bird migration patterns on weather radars

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Pergamon Phys. Chem. Earth (E), Vol. 25, No. 10-12, pp. 1185-I 193,200O 63 2000 Elsevier Science Ltd All rights reserved 1464- 1909/00/$ - see front matter PII: S1464-1909(00)00176-3 Bird Migration Patterns on Weather Radars J. Koistinen Finnish Meteorological Institute, P.O. Box 503, FIN-00101 Helsinki, Finland. E-mail: [email protected] Received 20 June 2000; accepted 1 I July 2000 Abstract. Widespread reflectivity and Doppler velocity patterns due to birds are common on weather radar images during preferred seasonal and diurnal periods. The knowledge of the main migration types and the forcing factors of them (wind, visibility, local topography) will make it quite easy for a human analyst to diagnose birds, insects and weather. Automatic diagnosis and elimination of spurious atmospheric winds due to bird migration is important for real time wind monitoring and for the quality of the Doppler winds assimilated into NWP models. 0 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Birds and insects are biological targets, which can be easily detected with operational weather radar systems. The field of radar ornithology was well established already 30 years ago (Eastwood, 1967). Point echoes from solitary bird flocks can be easily distinguished from weather phenomena. However, in heavy migration individual flocks merge commonly into more widespread structures which resemble weather echoes. Meteorological data users may easily misinterpret the reflectivities and Doppler “winds” which migration generates. For example drizzle, insects and nocturnal heavy bird migration have quite similar radar appearances. Boundary layer VVP or VAD winds from insects represent reasonably accurately actual winds (Riley, 1999) whereas birds have a much larger own velocity component. Thus the occurrence of bird migration introduces bias to atmospheric Doppler wind measure- ments, which can be fatal e.g. for the estimation of sensitive quantities like convergence. In the worst case automatic diagnosis packages interprete Doppler patterns from birds as dangerous weather phenomena, e.g. microbursts (Larkin, 1991) or low level jets (Andersson, 1994). On the other hand, birds and insects may help in detecting meso-scale meteorological phenomena, especially lines of wind shear like sea-breeze and gust fronts (Rider and Simpson, 1968; Puhakka et al., 1986). At present automatic diagnosis of bird migration is difficult and lacking in weather radar systems. For a human interpreter even a crude knowledge of the main reflectivity and Doppler velocity patterns and signal Correspondence to: Jarmo Koistinen 1185 properties of birds as well as the knowledge of favourable and unfavourable meteorological conditions for migration provide a good tool to diagnose migration in radar products. The aim of this paper is to present elements of such knowledge. 2. Basics of bird migration 2.1 Flying strategies Bird migration is an evolutionary adaptation to seasons. The food and water supplies are too scanty for many species during cold or dry seasons in the breeding area. Therefore birds avoid starvation or dehydration by migration to optimal feeding regions. It was only 240 years ago when Linnaeus maintained the view that swallows winter at the bottom of lakes. Since then the understanding of the phenomenon has developed revolutionary with the aid of field observations by binoculars and telescopes, ringing, laboratory experiments (e.g. in planetariums) and radars. Most recently satellite telemetry of birds equipped with a miniature radio transmitter has been very succesful. Simultaneously the concepts of flight mechanics, energy and water balance during the migration (including stopovers), wind drift and navigation have developed rapidly. Formerly these components were treated separately but nowadays they can be merged to an almost unified migration model taking into account both internal and external influencing factors. Like in meteorology, numerical models have helped a lot. A review of these issues can be found e.g. in Alerstam and Hedenstrijm (1998). Naturally the selected migration strategy varies largely between species and individual birds. Moreover, during migration the same individual varies its reaction to internal and external forcing (e.g. weather) with variable response in time and space. Birds migrate mainly by three flight methods. Large birds, which have relatively long and wide wings (e.g. hawks, eagles, storks, cranes) apply soaring flight in a convective boundary layer. They first soar circular path with stiff wings in a rising thermal. Between the thermals they glide and at the same time sink slowly until they find the next bubble of rising warm air. In this way external energy provides the work to evercome drag. Some larger oceanic birds (like albatrosses) use a modification of this strategy, called dynamic soaring. This technique applies

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Page 1: Bird migration patterns on weather radars

Pergamon Phys. Chem. Earth (E), Vol. 25, No. 10-12, pp. 1185-I 193,200O

63 2000 Elsevier Science Ltd All rights reserved

1464- 1909/00/$ - see front matter

PII: S1464-1909(00)00176-3

Bird Migration Patterns on Weather Radars

J. Koistinen

Finnish Meteorological Institute, P.O. Box 503, FIN-00101 Helsinki, Finland. E-mail: [email protected]

Received 20 June 2000; accepted 1 I July 2000

Abstract. Widespread reflectivity and Doppler velocity patterns due to birds are common on weather radar images during preferred seasonal and diurnal periods. The knowledge of the main migration types and the forcing factors of them (wind, visibility, local topography) will make it quite easy for a human analyst to diagnose birds, insects and weather. Automatic diagnosis and elimination of spurious atmospheric winds due to bird migration is important for real time wind monitoring and for the quality of the Doppler winds assimilated into NWP models. 0 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Birds and insects are biological targets, which can be easily detected with operational weather radar systems. The field of radar ornithology was well established already 30 years ago (Eastwood, 1967). Point echoes from solitary bird flocks can be easily distinguished from weather phenomena. However, in heavy migration individual flocks merge commonly into more widespread structures which resemble weather echoes. Meteorological data users may easily misinterpret the reflectivities and Doppler “winds” which migration generates. For example drizzle, insects and nocturnal heavy bird migration have quite similar radar appearances. Boundary layer VVP or VAD winds from insects represent reasonably accurately actual winds (Riley, 1999) whereas birds have a much larger own velocity component. Thus the occurrence of bird migration introduces bias to atmospheric Doppler wind measure- ments, which can be fatal e.g. for the estimation of sensitive quantities like convergence. In the worst case automatic diagnosis packages interprete Doppler patterns from birds as dangerous weather phenomena, e.g. microbursts (Larkin, 1991) or low level jets (Andersson, 1994). On the other hand, birds and insects may help in detecting meso-scale meteorological phenomena, especially lines of wind shear like sea-breeze and gust fronts (Rider and Simpson, 1968; Puhakka et al., 1986).

At present automatic diagnosis of bird migration is difficult and lacking in weather radar systems. For a human interpreter even a crude knowledge of the main reflectivity and Doppler velocity patterns and signal

Correspondence to: Jarmo Koistinen

1185

properties of birds as well as the knowledge of favourable and unfavourable meteorological conditions for migration provide a good tool to diagnose migration in radar products. The aim of this paper is to present elements of such knowledge.

2. Basics of bird migration

2.1 Flying strategies

Bird migration is an evolutionary adaptation to seasons. The food and water supplies are too scanty for many species during cold or dry seasons in the breeding area. Therefore birds avoid starvation or dehydration by migration to optimal feeding regions. It was only 240 years ago when Linnaeus maintained the view that swallows winter at the bottom of lakes. Since then the understanding of the phenomenon has developed revolutionary with the aid of field observations by binoculars and telescopes, ringing, laboratory experiments (e.g. in planetariums) and radars. Most recently satellite telemetry of birds equipped with a miniature radio transmitter has been very succesful. Simultaneously the concepts of flight mechanics, energy and water balance during the migration (including stopovers), wind drift and navigation have developed rapidly. Formerly these components were treated separately but nowadays they can be merged to an almost unified migration model taking into account both internal and external influencing factors. Like in meteorology, numerical models have helped a lot. A review of these issues can be found e.g. in Alerstam and Hedenstrijm (1998). Naturally the selected migration strategy varies largely between species and individual birds. Moreover, during migration the same individual varies its reaction to internal and external forcing (e.g. weather) with variable response in time and space.

Birds migrate mainly by three flight methods. Large birds, which have relatively long and wide wings (e.g. hawks, eagles, storks, cranes) apply soaring flight in a convective boundary layer. They first soar circular path with stiff wings in a rising thermal. Between the thermals they glide and at the same time sink slowly until they find the next bubble of rising warm air. In this way external energy provides the work to evercome drag. Some larger oceanic birds (like albatrosses) use a modification of this strategy, called dynamic soaring. This technique applies

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1186 J. Koistinen: Bird Migration Patterns

the large vertical wind shear in the marine boundary layer and lifts generated by sea waves. In flapping flight the beating wings provide the necessary lift and thrust. All small birds and considerable proportion of larger birds like waterfowl (ducks, geese, swans) use only flapping flight. An individual bird does not necessarily use a constant average air speed. It can adjust the air speed depending on the strategy of the flight i.e. minimum power consumption, maximum range reaching and minimum time of migration lead to different speeds applied. This results from the non- linear relation between power consumption and air speed of the bird.

The flying strategy which a migrating bird is using depends much on its refueling strategy. They may deposit only small fuel reserves at each stopover site and travel by numerous short flights or they may use only a few long flights associated with large fuel loads at departure. In the latter case a small bird can increase its body weight by 50 % within a week by hectic feeding. The fuel in the body is in the form of fat. On average, long distance migrants which winter in the tropics or even in the temperate zone of southern hemisphere, and which arrive late in spring, use the latter strategy. The birds moving e.g. from Scandinavia to Central Europe use typically the former strategy.

The resulting track vector of a migrating bird is the sum of the wind vector and the bird’s heading vector (Fig. 1). In order to compensate the wind drift and minimize the remaining distance to the final goal a bird can adjust both airspeed and heading (Liechti et al., 1994). By selecting an optimal combination of cruising altitudes during each flight a bird can minimize the energy and water cost per distance covered which means that it can allow some drift, especially in the beginning of the journey, and apply higher velocities of upper level winds. When approaching the goal, a bird may minimize the remaining distance by full compensation of the drift (Fig. la).Numerous field and radar studies reveal that full compensation and also partial and overcompensation of the drift occur. The concept of goal is well justified by ringing recoveries. Major part of migrating birds are not nomadic, i.e. they prefer fixed geographical breeding and wintering areas, quite often both of them are even the same fixed territories where the bird stays from year to year.

The crucial role of tailwinds in bird migration has a direct impact on the flying altitude distribution. The optimal cruising altitude is the highest altitude at which the bird is able to maintain its maximum range speed (Pennycuick, 1978). For example, radar studies of the nocturnal migration crossing the Saharan desert show that during the first hour after dusk the birds climb with vertical speeds of 0.1-4 m/s (average 0.7 m/s) and adjust flight levels to optimal wind conditions thereafter (Bruderer et al., 1995). Such vertical velocities will severely affect on the estimates of vertical air velocity from weather radar measurements. In good tailwind typical flight heights are 0.5 - 2 km in Northern Europe. In the Mediterranean region the maximum concentration can often be found also at 2-4 km. Quite commonly, especially in the subtropics, some birds cruise at altitudes of 5-6 km.

There are also observations of individual flocks flying at tropopause heights close to jet streams.

. . . . . remaining . .---._ ,. distance -....

C

S ,........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. G

Figure 1. Vectors showing the wind, the heading (airspeed of the bird) and the track of a bird with lengths corresponding to the movement during a whole flight in constant wind conditions. S denotes starting position of migration and G the final goal. A circle on top of the wind vector shows all possible end points of the flight depending on the heading direction. Angle 0 denotes the width of the sector in which the bird stays when windspeed is larger than the airspeed of the bird. (a) The bird compensates wind drift fully resulting to slower progress in migration. (b) The bird compensates wind drift partly, which minimizes the remaining migration distance but at the same time introduces need for full compensation during a later flight. (c) The bird flies a long trip assisted with a strong tailwind. In spite of maximum possible compensation the bird lands quite far from the shortest route, in a potentially inhospitable environment for further flights.

The physiological structures of bird body, which facilitate to perform the navigation and orientation required in the flight strategies described above, are out of the scope of this paper. The same is valid for the navigational tools. In short it can be said that scientific

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J. Koistinen: Bird Migration Patterns 1187

proof exists of the application of the following methods and tools: celestial compass based on the position of stars and the sun and the glow from the sun behind the horizon, magnetic compass based on the declination angle of the magnetic field of the earth, map memory, passive infrasonics, endogenous vector programmes, the pattern of polarized light of the sky, internal clock, photoperiodicity recorder, altimeter, velocity meter and recalibration of these tools during the migration. In spite of this impressive collection the present knowledge suggests the view that birds are unable to determine continuously their exact position in a fixed spherical coordinate system (Wehner, 1998; Wiltschko etal., 1998).

2.2 Seasonal and diurnal timing

In Europe the seasonal migration periods actually cover the whole year. The autumn migration of adult female waders and some male ducks begins in Northern Europe already in the beginning of June when the latest spring migrants are still on their way. December-February is a period when wintering birds in Central Europe may suddenly continue their migration either northwards or southwards in cases of exceptional warming, heavy snowfall or extreme frost. Speaking in very broad terms the average seasonal timing depends on the latitude and on the continentality (winter temperature and snow cover) of the region. The periods of most prominent migrations in weather radar measurements occur on average during April-early June and during August-October in Northern Europe and during March-May and September-November in Southern Europe.

The diurnal timing of migration is a result of the following four main components: (1) nocturnal migration consisting mostly of small songbirds, (2) morning migration of small and medium-sized landbirds, (3) soaring migration and (4) stream of waders or waterfowl which often follows coastlines (see Section 2.4). In some occasions we may observe that all types of migration are intensive within 24 hours, but more often only one or two are remarkable during the same day. In most occasions intensification of any of them requires favourable weather. Reflectivy and Doppler patterns of these four groups will be discussed in Section 3.

2.3 Intensity of migration and weather

Prior to radar studies in ornithology, when visual and audible observations of migration were dominating, the general opinion of optimal weather for bird migration was biased. Large numbers of birds were often seen migrating in headwind and sudden nocturnal falls of thousands of birds were detected in bad weather close to low pressure centers. It was concluded that birds prefer migrating headwind and low pressures act as a triggering mechanism. Radar studies revealed that on average tailwind migration is far more intensive - we just can’t see the birds easily as they are flying at higher altitudes. These studies also

revealed that good visibility and absence of hydrometeors i.e. anticyclonic weather favours the onset and continuation of heavy migration. Low pressures and weather fronts just bring the birds to our eyes as they tend to cease the migration originating far away. For a review of the influence of weather on birds see e.g. Elkins (1988) and Richardson (1978).

The theoretical aspects of flight strategy, shortly described in Section 2.1, implicate that wind at the cruising altitude is the most important meteorological factor for a migrating bird. In central Europe and the Mediterranean area migrating birds have to cope with windspeeds commonly ranging from 50 to 100 % of their normal airspeed in the boundary layer. Typical airspeed of birds is 7 - 20 m/s depending on the species involved. A bird migrating selectively during nights with following wind conditions speeds up its flight by 30 % on average compared to an individual disregarding the wind situation. Selecting the most profitable flight altitude may result in an additional gain of 40 % in flight speed (Liechti and Bruderer, 1998). This strategy is clearly visible in weather radar measurements. Heavy migrations can be seen when the 10 m height wind speed is O-8 m/s (calm to moderate breeze) and wind vector points towards the goal region of the migrants. If we neglect local topographical effects and bird species whose migration deviates from the overall pattern, on average heavy migration in weather radar measurements in Europe is most probable when the large scale boundary layer wind direction is northerly in autumn (from the sector NW-ENE), and southerly in spring (from the sector WSW-SE).

When the 10 m wind speed increases to strong breeze and gale (11-21 m/s) the intensity of migration usually decreases rapidly. There are at least two reasons to avoid onset of migration in such cases although a fast tailwind could in principle assist the bird very rapidly to the goal. Firstly, in large wind speeds also the wind drift increases. If the wind direction in strong winds is not very close to the exact goal direction the bird is unable to compensate the drift and the risk to be carried to inhospitable environment increases. According to Eq. (1) the width of the sector (Q, which the bird can reach by maximal compensation, becomes narrow (see Fig. lc):

8 = 2arcsin(Va/V), (1)

where Va denotes airspeed of the bird and V wind speed (Va < V). Secondly, strong winds are very turbulent and gusty close to the ground. In such conditions the risk increases that the takeoff trial ends to a fatal collision against ground or trees. In cases of headwind breeze or gale it is very likely that practically none of the echo pixels on a weather radar display are birds.

Wind conditions have a direct negative or positive effect on bird flight. The only other commonly limiting weather factor is visibility. Flying in heavy rainfall and snowfall or through a cloud with large content of supercooled droplets may directly influence to the flying capability of a bird. Obviously the main limiting factor is indirect: dense hydrometeor concentrations prohibit a bird

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1188 J. Koistinen: Bird Migration Patterns

to use all available navigation senses efficiently. This is manifestated by the fact that in spite of favourable wind conditions migration is weak in dense and thick layers of fog and in cases when the height of a dense Stratus or Stratocumulus cloud cover is around 200 m or less. In cases of higher level cloud layers radar studies show that birds avoid the layer and fly either above or below it. It should be noted that radiation fogs are often local or thin which doesnt prohibit heavy migration above the fog. A widespread advection fog is usually a real migration killer.

It is quite commonly thought that temperature is the triggering factor in bird migration. Many studies show that temperature has a negligible influence on flying height or migration intensity compared to the wind. The reason for a spurious temperature effect is well known in the atmospheric general circulation research i.e. in the northern hemisphere temperature (T) has a strong positive correlation with the southerly component of the wind speed (v):

[TV]‘> 0 (2)

where T’ is temperature anomaly compared to the time average [T], and v’ is anomaly in the southerly wind speed component compared to the time average [v]; brackets denotes time average. In practice this means that e.g. in spring intensive migration will be expected when wind turns southerly due to the passage of a cold front or a warm front. Naturally southerly winds tend to be more often warm than cold in spring (Fig. 2).

Figure 2. Typical weather charts in a favourable and unfavourable situation to detect intensive migration on weather radars. The map on the left represents favourable weather in autumn and unfavourable in spring whereas the map on the right represents the opposite cases.

As a summary of weather effects Fig. 2 exhibits typical weather charts of a favourable and unfavourable migration conditions. Although the maps are from Northern Europe similar weather patterns are valid for the rest of Europe as well. The most impressive migrations in weather radars occur when during the peak migration period unfavourable weather pattern with strong headwinds, rain and low clouds prevails several days and then rapidly changes to clear skies and moderate tailwinds.

The optimal meteorological conditions described above do not fully exclude cases when relatively strong migration is observed in less favourable weather. Night-migrating small passerines i.e. songbirds like warblers, flycatchers and thrushes as well as soaring migrants are most sensitive to stop migration in bad weather. On the other hand

shorebirds and waterfowl fly often long distances and arrive in adverse weather to areas without suitable feeding sites. In such cases they avoid landing and intensive migration can continue in rain and storm or above thick layers of lower clouds. After a longer spell of very bad weather quite intensive migration may start even in a moderate headwind. Then birds fly close to the ground or sea surface, where friction lowers wind speed enough so that the airspeed of the bird is at least a little bit larger than the heading wind component. In such occasions local topography will strongly affect on the track direction.

2.4 Topographical forcing

Over wide areas of homogeneous topography like sea or typical European flat landscape looking like a mixture of farmland and forests, the birds migrate as a broad front without specific horizontal concentrations. In constant tailwind conditions the average cruising altitude of migrants is defined largerly by the wind. Additionally, land breeding birds tend to fly higher in daylight when they have to cross wide sea areas. In this way they expand the range of their visual horizon, which gives better chances to reach land areas if the navigation applied seems to lead constantly over the sea. In the same way waterfowl and shorebirds often cross wide land areas at much higher altitudes than sea areas.

Millions of waterfowl and waders (e.g. sandpipers, Dunlins, Knots, Grey Plovers), which breed in the arctic regions between North Eastern Canada and Central Siberia winter mainly along the western coasts of Europe and Africa, where shallow shores offer the best feeding areas during the low tide period. Therefore these birds avoid both open ocean and large continental regions which has lead to a very pronounced concentration of migration following the eastern coast of the Atlantic ocean. From the North Sea a major branch of this arctic stream of migrants leads to the Baltic Sea, Gulf of Finland, White Sea and continues to the Siberian coast. Along this route enormous concentrations of migrating birds can be counted at a single spot. For example the largest daily sums of migrants following the southern coast of Finland in late May have been e.g. 100 000 Brent Geese and 0.5 million Long-tailed Ducks and Common Scoters (Bergman and Donner, 1964). It should be noted that similar but less intensive fixed migration routes exist along many European coasts and large rivers. In the same way soaring migrants apply daytime thermals in their migration and avoid crossing larger bodies of water, where convection is weaker than over land areas. This leads to huge concentrations of migration along the Eastern coast of Mediterranean in Israel and cross the narrow straits of Gibraltar, Bosporus and Messina.

When small landbirds are forced in daylight to fly against weak or moderate headwind at low altitude (typically lo-200 m) their visual range of horizon will diminish drastically compared to tailwind flying at the height of 500 m or more. If the flocks then encounter a coastline they turn to follow it. This is due to the combined

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J. Koistinen: Bird Migration Patterns 1189

result of avoiding too strong headwind aloft and not seeing the coastline on the opposite side of the sea or lake (e.g. Alerstam, 1990). In this kind of situations the stream along a long straight coastline or at the tip of a peninsula can be quite intensive. As the stream is narrow and all birds are flying at low altitudes it is easy to count them by binoculars. For example, at an optimal location along the eastern coast of the Baltic Sea it is sometimes possible to count 100 000 - 500 000 migrating small birds like Chaffinches, Bramblings and Siskins during one morning. On the other hand, weather radar will underestimate the migration except at close ranges to the radar as major part of the migrants stays below the lowest elevation beam. Such amounts require that during several previous days there have been no favorable tailwinds. This phenomenon led earlier to the wrong conclusion that migrating birds prefer headwinds (Section 2.3).

3. Diagnosis of migration on weather radars

3.1 Reflectivity patterns

Figure 3 gives a rough idea of typical diurnal timing and intensity of the four main migration types. Each curve represents only an order of magnitude in a typical intensive migration case in optimal weather and assuming that the pixel size of a radar image is 1 degree * 1 km. Deviations from these curves can of course be large in individual cases. It can be seen that on average the most quiet periods are from late afternoon to dusk and the dawn hour before sunrise. Of course any of the curves will rapidly attenuate when weather conditions become unfavourable. The arctic stream has more pronounced peaks locally but they can occur any time depending on the weather and the distance between the observation site and the take off site of the species involved.

Nocturnal migration can be seen commonly as a circular or elliptic disc concentrated around the radar (except when the radar is located close to large lakes or sea, which are empty of departuring birds). Elliptic shape denotes that larger part of birds have a common heading. The backscattering cross section of birds is larger when they are viewed from sides than from the head or the tail. There are often dozens of small and large bird species involved but the size distribution is very skew so that the numbers of small birds are several orders of magnitude larger than the frequency of larger species. As small bird individuals quite probably fly separately or in low density flocks the reflectivity field consists of horizontally homogeneous layers without any bands or other mesoscale

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0 Sunrise 12 sunset 24

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Figure 3. Number of migration pixels in a lowest level PPl or PseudoCAPPI image as a function of local time in a typical intensive migration, requiring favourable weather. Vertical lines denote times of sunset and sunrise. The four main migration types are morning migration (solid line), soaring migration (dashed line), nocturnal migration (dashed line) and arctic stream (dotted line).

structures. The density can be e.g. 50 - 200 birds/km3 (Bruderer et al., 1995) and typical reflectivities vary from -10 dBZe to +lO dBZe. Thus nocturnal migration is quite similar to the reflectivity fields of shallow weak precipitation, drizzle or insect migration. A good way to diagnose nocturnal small bird migration is to look at the time series as a movie loop. Nocturnal migration takes off one hour after the sunset, expands extremely rapidly on a PPI or CAPPI image during the next hour (as birds are climbing) and continues intensive through the whole night until dawn. Fig. 4. exhibits an example of the nocturnal takeoff.

The morning migration in tailwind over a homo- geneous terrain resembles nocturnal reflectivity pattern. It is often more spotty containing maxima of lo-20 dBZ as there are numerous dense flocks of medium-sized birds (e.g. Woodpigeons). Quite often in tailwinds morning migration seems to be more intensive over the sea as migrants fly at higher altitudes. If the radar is located at the coast of departure, the birds may fly almost parallel to the lowest level beam to the sea. In case of weak headwinds the morning migration approaching a coast will concentrate along it and major departures to the sea take place at the tips of peninsulas, Fig. 5.

The largest flocks of Brent Geese observed along the Gulf of Finland during spring migration have consisted of 2 000 - 10 000 individuals, which should be close to the results of Larkin (199 1) who measured that the horizontal span of large flocks of Canadian geese can be 1.5 -2.5 km and their reflectivity around 50 dBZe. Similar values should be expected in the soaring migration. For example in Israel it is possible to see a flock of 1000 buzzards and eagles or 10 000 storks, Thus the arctic migration stream and soaring migration consists of dots, resembling aircraft

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1190 J. Koistinen: Bird Migration Patterns

echoes, or small “showers” with reflectivities between O- 50 dBZ. In a warm airmass the dots are often embedded in the widespread insect echo. In heavy migration streams the individual dots can merge together forming a large banded structure, Fig. 6.

The air speeds of the insects generating wide reflectivity patterns on weather radars are usually less than 1 m/s. Riley (1999) points out that most insects are actively migrating although the practical result of the effort is advection by the wind. At least three main types of major insect departures can be detected in Finland: convective migration on sunny and warm days, nocturnal migration and continuous migration. The last one originates typically from the sector S-SE outside Finland from where insects are advected by a warm air mass and low level jet preceeding a cold front.

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1 Fi gure 4. An example or tne onset or nocturnal nugranon on both smes

of the Gulf of Finland 21 Apr. 1998 as a series of 500 m PseudcCAPPl images, range 100 km, light grey -lO...O dBZ, dark grey O-10 dBZ. (a) 17.45 UTC, note the sunset in WNW. (b) 18.30 UTC, first migrants appear. (c) 19.00 UTC, sudden outbreak of nocturnal migration. (d)19.30 UTC, the migration from Estonia is merged to that from Finland.

All reflectivity fields of insect migration resemble those of nocturnal and morning migration of birds. Nocturnal insect migration can be even more intense, e.g. Larkin (199 1) shows a case of moth migration in which the reflectivity factor maximum was 22 dBZe. The time evolution of nocturnal insect migration is also equal to that of birds. The difference of the two nocturnal reflectivity sources appears more clear in the Doppler velocities (see the next Section). During the day it is much easier to diagnose insects from the reflectivity pattern: Insects form a continuous field of reflectivity which is less spotty

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J. Koistinen: Bird Migration Patterns 1191

P‘igure 5. Intensive morning nugration in a weak headwmd. Note the stream of migrants over the sea, departuring from the peninsulas 30 km SW from the radar.

Figure 6. The stream of arctic migrants, mainly over the Gulf o Finland is intense during the peak migration, 17 May 2000, 3.15 UTC. PseudoCAPPI image at 500 m, range 150 km It is formed of individual flocks merged partly together. In this case major part of the migrants are Barnacle Geese.

than bird migration in daytime. Secondly, the substructures in a bird reflectivity field over land areas are random whereas boundary layer convection generates organised streets of maxima and minima (roll vortices) or cellular structures in the insect migration.

Temperature is strongly limiting insect migration. For example morning migration of birds occurs quite often close to zero temperatures. In Finland it has been observed that the maximum height (h) of the convective insect layer

(usually in the afternoon) can be estimated from the following rule of thumb:

h = lOO”T(850 hPa) (3)

where h is the maximum height (top) of the insect layer (m) and T(850 hPa) is the temperature ( C) at the pressure level 850 hPa. Most commonly simultaneous bird and insect migration takes place in a warm air mass in May (nocturnal migration) and in August-September (daytime soaring and convective migrations).

3.2 Doppler patterns

As a first guess one might expect that a collection of birds within a contribution volume would result to much broader spectrum widths than in precipitation. However, several comparisons between the spectrum widths from precipitation and nocturnal migration during spring 2000 in Southern Finland revealed that from the spectrum width image it is impossible to separate areas of rain, insects and birds, Fig. 7. Partly this can be spurious as a typical signal processor assumes Gaussian spectra which simplify the actual shape of bird spectra, especially in soaring migration. Fig. 8 exhibits a case when a flock of 4 White Storks was spotted from the window of the Department of Meteorology, University of Helsinki. The C-band Doppler radar beam was fixed manually to the flock and Doppler spectra were measured. Fig. 8 shows beautifully the individual spectrum of each bird when the flock was soaring circularly.

The real shapes of Doppler spectra and their time variations would certainly help in the diagnosis of the echo source at each individual measurement bin. Such methods have been used in tracking radars (e.g. Larkin and Eisenberg,l978; Bruderer et al., 1995) and are probably well developed in the military aircraft diagnostics. The Gaussian assumption will lead to decreasing ratios spectrum height/width This is a good side effect when birds are considered clutter, as then the probability increases that the bin data will not pass the Doppler signal quality threshold i.e. the coherency test. As a result the reflectivity or Doppler field in bird migration will contain rejected bins, visible as “holes” in the image, compared to the uniform fields of insects and rain. -Thus from the Doppler velocity image, applying a coherency quality control, it is often possible to diagnose areas of bird migration and areas of rain, Fig. 7..

The variable headings and speeds of birds at the same altitude will increase also the variance of the Doppler velocity field compared to a velocity field from insects or hydrometeors. This is often clearly visible e.g. in the PPI images of Doppler velocity which exhibit large number of individual bird pixels not fitting well to the average “wind” field. In the vertical wind sounding products like the time- height cross section of VVP, the large variance in bird migration often results to a spurious zero average vector i.e. calm. This effect can be calculated well by estimating

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1192 J. Koistinen: Bird Migration Patterns

igure 7. (a) Doppler spectrum width of simultaneous areas of noctuma 1 migration and rain on a 500 m PseudoCAPPI, 18 April 2000,02:00 UTC. Light grey denotes spectrum width less than 2 m/s and dark grey more than 2 m/s. (b) The same time moment as in(a) but showing the lowest elevation PPI of the Doppler velocity.

19 May 1988 12:26:37 14.50 km, 128 point FFT

14.7 m/s towards 0 m/s away 14.7 m/s

Figure 8. Doppler spectrum of a flock of 4 soaring White Storks, redrawn from a photograph (by Matti Leskinen, Univ. of Helsinki) of the analog display.

the linear VAD or VVP wind field in a normal way and then calculating the rms-variance of the individual pixel values compared to the linear average field. By setting a preselected rms-threshold it is often easy to eliminate the bird “winds”, Figs. 9 and 10.

are 9. Time-height cross-section (THVVP) of horizontal VVP wind barbs on 18-19 April 2000 during a period of nocturnal migration (19-00 UTC) followed by mid-level precipitation (00-04 UTC). No rms quality thresholding has been applied. Note the spatial inconsistency of bird vectors, their larger speed compared to the actual winds after 00 UTC and a group of vectors at low levels claiming calm (dots). Some vectors close to the edges of precipitation suffer also of bad quality.

4. Conclusions

Major part of bird migration is so weak in terms of reflectivity that it has no significant effects on the accumulated precipitation products used in hydrological applications. Only very large flocks of birds can temporarily induce small scale maxima resembling tiny showers. Widespread weak “precipitation” due to intensive bird migration can irritate customers when they see that the sky is clear. The easiest way is to use a lower dBZe threshold (e.g. 10 dBZe) for accumulation products. In snowfall the threshold should be 15-20 dB lower. Luckily widespread bird migrations are rare in winter simulataneously with snowfall. Application of a higher dBZ threshold in summer will eliminate large part of drizzles from radar images.

The increasing need of Doppler measurements as real time wind soundings or as an input to a NWP assimilation system implicates that weather radars should be as sensitive as possible in order to maximize the amount of clear air Doppler data. Insects are quite useful in this respect whereas birds introduce a problem as the “wind” field from bird migration can1 be used as an estimate of the atmospheric winds. Thus detection and elimination of migration from Doppler data is preferable. For a human analyst that is relatively easy but automatic diagnosis is more demanding. It could be based at least on the following aspects:

Page 9: Bird migration patterns on weather radars

J. Koistinen: Bird Migration Patterns 1193

I. + * * + + f + f +J, .+J, J#-/, 1 + +

I + l + + + * l + + * j ;+ -/+ J+ 5 f + + : time (u_?(z) .: ./ .,

ah 2100 2m <3m oooa 0100 ox)0 0300 0400 , I Figure 10. As in Fig. 9 but a rms-error threshold has been applied for VVP wind data.

1. Bird migration patterns have known average seasonal and diurnal rhytms.

2. Although birds, and insects move, their reflectivity patterns are often quasi-stationary.

3. Intensive migration requires favourable weather (weak wind or moderate tailwind, no low clouds or fog).

4. If the boundary layer wind deviates too much from the NWP wind in a weak pressure gradient, the source of difference can be birds.

5. The rms-difference between a linearly fitted VVP or VAD wind and the individual bin values is larger in bird migration than in rain or insect migration.

6. The spectrum width of bird migration based on the Gaussian assumption does not deviate much

from that in precipitation. Recording more moments and possibly even the time series of the spectra would quite probably give tools to diagnose birds at individual measurement bins.

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