Layers of insect echoes near a thunderstorm and implications for the interpretation of radar data in terms of airflow

Abstract

Insects can serve as useful radar targets for determining aspects of the structure and kinematics of the atmosphere, but it is necessary sometimes to know more about insect behaviour in order to have confidence in the meteorological interpretations. A variety of meteorological radars situated at Chilbolton in southern England have been used to investigate the nature of multiple shallow clear-air echo layers that were detected at heights between 1.0 and 2.5 km in the vicinity of an elevated daytime thunderstorm (mesoscale convective system). Multi-wavelength and polarization measurements were used to confirm that the layers were due in large part to insects. The layers were within potentially warm air near the top of a cold undercurrent of surface air. The convective updraught of the thunderstorm was fed by air from just above the cold undercurrent. Some of the insect layers were within air destined to ascend into the storm's convective updraught and some were within undercurrent air that was forced to ascend only temporarily as the storm approached. Initially the layer echoes ascended with the airflow but then, close to the storm, their intensity weakened despite continuing updraughts, mainly as a result of the insects dropping downwards. Far ahead of the storm the insects showed a strong preference to remain within their individual shallow layers but, even close to the storm, where they started dropping out, insects still retained a preference to be within these layers. As a result, some layers continued to be discernible as they followed the ascending airflow towards the storm, thereby continuing to serve as useful meteorological tracers of the perturbed flow in the vicinity of the thunderstorm.

Clear-Air Echoes Observed by Ka-band Polarimetric Cloud Radar: A Case Study on Insect Echoes in the Tokyo Metropolitan Area, Japan

“…Therefore, it is inferred that the widespread echoes observed in the present study were due to signals from insects. Values of of insect echoes observed by Ka-band radars can be up to 0 dBZ (Luke et al 2008;Browning et al 2011), i.e., larger than observed in the present study (<−10 dBZ). It is considered that the difference in maximum reflects the difference in the backscattering cross section depending on the observed insect (Takeda and Murabayashi 1981;Martin and Shapiro 2007).…”

“…insects (Zrnic and Ryzhkov 1998;Browning et al 2011;Melnikov and Zrnić 2017;Hubbert et al 2018), the return signal is highly horizontally polarized ( = ~5 dB, sometimes >10 dB), and the value of is typically small (<~0.9) (Zrnic and Ryzhkov 1998;Minda et al 2008;Van Den Broeke 2013;Hubbert et al 2018). The total differential phase (Ψ )…”

Environmental effects on flying migrants revealed by radar

“…Despite the limitations of radar technology to track flying birds and insects in rain (Box 2), some data exist regarding flight behavior in precipitating conditions. Under convective rain, insect flight can continue outside the precipitating cumulonimbus cells (Browning et al 2011, Leskinen et al 2011, Drake and Reynolds 2012. Moreover, large insects can continue flying in light rain (Drake et al 1981).…”

“…In birds, the effects of rain may be indirect via wetting the plumage, leading to increased weight and by impeding visibility (Emlen andDemong 1978, Liechti 1986). Insects, and probably birds as well, avoid heavy rain events by tumbling downward before reaching the powerful updrafts associated with thunderstorms that can cause mortality due to freezing (Browning et al 2011). Precipitation is known to induce flight termination in migrating insects (chapter 11 in Reynolds 2012, Reynolds et al 2018), but evidence from birds is rare.…”

Characterizing animal anatomy and internal composition for electromagnetic modelling in radar entomology

“…15 in Drake and Reynolds 2012), atmospheric research radars have significantly contributed to our knowledge of insect migration (e.g. Russell and Wilson 1997;Geerts et al 2006;Browning et al 2011;see Chapt. 11 and 15 in Drake and Reynolds 2012).…”