Iwan Holleman

Radboud University Nijmegen, Nymegen, Gelderland, Netherlands

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Publications (69)68.16 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A method for the operational monitoring of the weather radar antenna mechanics and signal processing is presented. The method is based on the analysis of sun signals in the polar volume data produced during the operational scanning of weather radars. Depending on the hardware of the radar, the volume coverage pattern, the season, and the latitude of the radar, several tens of sun hits are found per day. The method is an extension of that for determining the weather radar antenna pointing and for monitoring the receiver stability and the differential reflectivity offset. In the method the width of the sun image in elevation and in azimuth is analyzed from the data, together with the center point position and the total power, analyzed in the earlier methods. This paper describes how the width values are obtained in the majority of cases without affecting the quality of the position and power values. Results from the daily analysis reveal signal processing features and failures that are difficult to find out otherwise in weather data. Moreover, they provide a tool for monitoring the stability of the antenna system, and hence the method has great potential for routine monitoring of radar signal processing and the antenna mechanics. Hence, it is recommended that the operational solar analysis be extended into the analysis of the width.
    Journal of Atmospheric and Oceanic Technology 08/2014; 31(8):1704-1712. DOI:10.1175/JTECH-D-13-00246.1 · 1.82 Impact Factor
  • Bulletin of the American Meteorological Society 06/2014; 95(6):897-907. DOI:10.1175/BAMS-D-12-00216.1 · 11.57 Impact Factor
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    ABSTRACT: From literature five methods for the detection of summer hail have been selected. These five different methods have been tested on severe weather events in the Netherlands that occurred during the summer of 1999. The general trends in the scoring parameters of the detection methods as a function of the warning threshold are rather similar, but there are substantial quantitative differences. Using a simple model, the effects of missing ground truth data on the scoring parameters of the detection methods has been described qualitatively. It is concluded that, of all hail detection methods considered, the method of Waldvogel performs best and is suited best for display of the “probability of hail”.
    Physics and Chemistry of the Earth Part B Hydrology Oceans and Atmosphere 07/2013; 25(s 10–12):1293–1297. DOI:10.1016/S1464-1909(00)00197-0
  • Iwan Holleman, Asko Huuskonen
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    ABSTRACT: [1] We present new analytical formulas for the atmospheric refraction of radiowaves from exoatmospheric sources, such as the Sun. The refraction formulas are derived from the so-called Effective Earth's Radius Model (k-Model) or 4/3-Model which is widely used for atmospheric radars. The new formulas for the refraction angle as a function of elevation are compared to numerical results from the atmospheric refraction routines of the Starlink positional astronomy library and to refraction observations of operational weather radars in the Netherlands and Finland. It is concluded that the refraction formulas from the k-Model are in good agreement (within 0.02°) with the reference calculations and radar observations. As the k-Model is used in numerous radar applications, it is expected that these easy-to-use refraction formulas, which are consistent with this physical model, can be of wide use.
    05/2013; 48(3). DOI:10.1002/rds.20030
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    ABSTRACT: Common swifts are specialist flyers spending most of their life aloft, including night-time periods when this species roosts on the wing. Nocturnal roosting is preceded by a vertical ascent in twilight conditions towards altitudes of up to 2.5 km, behaviour previously explained as flight altitude selection for sleeping. We examined the nocturnal flight behaviour of swifts, as uniquely identified by a Doppler weather radar in central Netherlands using continuous measurements during two consecutive breeding seasons. Common swifts performed twilight ascents not only at dusk but also at dawn, which casts new light on the purpose of these ascents. Dusk and dawn ascents were mirror images of each other when time-referenced to the moment of sunset and sunrise, suggesting that the acquisition of twilight-specific light-based cues plays an important role in the progression of the ascents. Ascent height was well explained by the altitude of the 280 K isotherm, and was not significantly related to wind, cloud base height, humidity or the presence of nocturnal insects. We hypothesize that swifts profile the state of the atmospheric boundary layer during twilight ascents and/or attempt to maximize their perceptual range for visual access to distant horizontal landmarks, including surrounding weather. We compare twilight profiling by swifts with vertical twilight movements observed in other taxa, proposed to be related to orientation and navigation.
    Animal Behaviour 03/2013; 85(3):545–552. DOI:10.1016/j.anbehav.2012.12.006 · 3.07 Impact Factor
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    ABSTRACT: At temperate latitudes the synoptic patterns of bird migration are strongly structured by the presence of cyclones and anticyclones, both in the horizontal and altitudinal dimensions. In certain synoptic conditions, birds may efficiently cross regions with opposing surface wind by choosing a higher flight altitude with more favourable wind. We observed migratory passerines at mid-latitudes that selected high altitude wind optima on particular nights, leading to the formation of structured migration layers at varying altitude up to 3 km. Using long-term vertical profiling of bird migration by C-band Doppler radar in the Netherlands, we find that such migration layers occur nearly exclusively during spring migration in the presence of a high-pressure system. A conceptual analytic framework providing insight into the synoptic patterns of wind assistance for migrants that includes the altitudinal dimension has so far been lacking. We present a simple model for a baroclinic atmosphere that relates vertical profiles of wind assistance to the pressure and temperature patterns occurring at temperate latitudes. We show how the magnitude and direction of the large scale horizontal temperature gradient affects the relative gain in wind assistance that migrants obtain through ascending. Temperature gradients typical for northerly high-pressure systems in spring are shown to cause high altitude wind optima in the easterly sectors of anticyclones, thereby explaining the frequent observations of high altitude migration in these synoptic conditions. Given the recurring synoptic arrangements of pressure systems across temperate continents, the opportunities for exploiting high altitude wind will differ between flyways, for example between easterly and westerly oceanic coasts.
    PLoS ONE 01/2013; 8(1):e52300. DOI:10.1371/journal.pone.0052300 · 3.53 Impact Factor
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    ABSTRACT: A fully automated method for the detection and quantification of bird migration was developed for operational C-band weather radar, measuring bird density, speed and direction as a function of altitude. These weather radar bird observations have been validated with data from a high-accuracy dedicated bird radar, which was stationed in the measurement volume of weather radar sites in The Netherlands, Belgium and France for a full migration season during autumn 2007 and spring 2008. We show that weather radar can extract near real-time bird density altitude profiles that closely correspond to the density profiles measured by dedicated bird radar. Doppler weather radar can thus be used as a reliable sensor for quantifying bird densities aloft in an operational setting, which--when extended to multiple radars--enables the mapping and continuous monitoring of bird migration flyways. By applying the automated method to a network of weather radars, we observed how mesoscale variability in weather conditions structured the timing and altitude profile of bird migration within single nights. Bird density altitude profiles were observed that consisted of multiple layers, which could be explained from the distinct wind conditions at different take-off sites. Consistently lower bird densities are recorded in The Netherlands compared with sites in France and eastern Belgium, which reveals some of the spatial extent of the dominant Scandinavian flyway over continental Europe.
    Journal of The Royal Society Interface 01/2011; 8(54):30-43. DOI:10.1098/rsif.2010.0116 · 3.86 Impact Factor
  • Treatise on Water Science, Edited by P. Wilderer, 01/2011: pages 351-399; Academic Press., ISBN: 978-0-444-53193-3
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    ABSTRACT: The Cabauw Experimental Site for Atmospheric Research (CESAR) observatory hosts a unique collection of instruments related to precipitation measurement. The data collected by these instruments are stored in a database that is freely accessible through a Web interface. The instruments present at the CESAR site include three disdrometers (two on the ground and one at 200 mabove ground level), a dense network of rain gauges, three profiling radars (1.3, 3.3, and 35 GHz), and an X-band Doppler polarimetric scanning radar. In addition to these instruments, operational weather radar data from the nearby (;25 km) De Bilt C-band Doppler radar are also available. The richness of the datasets available is illustrated for a rainfall event, where the synergy of the different instruments provides insight into precipitation at multiple spatial and temporal scales. These datasets, which are freely available to the scientific community, can contribute greatly to our understanding of precipitation-related atmospheric and hydrologic processes.
    Journal of Hydrometeorology 12/2010; DOI:10.1175/2010JHM1245.1 · 3.57 Impact Factor
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    ABSTRACT: An 11 year high-quality radar rainfall data set is used to abstract annual maximum rainfall depths for durations of 15 min to 24 h and area sizes of 6 to 1.7 × 103 km2 for the Netherlands. Generalized extreme value (GEV) distributions are fitted to the annual maxima. A new method is presented to describe the distribution of extreme areal rainfall depths by modeling GEV parameters as a function of both duration and area size. This leads to a semiempirical expression from which quantiles of extreme rainfall depths can be obtained for a chosen duration, area size, and return period. The uncertainties in these quantiles are calculated using the bootstrap method. Radar-based areal reduction factors (ARFs) are derived. These ARFs are comparable to those based on high-density rain gauge networks derived from the literature. It is concluded that radar data, after careful quality control, are suitable to estimate extreme areal rainfall depths.
    Water Resources Research 09/2010; 46(9):10-10. DOI:10.1029/2009WR008517 · 3.71 Impact Factor
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    ABSTRACT: A method for the daily monitoring of the differential reflectivity bias for polarimetric weather radars is presented. Sun signals detected in polar volume data produced during operational scanning of the radar are used. This method is an extension of that for monitoring the weather radar antenna pointing at low elevations and the radar receiving chain using the sun. This "online" method is ideally suited for routine application in networks of operational radars. The online sun monitoring can be used to check the agreement between horizontal and vertical polarization lobes of the radar antenna, which is a prerequisite for high-quality polarimetric measurements. By performing both online sun monitoring and rain calibration at vertical incidence, the differential receiver bias and differential transmitter bias can be disentangled. Results from the polarimetric radars in Trappes (France) and Bornholm (Denmark), demonstrating the importance of regular monitoring of the differential reflectivity bias, are discussed. It is recommended that the online sun-monitoring method, preferably in combination with rain calibration, is routinely performed on all polarimetric weather radars because accurate calibration is a prerequisite for most polarimetric algorithms.
    Journal of Atmospheric and Oceanic Technology 05/2010; 27(5):881-887. DOI:10.1175/2010JTECHA1381.1 · 1.82 Impact Factor
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    ABSTRACT: We present the evaluation of the daylight cycle of precipitation occurrence and intensity as predicted by the Regional Atmospheric Climate Model (RACMO) using corresponding information retrieved from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on-board METEOSAT-8, and observations from the European weather radar network (OPERA). SEVIRI is the first satellite instrument with the potential to provide accurate information on daylight cycles of precipitation. Roebeling and Holleman (2009) presented a method to detect precipitation and retrieve rain rates from SEVIRI observation during daylight hours. They showed that these retrievals agree very well with weather radar observations over the Netherlands. The objectives of this study are twofold. First, we will show the validity of the SEVIRI retrievals of precipitation occurrence and intensity over Europe, through a comparison against corresponding observations from about 100 weather radars of the OPERA network. Second, we will evaluate RACMO predicted daylight cycles of precipitation occurrence and intensity for the ocean, continental and Mediterranean climate regimes of Europe during summer. The daylight cycles of the above mentioned precipitation properties are analysed in their mean values, the time of the daylight maximum, and the daylight normalized amplitude. The first results show good agreement between SEVIRI and OPERA precipitation properties. Part of the differences are explained by parallax shifts in the SEVIRI data, by uncertainties in the OPERA data due to differences in the distance from the weather radar to the observed location, and due to differences in methods used to calibrate and process the observations of the different weather radars. In the evaluation of precipitation intensity as predicted by the RACMO model we show that the model predicted intensities are up to 50% higher than the intensities retrieved from SEVIRI or OPERA over Europe. In conclusion, this study shows that the SEVIRI and OPERA dataset of precipitation properties proves to be a powerful tool for evaluating parametrizations of cloud processes in weather and climate prediction models, and thus helps to increase the confidence in these models.
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    ABSTRACT: Amethodforoperationalmonitoringofaweatherradarreceivingchain,includingtheantennagainandthe receiver,is presented.The''online''methodis entirelybasedon the analysisofsunsignalsin the polarvolume data produced during operational scanning of weather radars. The method is an extension of that for de- termining the weather radar antenna pointing at low elevations using sun signals, and it is suited for routine application. The solar flux from the online method agrees very well with that obtained from ''offline'' sun tracking experiments at two weather radar sites. Furthermore, the retrieved sun flux is compared with data from the Dominion Radio Astrophysical Observatory (DRAO) in Canada. Small biases in the sun flux data from the Dutch and Finnish radars (between20.93 and 10.47 dB) are found. The low standard deviations of these sun flux data against those from DRAO (0.14-0.20 dB) demonstrate the stability of the weather radar receiving chains and of the sun-based online monitoring. Resultsfromadailyanalysisofthesunsignalsinonlineradardatacanbeusedformonitoringthealignment of the radar antenna and the stability of the radar receiver system. By comparison with the observations from a sun flux monitoring station, even the calibration of the receiving chain can be checked. The method pre- sented in this paper has great potential for routine monitoring of weather radars in national and international networks.
    Journal of Atmospheric and Oceanic Technology 01/2010; 27(1). DOI:10.1175/2009JTECHA1213.1 · 1.82 Impact Factor
  • A. Overeem, T. A. Buishand, I. Holleman
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    ABSTRACT: Rain gauge data are often employed to estimate the rainfall depth for a given return period. However, the number of rain gauge records of short-duration rainfall, such as 15 min, is sparse. The obvious advantage of radar data over most rain gauge networks is their higher temporal and spatial resolution. Furthermore, the current quality of quantitative precipitation estimation with radar and the length of the available time series make it feasible to calculate radar-based extreme rainfall statistics. In this paper an 11-year radar data set of precipitation depths for durations of 15 min to 24 h is derived for the Netherlands (3.55 × 104 km2). The radar data are adjusted using rain gauges by combining an hourly mean-field bias adjustment with a daily spatial adjustment. Assuming a generalized extreme value (GEV) distribution, the index flood method is used to describe the distribution of the annual radar rainfall maxima. Regional variability in the GEV location parameter is studied. GEV parameters based on radar and rain gauge data are compared and turn out to be in reasonable agreement. Furthermore, radar rainfall depth-duration-frequency (DDF) curves and their uncertainties are derived and compared with those based on rain gauge data. Although uncertainties become large for long durations, it is shown that radar data are suitable to construct DDF curves.
    Water Resources Research 10/2009; 45(10). DOI:10.1029/2009WR007869 · 3.71 Impact Factor
  • Aart Overeem, Iwan Holleman, Adri Buishand
    Journal of Applied Meteorology and Climatology 07/2009; 48(7):1448-1463. DOI:10.1175/2009JAMC1954.1 · 2.10 Impact Factor
  • Haan, Iwan Holleman, A.A.M. Holtslag
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    ABSTRACT: In this paper the construction of real-time integrated water vapor (IWV) maps from a surface network of global positioning system (GPS) receivers is presented. The IWV maps are constructed using a twodimensional variational technique with a persistence background that is 15 min old. The background error covariances are determined using a novel two-step method, which is based on the Hollingsworth¿Lonnberg method. The quality of these maps is assessed by comparison with radiosonde observations and IWV maps from a numerical weather prediction (NWP) model. The analyzed GPS IWV maps have no bias against radiosonde observations and a small bias against NWP analysis and forecasts up to 9 h. The standard deviation with radiosonde observations is around 2 kg m-2, and the standard deviation with NWP increases with increasing forecast length (from 2 kg m-2 for the NWP analysis to 4 kg m-2 for a forecast length of 48 h). To illustrate the additional value of these real-time products for nowcasting, three thunderstorm cases are discussed. The constructed GPS IWV maps are combined with data from the weather radar, a lightning detection network, and surface wind observations. All cases show that the location of developing thunderstorms can be identified 2 h prior to initiation in the convergence of moist air.
    Journal of Applied Meteorology and Climatology 07/2009; 48(7). DOI:10.1175/2008JAMC2024.1 · 2.10 Impact Factor
  • A. Overeem, T. A. Buishand, I. Holleman
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    ABSTRACT: Rain gauge data are often utilized to estimate the probabilities of extreme rainfall. However, the number of rain gauge records of short-duration rainfall, such as 5 minutes, is sparse. The obvious advantage of radar data with respect to most rain gauge networks is their higher temporal and spatial resolutions. Further, the current quality of quantitative precipitation estimation with radar and the length of the available time series make it feasible to calculate radar-based extreme rainfall statistics. In this paper a 10-year radar data set of precipitation depths for durations of 5 min to 24 hour is derived for the Netherlands (3.55 s 104 km2). The radar data are adjusted by combining an hourly mean-field bias adjustment using an automatic rain gauge network with a daily spatial adjustment employing a dense manual gauge network. A regional frequency analysis, assuming a GEV distribution, is used to describe the distribution of the annual radar rainfall maxima. Regional variability in extreme rainfall statistics is studied. Further, radar rainfall depth-duration-frequency (DDF) curves are derived and compared with those based on rain gauge data. DDF curves describe rainfall depth as a function of duration for given return periods or probabilities of exceedance. Key words: Rainfall, DDF curves, radar, GEV distribution
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    R A Roebeling, I Holleman
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    ABSTRACT: 1] This paper presents and validates a new algorithm to detect precipitating clouds and estimate rain rates from cloud physical properties retrieved from the Spinning Enhanced Visible and Infrared Imager (SEVIRI). The precipitation properties (PP) algorithm uses information on cloud condensed water path (CWP), particle effective radius, and cloud thermodynamic phase to detect precipitating clouds, while information on CWP and cloud top height is used to estimate rain rates. An independent data set of weather radar data is used to determine the optimum settings of the PP algorithm and calibrated it. For a 2-month period, the ability of SEVIRI to discriminate precipitating from nonprecipitating clouds is evaluated using weather radar over the Netherlands. In addition, weather radar and rain gauge observations are used to validate the SEVIRI retrievals of rain rate and accumulated rainfall across the entire study area and period. During the observation period, the spatial extents of precipitation over the study area from SEVIRI and weather radar are highly correlated (correlation % 0.90), while weaker correlations (correlation % 0.63) are found between the spatially mean rain rate retrievals from these instruments. The combined use of information on CWP, cloud thermodynamic phase, and particle size for the detection of precipitation results in an increase in explained variance ($10%) and decrease in false alarms ($15%), as compared to detection methods that are solely based on a threshold CWP. At a pixel level, the SEVIRI retrievals have an acceptable accuracy (bias) of about 0.1 mm h À1 and a precision (standard error) of about 0.8 mm h À1 . It is argued that parts of the differences are caused by collocation errors and parallax shifts in the SEVIRI data and by irregularities in the weather radar data. In future studies we intend to exploit the observations of the European weather radar network Operational Programme for the Exchange of Weather Radar Information (OPERA) and extend this study to the entirety of Europe.
    Journal of Geophysical Research Atmospheres 01/2009; 114(D21). DOI:10.1029/2009JD012102 · 3.44 Impact Factor
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    ABSTRACT: Wind profiles from an operational C-band Doppler radar have been combined with data from a bird tracking radar to assess the wind profile quality during bird migration. The weather radar wind profiles (WRWPs) are retrieved using the well-known volume velocity processing (VVP) technique. The X-band bird radar performed range–height scans perpendicular to the main migration direction and bird densities were deduced by counting and normalizing the observed echoes. It is found that the radial velocity standard deviation (r) obtained from the VVP retrieval is a skillful indicator of bird migration. Using a threshold of 2 m s 1 on r , more than 93% of the bird-contaminated wind vectors are rejected while over 70% of the true wind vectors are accepted correctly. For high bird migration densities the raw weather radar wind vectors have a positive speed bias of 8.6 3.8 m s 1 , while the quality-controlled wind vectors have a negligible speed bias. From the performance statistics against a limited area numerical weather prediction model, it is concluded that all (significant) bird contamination is removed and that high-quality weather radar wind profiles can be obtained, even during the bird migration season.
    Journal of Atmospheric and Oceanic Technology 12/2008; 25(12). DOI:10.1175/2008JTECHA1067.1 · 1.82 Impact Factor