Polarimetric Radar Observations of Biological Scatterers in the Eye of Typhoon Lekima (2019)

Tropical cyclones not only cause strong winds and heavy rainfall, but they can also facilitate the transport of birds and insects from tropical regions to areas along their paths. Before super Typhoon Lekima made landfall in 2019, an operational polarimetric radar in Wenzhou City observed biological scatterers in the typhoon's eye. These scatterers were likely birds and insects that were trapped in the calm center of the typhoon by strong winds and heavy rain. The polarimetric variables of these biological scatterers had specific characteristics: low reflectivity factor (ZH) values with a median of 7.5 dBZ, low cross-correlation coefficient (& rho;hv) values with a median of 0.65, large differential reflectivity (ZDR) values with a median of 2.8 dB and a maximum of & SIM;7.8 dB, and widely-distributed differential phase (& phi;DP) values with 25th and 75th percentiles ranging from 1.8 & DEG; to 36.0 & DEG; and a median of 20.1 & DEG;. When the edge of the eye reached the coastline, the birds and insects landed, with the biological scatterer signature changing from a circular shape to a band shape. We further compared Lekima with Atlantic Hurricane Irene which happened in 2011, and both storms had similar polarimetric characteristics contributed by both birds and insects. However, the biological echoes in Lekima better exhibited the characteristics of birds with a larger proportion of samples with ZDR lower than 5 dB as well as large & phi;DP and low & rho;hv values. This finding could help to understand the role of typhoons in driving biological migration between oceans and/or islands and continents. Tropical cyclones can facilitate the transport of tropical birds and insects to areas along their paths, in addition to causing strong winds and heavy rainfall. A polarimetric radar was used to observe biological scatterers, likely birds and insects, in the eye of super Typhoon Lekima before landfall near Wenzhou City in 2019. These organisms were probably brought into the typhoon's calm center by strong winds. The radar characteristics of the scatterers were low reflectivity factor, low correlation coefficient, large differential reflectivity, and widely distributed differential phase. As the typhoon approached the coastline, the shape of the biological scatterer signature changed from a circular shape to a band shape. After the landfall of the typhoon, the echoes within the eye disappeared, likely because the birds and insects may fly near or land on the ground for a rest. This study's findings can help us better understand how typhoons can transport biological matter between oceans and/or islands and continents, which can affect ecosystems and the spread of diseases between different regions. Biological scatterers were observed by a polarimetric radar in the eye of Typhoon LekimaThe biological echo showed low ZH, low & rho;hv, and high ZDR values with a wide distribution of & phi;DPThe biological echoes within the eye of Lekima originated from a larger proportion of bird scatterers compared to those in Irene (2011)