Identification and use of bioscatter in weather radar data

Scanning millimeter-wave radar has been convincingly demonstrated as a highly effective means of detecting and tracking weather systems, estimating amounts of precipitation reaching the Earth's surface, and producing many other valuable atmospheric parameters. However, these weather radars also regularly detect scatter from airborne biological organisms (so-called “bioscatter”). If not properly identified, bioscatter can be erroneously attributed to atmospheric phenomena and adversely affect radar-derived meteorological data. In recent years, a variety of techniques and algorithms have been developed to discriminate bioscatter from weather signals in radar data. However, considerably more research is needed, especially emphasizing validation. Collaborations among meteorologists, atmospheric scientists, biologists, radar and computer scientists are needed to: i) identify radar echoes of biological origin, ii) verify echoes from bioscatter using independent observations, and iii) remove the bioscatter with minimal impact to the underlying atmospheric signals.

Once identified as bioscatter, radar echoes can be used to detect and characterize the presence and movements of birds, bats, and insects, which then can be integrated into a host of biologically and ecologically motivated studies. This is an example of the emerging scientific discipline of radar aeroecology. The objective of aeroecology is to broaden understanding of ecological patterns and processes that result from the behavior of organisms in the aerosphere (boundary layer and lower free troposphere). Radar aeroecology is an extension of aeroecology with an emphasis on the use of radar.

We have conducted a series of experiments both in the laboratory and field geared towards improving our understanding of how radio waves interact with flying animals and how to utilize bioscatter to study their movements and behavior. In particular, we have focused on the use of radar signals from the network of weather surveillance radars (WSR-88D) operating in the U.S. collectively known as NEXRAD. Using data from NEXRAD we have been investigating the emergence, dispersal and foraging behavior of two colonial-roosting species: Brazilian free-tailed bats (Tadarida brasiliensis) and purple martins (Progne subis). Both species are insectivorous and feed on the wing, although bats emerge at sunset and the martins emerge at sunrise. Also, both can be detected using NEXRAD. Complementary to the NEXRAD data, we have used dual-polarimetric X-band mobile weather radars to observe the Brazilian free-tailed bats and purple martins at particular roosting sites. Although NEXRAD provides near continuous observations over a wide spatial domain, they cannot fully sample the airspace near the Earth's surface. Thus, mobile weather radar is needed to validate the observations using NEXRAD and to assess temporal and spatial patterns of bats and birds being sampled.

We will present our findings and make recommendations on how certain types of bioscatter (derived from movements of Brazilian free-tailed bats and purple martins) can be identified in NEXRAD data and how this bioscatter can be appropriately assessed and interpreted in the context of how these volant animals use the aerosphere. For example, NEXRAD data can be used to address certain fundamental questions such as: How Brazilian free-tailed bats and purple martins use the aerosphere when foraging? What are the principle drivers affecting their behavior while in flight? How is the behavior of these animals impacted by prevailing and pending meteorological conditions? We introduce several analytical tools that can be used to monitor bats and birds using weather radar and discuss how they can be used to retrieve biologically meaningful data.