Characteristics of Chaff Echo Observed by NIMR X-band Dual Polarization Radar

Chaff is the material that is a coated, thin foil, and it is dropped in the air by air forces to disrupt the position of combat planes. Since the echo by the chaff can cause confusion in analyzing rainfall echo in a radar image although the reflectivity may be low, we investigated the chaff echo characteristics to distinguish the rainfall echo from the chaff echo using the dual polarization parameters such as the reflectivity, the differential reflectivity, and the cross correlation coefficient observed the NIMR X-band dual polarization radar located at Muan of the Korean Peninsula. Most chaff echoes move horizontally and fall to the ground slow, and they are distributed over time as a long line in the same direction of the horizontal wind field. For the chaff echo analysis, we firstly performed the statistical analysis of the chaff echo occurrence from July to December in 2009 using the observation data from the NIMR X-band dual polarization radar. The result showed that chaff echo was observed about 40% of the days during the analyzed period. The operational radar may be observed chaff echo in more days since its observation radius is about 240 km that is larger than that of the NIMR X-band dual polarization radar. To set up the dual polarization parameter ranges in chaff echo, we analyzed the characteristics found in the chaff echo, chaff-rainfall echo, snowfall echo, and bright band echo. In the chaff echo, the differential reflectivity was distributed in the range of -10 ~ 20 dB, and the reflectivity was distributed mostly below 0 dBZ but the maximum was 35 dBZ. The cross correlation coefficient was 0.7 or less, but the snowfall, ice crystals, and the mixed layer echo also showed the same value, which indicates the improved method is necessary for accurate analysis of the cross correlation coefficient. When the chaff echo and rainfall echo coexisted, the rainfall echo showed the differential reflectivity around -0.5 ~ 1.0 dB and the cross correlation coefficient of 0.9 or higher, whereas the chaff echo showed the various differential reflectivity ranging from the very low value of -4.0 dB to the very high value of 10.0 dB and the cross correlation coefficient of 0.6 or less. Therefore, since it is possible to distinguish the chaff echo through the dual polarization parameters analysis even when the chaff echo and the rainfall echo coexist, the dual polarization radar data can be used to developing the chaff echo elimination algorithm in the future.