. To avoid double counting, we perform an event-based analysis and consider the hourly peak intensity, rather than all hourly data. Atmospheric large-scale profiles consistently show a clear separation between precipitation deciles, characterized by increasing instability and moisture content of the atmosphere for more extreme precipitation. Furthermore, stronger events are characterized by larger atmospheric forcing preceding the event, which primarily relates to vertical motions. Based on these results, four atmospheric parameters, describing atmospheric moisture, stability and large-scale convergence, are analyzed as potential indicators of strong precipitation events. Despite positive relations between these parameters and the peak intensity, their correlations are found to be weak.

Expansion of urban areas has profound effects on land surface characteristics. As such, the land surface can exert influence on atmospheric parameters that might alter precipitation amounts or patterns. In this study, precipitation observations near urban areas along the West coast of the Netherlands are investigated throughout the 1951–2010 period. An innovative analysis methodology is used to deal with the small and fragmented urban areas in the Netherlands. The results show that daily precipitation totals downwind of urban areas are, on average, about 7% higher than precipitation in the rest of the Dutch West coast. Precipitation enhancements up to 20% are found depending on wind direction and time period. These results are comparable with studies from around the globe and show that the influence of relatively small fragmented urban areas, as are present in the Netherlands, can be similar to the influence of large metropolitan areas on precipitation.

Significant increases in precipitation have been observed in The Netherlands over the last century. At the same time persistent spatial variations are apparent. The objective of this study is to analyse and explain these spatial patterns, focussing on changes in means and extremes for the period 1951–2009. To investigate different possibilities for the causes of spatial variations, a distinction was made between six regions based on mean precipitation, soil type and elevation, and four zones at different distances to the coast. Spatial maxima in mean precipitation inland and over elevated areas are mainly formed in winter and spring, while maxima along the coast are generated in autumn. Daily precipitation maxima are found in the central West coast and over elevated areas. Upward trends in daily precipitation are highest from February to April and lowest from July to September. The strongest and most significant increases are found along the coast. For several seasonal and climatological periods diverging behaviour between coastal and inland zones is observed. We find that distance to the coast gives a more consistent picture for the seasonal precipitation changes than a classification based on surface characteristics. Therefore, from the investigated surface factors, we consider sea surface temperature to have the largest influence on precipitation in The Netherlands.

Het KNMI regionaal klimaat model RACMO wordt in toenemende mate gebruikt bij de detaillering van Klimaatscenario’s. Voorbeelden zijn de frequentie en intensiteit van hittegolven en de veranderingen daarin. Of te verwachten wijzigingen in het optreden van lokale neerslagextremen. In dit project zijn een aantal componenten van RACMO verder ontwikkeld. De bodemhydrologie van het model is verder verfijnd door ruimtelijke heterogeniteit in te voeren voor een aantal bodemparameters, zoals bodemtype en worteldiepte. Deze aanpassing resulteert in meer uitgesproken ruimtelijke structuren op regionale schaal.

The impact of climate change will manifest itself in our future weather. In the project Future Weather we investigated a number of these impact relevant weather conditions in the (present and) future climate. We focussed primarily on changes in precipitation extremes on different scales ranging from intense showers at local scales to multi-day precipitation extremes over the Rhine catchment area. On an intermediate scale, regional differences in precipitation within the Netherlands are studied. Finally, we considered a worst case scenario of a combined wind and discharge extreme.

Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under well-controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the three-dimensional (3D) atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parametrization schemes in SCM simulations or is the spread due to uncertainties in the forcings too large? This study investigates this question for the nocturnal low-level jet (LLJ) at Cabauw. First, we analyse a single LLJ case that has been used for the third GEWEX Atmospheric Boundary Layer Studies (GABLS3) intercomparison study. To estimate the forcings, for this case a blend of local observations and 3D model output has been used. A sensitivity study to both the forcings and the turbulence formulation is performed by using the SCM version of the ECMWF model. The sensitivity to the turbulence parametrization is largest in the bulk of the stable boundary layer (SBL). The influence of the forcings manifests itself mainly in the upper part of the SBL and above. Second, an ensemble of eight comparable LLJ cases is considered. Using forcings derived from 3D model integrations, SCM results of the separate cases show significant deviations from the observations. However, the mean of the SCM simulations agrees well with the mean of the observations. Based on the eight selected LLJ cases, a composite SCM case is defined from the mean forcings of the 3D model. This improves the signal-to-noise ratio, which enables a better judgement of the quality of boundary-layer parametrizations in a comparison with the observations

We will present a quantification of how the North Sea surface temperature (SST) may alter the spatial distribution of precipitation extremes within the Netherlands. Recent research has shown that the coastal area climate experiences a strong dependency to SST under specific atmospheric circulation conditions. With a model of intermediate resolution (in both time and space) results obtained for August 2006 showed already that the influence of a warm North Sea can be significant, even leading to an increase in coastal precipitation of 15 % per degree warming of the North Sea (Lenderink et al. 2008). However, these models of intermediate resolution have difficulties in simulating the right amount of precipitation. On a 25 km resolution, the precipitation is underestimated in summertime circumstances for coastal area in the Netherlands. The summertime precipitation is characterized by convective showers, which are difficult to capture at this resolution. A new generation of regional climate models, which use the non-hydrostatic approach, have the ability to simulate precipitation on a resolution as small as 1 km. This is achieved by actually resolving the microphysics of the atmosphere instead of parameterizing convection. It is also hypothesized that precipitation extremes are much better captured. With the results of the simulations presented extreme events of climate scenarios for the Netherlands will be improved in many aspects. This will provide better quantification of changes in variability at different time scales and a better representation of the uncertainty.

De komende decennia worden er tussen de 500.000 en 1.500.000 woningen gebouwd waarvan een groot deel in laag Nederland. Deze studie laat zien dat door deze woningen overstromingsbestendig te bouwen schadereductie mogelijk is. Het schaderisico wordt dan nog eens een factor 2 minder als naast een Business as Usual variant nieuwbouwwoningen worden opgehoogd tot +5 m NAP. De kosten van opgehoogde nieuwbouwhuizen zijn hoger en variëren tussen de 0,4 en 1.7 miljard euro/jaar, hetgeen overeenkomt met 0,1-0,5% van het BNP. Dijkversterking levert de hoogste reductie op in het schaderisico bij de gehanteerde scenario’s. Gevolgbeperkende maatregelen in de ruimtelijk ordening als additionele oplossingsrichting zijn echter goed mogelijk als er ook een economische perspectief is bijvoorbeeld door middel van multifunctioneel ruimtegebruik.

Het IPCC brengt verspreid over 2007 in vier delen het vierde klimaatrapport uit (Fourth Assessment Report - AR4). Het geeft daarmee de nieuwste inzichten op het gebied van klimaatverandering in de wereld. Aan het AR4 hebben honderden gerenommeerde klimaatonderzoekers uit de hele wereld meegewerkt, waaronder verschillende uit Nederland. De hoofdrapporten met de wetenschappelijke onderbouwing komen dit jaar stapsgewijs beschikbaar. Eind november volgt het zogeheten ‘synthesis report’ van het IPCC. In dit rapport zullen de bevindingen en conclusies van de drie werkgroeprapporten in samenhang beschouwd worden om zo een integraal wetenschappelijk beeld te geven van ons huidige begrip van klimaatverandering. Deze stand van zaken rond klimaatverandering en Nederland wordt gegeven door: MNP, KNMI, VU Amsterdam, Universiteit Utrecht, NWO, Wageningen UR en ECN

In statistical cloud schemes the cloud fraction is directly related to the normalized saturation deficit (Q1). This Q1 is determined by the distance of the (gridbox) mean state to the mean saturation normalized by the horizontal variance of the atmospheric state. Estimates of the horizontal variance of the conserved atmospheric state variables (e.g. liquid water potential temperature and total water) are therefore needed. Up to now, statistical cloud schemes have been used frequently in combination with diffusion schemes for turbulent transports. In this approach horizontal variances are determined by the turbulent flux and a typical convective velocity scale. At present mass flux schemes have proven to be more succesfull than diffusion schemes to represent turbulent fluxes in shallow cumulus layers. Therefore it is natural to combine the mass flux approach with a statistical cloud scheme. In order to do this estimates of the horizontal variance due to mass flux activity are needed. We obtained estimates of the variances from a simplified budget equations for the horizontal variance We show the result of the scheme for the intercomparison cases in GCSS working group 1 of Cumulus clouds (BOMEX and ARM) and Cumulus rising into Stratocumulus (ATEX). Results of the scheme show a good agreement with the LES results for these cases.