How does Weather Impact Spring Migration? (Credit: BirdCast, TropicalTidbits, codmeteorology, RadarScope)
Photo Credit: Pixcove
Discussion: Many of us understand that radars are used to monitor hydro-meteorological phenomena, whether that be rain, snow, hail, and anywhere in between. However, radars do not just focus on capturing weather. Anything that flies into the range of the radar beam can appear as reflectivity. Migratory birds are one non-meteorological feature that are commonly seen on radar.
A small group of birds on radar can show up as what appears to be small rain showers whereas large migratory groups are visualized as a larger mass of light to potentially moderate rain. There are other radar parameters that can be used to decipher actual rain showers from birds such as correlation coefficient (a measure of how uniform the object being observed is) where typical numbers at or below 0.8 are common for birds with rain closer to 1.
Here is an example of birds on radar migrating from Cuba to Key West, FL. The returns on the radar are close to 5-10 dBZ perhaps around a few lower teens in a few spots. This relates to a light to perhaps nearing a low-end moderate migration. The radar reflectivity correlation to the size classification of a migration is according to Gauthreaux et al. (1999) and stated on birdcast.info/research/review/.
As mentioned before, typical correlation coefficient values around 0.8 and below relate to objects other than rain. As seen above the cc values are near 0.8-0.9 and lower in some areas signifying a migration of birds from Cuba to Key West, Florida.
There are different weather factors that can trigger a bird migration. The major assisting factor is the wind direction and speed. Birds favor a tailwind regardless of the time of the year. During the spring migration, when birds are traveling north across the United States, a southerly flow is preferable. The forecast from the website birdcast.info shown above (this website has other information including a live map) the night of April 9th into the 10th features a sizeable migration from Texas on northward into the Mississippi Valley. The GFS MSLP forecast image for 06Z April 10th (below), during the nighttime hours, shows a strong low pressure across the central High Plains. Notice the similarities between the forecast medium migration region and where the pressure gradient (tight, packed close together lines running southwest to northeast) aka relatively strong southerly winds reside (Texas, Oklahoma, into the Mississippi Valley). This tailwind will trigger a low and in portions a medium migration according to the forecast.
Precipitation can act as a limiting factor. This is where uncertainty develops when forecasting migration. On the previous Birdcast image, the precipitation is outlined over the forecast. The site mentions where precipitation and favorable migration conditions coexist, variability and significant unknowns are present in the forecast.
Above, a “flying eagle” supercell typically known as a V-Notch. The shape looks like an eagle with the wings pointed upwards. Image courtesy of @tornadotrackers via radarscope. Radar imagery valid June 19, 2018.
On a loosely related note, there is a type of supercell that looks like a bird on radar. The image above shows a strong supercell thunderstorm with a configuration on radar that is typically known as a V-Notch or “Flying Eagle”. This is typically present with the strongest and tallest updrafts. Radar has great application to not only detecting powerful and dangerous supercells like the one shown above but other important features such as bird migratory patterns.
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©2019 Meteorologist Joe DeLizio