Estimating animal densities in the aerosphere using weather radar: To Z or not to Z?

Phillip B Chilson,Winifred F Frick,J Ryan Shipley,Thomas H Kunz,Phillip M Stepanian,Jeffrey F Kelly

doi:10.1890/es12-00027.1

Abstract

Weather radars provide near‐continuous recording and extensive spatial coverage, which is a valuable resource for biologists, who wish to observe and study animal movements in the aerosphere over a wide range of temporal and spatial scales. Powerful biological inferences can be garnered from radar data that have been processed primarily with the intention of understanding meteorology. However, when seeking to answer certain quantitative biological questions, e.g., those related to density of animals, assumptions made in processing radar data for meteorological purposes interfere with biological inference. In particular, values of the radar reflectivity factor (Z) reported by weather radars are not well suited for biological interpretation. The mathematical framework we present here allows researchers to interpret weather radar data originating from biological scatterers (bioscatterers) without relying on assumptions developed specifically for meteorological phenomena. The mathematical principles discussed are used to interpret received echo power as it relates to bioscatterers. We examine the relationships among measurement error and these bioscatter signals using a radar simulator. Our simulation results demonstrate that within 30–90 km from a radar, distances typical for observing aerial vertebrates such as birds and bats, measurement error associated with number densities of animals within the radar sampling volume are low enough to allow reasonable estimates of aerial densities for population monitoring. The framework presented for using radar echoes for quantifying biological populations observed by radar in their aerosphere habitats enhances use of radar remote‐sensing for long‐term population monitoring as well as a host of other ecological applications, such as studies on phenology, movement, and aerial behaviors.

Highlights

There is a long tradition of incorporating radar technology into biological studies as a means of observing movements of airborne animals and quantifying their numbers in the lower atmosphere (Gauthreaux 2006)
Simplified radar equation To demonstrate the importance of the distinctions between radar reflectivity and the radar reflectivity factor and their relationships to bioscatter, we examine the basic radar equation, which is used to calculate the power of the backscattered electromagnetic radiation received by a radar
Past efforts to quantify aerial densities based on radar observations have typically relied on comparing relative values of Z (Horn and Kunz 2008, Buler and Moore 2011) or used linear regression to calibrate densities estimated from individual bioscatterers from marine radar to reported values of Z from NEXt generation RADar (NEXRAD) installations (Diehl et al 2003, van Gasteren et al 2008, Buler and Diehl 2009)

Summary

Introduction

There is a long tradition of incorporating radar technology into biological studies as a means of observing movements of airborne animals and quantifying their numbers in the lower atmosphere (Gauthreaux 2006). Radars transmit electromagnetic radiation in the form of radio waves through the use of an antenna. For the case of scatter from a single object located at a distance r (m) from a monostatic radar, the received power Pr (W) is given by a simplified form of the radar equation: Pr 1⁄4 Pt G2k2r 64p3r4. Ð1Þ where Pt (W) is the transmit power of the radar, G is the gain of the antenna, k (m) is the wavelength of the radio waves, and r (m2) is the radar cross section (RCS) of the scatterer (Rinehart 2004). Antenna gain is basically a measure of an antenna’s capacity to amplify signal power (sensitivity) as a function of orientation and can be related to the radar wavelength and the effective area of the antenna. The value of r generally is a function of the scattering angle with respect to incident angle and since all operational weather radars are monostatic, the scattering angle is oriented in the opposite direction of the incident angle for backscatter

Methods

Results

Conclusion