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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.

Poelman D.R.,Royal Meteorological Institute of Belgium
Monthly Weather Review | Year: 2014

Temporal and spatial distributions of cloud-to-ground (CG) lightning in Belgium are analyzed. Based on data from the European Cooperation for Lightning Detection (EUCLID) network, spanning a period of 10 years between 2004 and 2013, mean CG flash densities vary between 0.3km-2 yr-1 in the west up to 2.4km-2 yr-1 toward the east of Belgium, with an average flash density of 0.7km-2 yr-1. The same behavior is found in terms of thunderstorm days and hours, where in the east most of the activity is observed, with a dropofftoward the coast. The majority of lightning activity takes place in the summer months between May and August, accounting for nearly 90% of the total activity. Furthermore, the thunderstorm season reaches its highest activity in July in terms of CG detections, while the diurnal cycle peaks between 1500 and 1600 UTC. A correlation is found between the estimated peak currents and altitude, with on average higher absolute peak currents at lower elevations and vice versa. In addition, a cell tracking algorithm is applied to the data to monitor the behavior of the individual cells. It is found that the lightning cells travel at an average speed of about 25kmh-1, with a preferred northeasterly direction of movement. At last, CG flash rates are strongly related to the cell area. © 2014 American Meteorological Society.

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.

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.

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