[show abstract][hide abstract] ABSTRACT: Highlights
► Flight altitudes of common swifts were monitored full-season by weather radar. ► Common swifts undertake twilight ascents both at dusk and dawn. ► Twilight ascents show a temporal mirror symmetry referenced to sunset/sunrise. ► Ascent height is positively correlated with atmospheric temperature. ► Twilight ascents are probably associated with orientation challenges.
[show abstract][hide abstract] 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. · 3.73 Impact Factor
[show abstract][hide abstract] 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. · 4.91 Impact Factor
[show abstract][hide abstract] 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.
[show abstract][hide abstract] 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). · 1.69 Impact Factor
[show abstract][hide abstract] 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 01/2010; 46(9):10-10. · 3.15 Impact Factor
[show abstract][hide abstract] 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
[show abstract][hide abstract] 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 48 (2009) 7. 01/2009;
[show abstract][hide abstract] 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 01/2009; 114. · 3.17 Impact Factor
[show abstract][hide abstract] 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 - WATER RESOUR RES. 01/2009; 45(10).
[show abstract][hide abstract] ABSTRACT: Although radar has been used in studies of bird migration for 60 years, there is still no network in Europe for comprehensive monitoring of bird migration. Europe has a dense network of military air surveillance radars but most systems are not directly suitable for reliable bird monitoring. Since the early 1990s, Doppler radars and wind profilers have been introduced in meteorology to measure wind. These wind measurements are known to be contaminated with insect and bird echoes. The aim of the present research is to assess how bird migration information can be deduced from meteorological Doppler radar output. We compare the observations on migrating birds using a dedicated X-band bird radar with those using a C-band Doppler weather radar. The observations were collected in the Netherlands, from 1 March to 22 May 2003. In this period, the bird radar showed that densities of more than one bird per km3 are present in 20% of all measurements. Among these measurements, the weather radar correctly recognized 86% of the cases when birds were present; in 38% of the cases with no birds detected by the bird radar, the weather radar claimed bird presence (false positive). The comparison showed that in this study reliable altitudinal density profiles of birds cannot be obtained from the weather radar. However, when integrated over altitude, weather radar reflectivity is correlated with bird radar density. Moreover, bird flight speeds from both radars show good agreement in 78% of cases, and flight direction in 73% of cases. The usefulness of the existing network of weather radars for deducing information on bird migration offers a great opportunity for a European-wide monitoring network of bird migration.
[show abstract][hide abstract] ABSTRACT: During thunderstorm activity, electromagnetical and infrasonic signals are emitted due to the process of lightning and thunder. It is shown that correlating infrasound detections with results from a electromagnetical lightning detection network is successful up to distances of 50 km from the infrasound array. Infrasound recordings clearly show blast wave characteristics, relatable to CG discharges, with a dominant frequency between 1-5 Hz. Amplitude measurements can partly be explained by the beam pattern of a line source. The ability to measure thunderstorm activity with infrasound arrays has both positive and negative implications for infrasound verification purposes.
The Journal of the Acoustical Society of America 06/2008; 123(5):3839. · 1.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: When operating a network of weather radars for monitoring of (severe) precipitation and (strong) wind, data quality and network homogeneity are of crucial importance. Using sun for offline calibration of the antenna alignment is a well- established method and tools for this are included in the software packages provided by the radar manufacturers. This offline calibration is typically performed during radar mainte- nance. Solar signals can, however, be detected automatically in polar reflectivity data produced operationally by a weather radar. Recently we have published a method to determine the an- gular biases of the radar antenna using solar signals observed by a scanning weather radar (Huuskonen and Holleman, 2007). Data recorded at low elevations, where atmospheric refraction has a significant effect on the propagation of radio waves, are used and a method to take the effect of the refraction into account in the analysis was presented. Using a linear model the detected solar signals can be analyzed quantitatively, and the azimuth and elevation biases of the antenna reading are obtained. The method is applied to datasets based on operational measurements at FMI and KNMI. Here we present an extension of the online method towards monitoring of the weather radar receivers (Holleman et al, 2008). The solar monitoring method can be used for mon- itoring of the day-to-day stability of a single receiver, for monitoring of the network homogeneity, and for checking the absolute calibration. For this the maximum power of the sun signals detected by a radar is converted to solar flux units and then compared to observations from the DRAO solar flux monitoring station in Canada. Here we introduce the online sun method and describe all conversion steps. In addition a comparison between online and offline sun monitoring results is discussed. Finally operational results from the FMI and KNMI radar networks are highlighted. II. SUN SIGNALS IN RADAR DATA A method for monitoring radar antenna pointing and re- ceiver calibration using sun signals detected in operational scan data is presented. Weather radars scanning at low eleva- tion angles regularly detect signals from the sun. These signals are most usually seen around sun rise and sun set and they can be recognized in the images as spokes in the direction of
[show abstract][hide abstract] ABSTRACT: Rainfall depth-duration-frequency (DDF) curves describe rainfall depth as a function of duration for given return periods and are important for the design of hydraulic structures. This paper focusses on the effects of dependence between the maximum rainfalls for different durations on the estimation of DDF curves and the modelling of uncertainty of these curves. For this purpose the hourly rainfall depths from 12 stations in the Netherlands are analysed. The records of these stations are concatenated to one station-year record, since no geographical variation in extreme rainfall statistics could be found and the spatial dependence between the maximum rainfalls appears to be small. A generalized extreme value (GEV) distribution is fitted to the 514 annual rainfall maxima from the station-year record for durations of 1, 2, 4, 8, 12 and 24 h. Subsequently, the estimated GEV parameters are modelled as a function of duration to construct DDF curves, using the method of generalized least squares to account for the correlation between GEV parameters for different durations. A bootstrap estimate of the covariance matrix of the estimated GEV parameters is used in the generalized least squares procedure. It turns out that the shape parameter of the GEV distribution does not vary with duration. The bootstrap is also used to obtain 95%-confidence bands of the DDF curves. The bootstrap distribution of the estimated quantiles can be described by a lognormal distribution. The parameter σ of this distribution (standard deviation of the underlying normal distribution) is modelled as a function of duration and return period.
Journal of Hydrology 01/2008; · 2.96 Impact Factor
[show abstract][hide abstract] ABSTRACT: During thunderstorm activity in the Netherlands, electromagnetic and infrasonic signals are emitted due to the process of lightning and thunder. It is shown that correlating infrasound detections with results from a electromagnetic lightning detection network is successful up to distances of 50 km from the infrasound array. Infrasound recordings clearly show blastwave characteristics which can be related to cloud-ground discharges, with a dominant frequency between 1-5 Hz. Amplitude measurements of CG discharges can partly be explained by the beam pattern of a line source with a dominant frequency of 3.9 Hz, up to a distance of 20 km. The ability to measure lightning activity with infrasound arrays has both positive and negative implications for CTBT verification purposes. As a scientific application, lightning studies can benefit from the worldwide infrasound verification system.
Geophysical Research Letters 01/2008; · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper presents a method to detect precipitation and estimate rain rates using cloud physical properties retrieved from the Spinning Enhanced Visible and Infrared Imager (SEVIRI). The method calculates rain rates from cloud liquid water path (LWP), particle effective radius, cloud thermodynamic phase and cloud top height. The accuracy of the precipitation detection and rain rate retrievals from SEVIRI is evaluated with Weather Radar observations. Rain rates from SEVIRI are compared against Weather Radar observations for an area over Northern Europe and a two month period. The Weather Radar observations are used to validate the instantaneous rain rate retrievals and accumulated rainfall sums (precipitation depths) across the entire study area and period. In addition, we evaluate the ability of SEVIRI to discriminate precipitating from non-precipitating clouds. The results show very high agreement (corr. ~0.90) between amounts of precipitating clouds detected from Weather Radar and SEVIRI observations. Although weaker correlations (corr. ~0.63) are found between the rain rate retrievals from Weather Radar and SEVIRI, the SEVIRI-retrievals still have an acceptable accuracy of about 0.2 mm hr -1 and a precision of about 0.7 mm hr -1 . Part of the differences between Weather Radar and SEVIRI are explained by irregularities in the Weather Radar data due to residual sea clutter, and parallax shifts in the SEVIRI data. In conclusion, the results of this study show the potential of SEVIRI retrieved cloud physical properties for the detection of precipitation and the retrieval of realistic rain rates. In future studies we intend to exploit the observations of the European Weather Radar network (OPERA) and extend this study to entire Europe.