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, 8 (54), 30-43

Bird Migration Flight Altitudes Studied by a Network of Operational Weather Radars

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Bird Migration Flight Altitudes Studied by a Network of Operational Weather Radars

Adriaan M Dokter et al. J R Soc Interface.

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.

Figures

Figure 1.
Figure 1.
Map of operational weather radars for part of western Europe. Radar sites are indicated by bullets, the weather radars used in this study are labelled and coloured red. The Opera reflectivity composite is overlaid for 19 April 2008 19.30 UTC.
Figure 2.
Figure 2.
(a) Plan position indicators (PPIs) for reflectivity factor and radial velocity for a bird migration event ((i) 5 October 2007, 00.02 UTC) and an event with weak convective showers ((ii) 26 September 2007, 16.32 UTC). The PPIs show the 1.2° elevation scan up to 25 km range. Borders of identified reflectivity cells are indicated in red. (b) Radial velocity standard deviation σcell as a function of cell-averaged reflectivity factor Zcell for reflectivity cells detected during intense bird migration events (green bullets, σcell = 9 ± 2 m s−1) and during events with convective showers (blue bullets, σcell = 0.5 ± 0.2 m s−1) for the Wideumont weather radar. Selected intense migration events were 4, 5, 6, 7 and 13 October 2007 from 17.00 to 09.00 UTC next day. Selected convective shower events were 24 September 2007, 9.00 to 18.00 UTC; 26 September 2007, 07.00 to 18.00 UTC; 17 October 2007, 06.00 to 17.00 UTC; 18 October 2007, 09.00 to 17.00 UTC. Only cells consisting of over 800 resolution volumes are shown to limit the number of scatter points. For the largest reflectivity cell in each PPI, a connecting solid line is drawn to its corresponding scatter point. Cells inside the yellow shaded segments (σcell < 5 m s−1 or Zcell > 15 dBZe) are classified as non-bird reflectivity cells and removed from the scan.
Figure 3.
Figure 3.
Comparison of the bird density altitude profiles determined by (a) bird radar and (b) weather radar. (c) Height-integrated bird densities are displayed for both weather radar (red) and bird radar (blue). Weather radar reflectivities were converted to bird density by assuming a constant weather radar cross section at C-band of σbird = 11 cm2 (see legends for figures 5 and 6). The period between sunset and sunrise is shaded in grey. Wind barbs in (c) show the wind profile from the Hirlam numerical weather prediction model. Each half barb represents 10 km h−1 and each full barb 20 km h−1.
Figure 4.
Figure 4.
Radial velocity standard deviation σr as a function of raw reflectivity (a, reflectivity including non-bird echoes) and as a function of bird reflectivity (b, reflectivity cleaned from non-bird echoes). Each scatter point refers to a time–height layer, i.e. a specific height layer measured at a specific time. To limit the number of scatter points, only data for the Wideumont campaign are shown (22 September–21 October 2007, 14 742 detected non-empty time–height layers). In colour, the bird density is indicated as measured simultaneously by the bird radar. The large majority of non-zero bird densities are observed for time–height layers with σr > 2 m s−1.
Figure 5.
Figure 5.
Correlation weather radar reflectivity and bird radar density in Wideumont, Belgium, corresponding to nightly (18.00–06.00 UTC) bird density estimates for 14 742 time–height layers recorded at 15 min time interval and 0.2 km height interval for the continuous period of 22 September–21 October 2007. We find σbird = 11 ± 6 cm2 and a correlation coefficient R2 = 0.73 (as calculated from a set of statistically independent bootstrap samples). Solid black line; best fit: η = 11ρbird
Figure 6.
Figure 6.
Seasonal trend in bird radar cross-section σbird at C-band. During autumn, the cross section increases in time (a, Wideumont campaign), while in spring, the cross section decreases (b, Trappes campaign). These trends reflect the increasing/decreasing proportion of larger bird species (mainly Turdus thrushes) during the migratory season.
Figure 7.
Figure 7.
Bird densities as a function of time and altitude at different weather radar sites (figure 1) for the night of 19 April 2008; (b) shows the height-integrated bird densities over 0.2–4 km AGL. The period between sunset and sunrise is shaded in grey and civil twilight is shaded in light grey. The pink boxed inset on the right-hand side of each panel shows the Hirlam wind profile at 00.00 UTC. Bird flight speed and directions are indicated as barbs overlaid on the altitude profile. Each half barb represents 10 km h−1 and each full barb 20 km h−1. Around 00.00 UTC, a double-layered bird density profile is observed in Wideumont. We attribute the top band to birds that departed in the vicinity of Trappes, while the lower band results from birds departing more locally.

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