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Vannitsem S.,Royal Meteorological Institute of Belgium
Chaos | Year: 2017

The deterministic equations describing the dynamics of the atmosphere (and of the climate system) are known to display the property of sensitivity to initial conditions. In the ergodic theory of chaos, this property is usually quantified by computing the Lyapunov exponents. In this review, these quantifiers computed in a hierarchy of atmospheric models (coupled or not to an ocean) are analyzed, together with their local counterparts known as the local or finite-time Lyapunov exponents. It is shown in particular that the variability of the local Lyapunov exponents (corresponding to the dominant Lyapunov exponent) decreases when the model resolution increases. The dynamics of (finite-amplitude) initial condition errors in these models is also reviewed, and in general found to display a complicated growth far from the asymptotic estimates provided by the Lyapunov exponents. The implications of these results for operational (high resolution) atmospheric and climate modelling are also discussed. © 2017 Author(s).

Demaeyer J.,Royal Meteorological Institute of Belgium | Vannitsem S.,Royal Meteorological Institute of Belgium
Quarterly Journal of the Royal Meteorological Society | Year: 2017

A stochastic subgrid-scale parametrization based on the Ruelle's response theory and proposed by Wouters and Lucarini is tested in the context of a low-order coupled ocean–atmosphere model for which a part of the atmospheric modes is considered as unresolved. A natural separation of phase-space into an invariant set and its complement allows for an analytical derivation of the different terms involved in the parametrization, namely the average, fluctuation and long memory terms. In this case, the fluctuation term consists of additive stochastic noise. Its application to the low-order system reveals that a considerable correction of the low-frequency variability along the invariant subset can be obtained, provided that the coupling is sufficiently weak. This new approach of scale separation opens new avenues of subgrid-scale parametrizations in multiscale systems used for climate forecasts. © 2016 Royal Meteorological Society

Vannitsem S.,Royal Meteorological Institute of Belgium | De Cruz L.,Royal Meteorological Institute of Belgium
Geoscientific Model Development | Year: 2014

A new low-order coupled ocean-atmosphere model for midlatitudes is derived. It is based on quasi-geostrophic equations for both the ocean and the atmosphere, coupled through momentum transfer at the interface. The systematic reduction of the number of modes describing the dynamics leads to an atmospheric low-order component of 20 ordinary differential equations, already discussed in Reinhold and Pierrehumbert (1982), and an oceanic low-order component of four ordinary differential equations, as proposed by Pierini (2011). The coupling terms for both components are derived and all the coefficients of the ocean model are provided. Its dynamics is then briefly explored, through the analysis of its mean field, its variability and its instability properties. The wind-driven ocean displays a decadal variability induced by the atmospheric chaotic wind forcing. The chaotic behavior of the coupled system is highly sensitive to the ocean-atmosphere coupling for low values of the thermal forcing affecting the atmosphere (corresponding to a weakly chaotic coupled system). But it is less sensitive for large values of the thermal forcing (corresponding to a highly chaotic coupled system). In all the cases explored, the number of positive exponents is increasing with the coupling. Two codes in Fortran and Lua of the model integration are provided as Supplement. © Author(s) 2014.

Van De Vyver H.,Royal Meteorological Institute of Belgium
Water Resources Research | Year: 2012

Quantification of precipitation extremes is important for flood planning purposes, and a common measure of extreme events is the T year return level. Extreme precipitation depths in Belgium are analyzed for accumulation durations ranging from 10 min to 30 days. Spatial generalized extreme value (GEV) models are presented by considering multisite data and relating GEV parameters to geographical/climatological covariates through a common regression relationship. Methods of combining data from several sites are in common use, and in such cases, there is likely to be nonnegligible intersite dependence. However, parameter estimation in GEV models is generally done with the maximum likelihood estimation method (MLE) that assumes independence. Estimates of uncertainty are adjusted for spatial dependence using methodologies proposed earlier. Consistency of GEV distributions for various durations is obtained by fitting a smooth function to the preliminary estimations of the shape parameter. Model quality has been assessed by various statistical tests and indicates the relevance of our approach. In addition, a methodology is applied to account for the fact that measurements have been made in fixed intervals (usually 09:00 UTC-09:00 UTC). The distribution of the annual sliding 24 h maxima was specified through extremal indices of a more than 110 year time series of 24 h aggregated 10 min rainfall and daily rainfall. Finally, the selected models are used for producing maps of precipitation return levels. © 2012. American Geophysical Union. All Rights Reserved.

In this letter, the urban heat island effects on the temperature time series of Uccle (Brussels, Belgium) during the summers months 1960-1999 was estimated using both ground-based weather stations and remote sensing imagery, combined with a numerical land surface scheme including state-of-the-art urban parameterization, the Town Energy Balance Scheme. Analysis of urban warming based on remote sensing method reveals that the urban bias on minimum temperature is rising at a higher rate, 2.5 times (2.85 ground-based observed) more, than on maximum temperature, with a linear trend of 0.15 °C (0.19 °C ground-based observed) and 0.06 °C (0.06 °C ground-based observed) per decade respectively. The results based on remote sensing imagery are compatible with estimates of urban warming based on weather stations. Therefore, the technique presented in this work is a useful tool in estimating the urban heat island contamination in long time series, countering the drawbacks of a ground-observational approach. © 2010 by the authors.

Van Schaeybroeck B.,Royal Meteorological Institute of Belgium | Indekeu J.O.,Catholic University of Leuven
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2015

An ultralow-temperature binary mixture of Bose-Einstein condensates adsorbed at an optical wall can undergo a wetting phase transition in which one of the species excludes the other from contact with the wall. Interestingly, while hard-wall boundary conditions entail the wetting transition to be of first order, using Gross-Pitaevskii theory we show that first-order wetting as well as critical wetting can occur when a realistic exponential optical wall potential (evanescent wave) with a finite turn-on length λ is assumed. The relevant surface excess energies are computed in an expansion in λ/ξi, where ξi is the healing length of condensate i. Experimentally, the wetting transition may best be approached by varying the interspecies scattering length a12 using Feshbach resonances. In the hard-wall limit, λ→0, exact results are derived for the prewetting and first-order wetting phase boundaries. © 2015 American Physical Society.

Journee M.,Royal Meteorological Institute of Belgium | Bertrand C.,Royal Meteorological Institute of Belgium
Remote Sensing of Environment | Year: 2010

Appropriate information on solar resources is very important for a variety of technological areas, such as: agriculture, meteorology, forestry engineering, water resources and in particular in the designing and sizing of solar energy systems. However, the availability of observed solar radiation measurements has proven to be spatially and temporally inadequate for many applications. In this paper we propose to merge the global solar radiation measurements from the Royal Meteorological Institute of Belgium solar measurements network with the operationally derived surface incoming global short-wave radiation products from Meteosat Second Generation satellites imageries to improve the spatio-temporal resolution of the surface global solar radiation data over Belgium. We evaluate several merging methods with various degrees of complexity (from mean field bias correction to geostatistical merging techniques) together with interpolated ground measurements and satellite-derived values only. The performance of the different methods is assessed by leave-one-out cross-validation. © 2010 Elsevier Inc.

Hamid K.,Royal Meteorological Institute of Belgium
Atmospheric Research | Year: 2012

From the 7th July, 2010 until 14th July, 2010, a heat wave dominated the weather in Belgium. Three major storm situations occurred during this period and all three caused severe damage. In this paper, we discuss the last storm case of 14th July. That day, a quasi-linear convective system (QLCS) crossed parts of France, Belgium and the Netherlands and caused very important wind damages and at least 2 people were killed. Most of the damage was caused by straight-line wind but at least one tornado was observed over the north of the Netherlands. This complex was induced in a very favorable synoptic configuration for severe weather and also the timing of the storm was favorable. In the frame of the Supercell project at the RMI, a damage survey was made of one of the most affected regions, which yielded useful information about the origin of the losses.Careful examination of the radar imagery revealed some connections between internal mesovortices and tracks of enhanced damage. As far as known, this paper is the first description of a derecho in Belgium. It is also the first time a connection is shown between some mesovortices and several damage tracks over the country. During the evolution of the QLCS, several examples of successive vortices were found in front of the system. Several of the vortices were accompanied by a tornado or a funnel cloud and the damage was typically concentrated along elongated tracks. © 2012 Elsevier B.V.

van de Vyver H.,Royal Meteorological Institute of Belgium
Theoretical and Applied Climatology | Year: 2012

The objective of the present study is to apply a wide range of efficient trend estimation methods for understanding how temperature extremes are locally changing. Temporal patterns of changes in extreme daily maximum or minimum temperature at homogeneous climate stations located in Belgium and their associations with changes in climate means are examined for the period 1952/1953 until present. A considerable amount of work is devoted to the formulation of extreme value models in the presence of non-stationarity. The covariate process is considered to be linear in time or/and in the North Atlantic Oscillation index as well. Additional insights on historical changes in frequency and amplitude of temperature extremes are obtained with the non-parametric quantile-perturbation approach. © 2011 Springer-Verlag.

Goudenhoofdt E.,Royal Meteorological Institute of Belgium | Delobbe L.,Royal Meteorological Institute of Belgium
Journal of Applied Meteorology and Climatology | Year: 2013

High-resolution volumetric reflectivity measurements from a C-band weather radar are used to study the characteristics of convective storms in Belgium. After clutter filtering, the data are processed by the stormtracking system Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN) using a 40- dBZ reflectivity threshold. The 10-yr period of 5-min data includes more than 1 million identified storms, mostly organized in clusters. A storm is observed at a given point 6 h yr-1 on average. Regions of slightly higher probability are generally correlated with orographic variations. The probability of at least one storm in the study area is 15%, with a maximum of 35% for July and August. The number of storms, their coverage, and their watermass are limitedmost of the time. The probability to observe a high number of storms reaches a maximum in June and in the early afternoon in phase with solar heating. The probability of large storm coverage and large water mass is highest in July and in the late afternoon. Convective storms are mostly small and weak. Deeper ones are found mainly in the afternoon whereas bigger and more intense ones also appear in the evening. The occurrence of the most intense storms does not vary along the day. Simple tracks have a mean duration of 25 min. Complex tracks, involving splitting or merging, last 70 min on average. Most convective storms move in the northeast direction, with a median speed of 30 km h-1. Their motion is slower in summer and in the afternoon. Regions with slightly higher convective initiation are related to orography. © 2013 American Meteorological Society.

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