Characteristics of the Bright Band Based on Quasi-Vertical Profiles of Polarimetric Observations from an S-Band Weather Radar Network

• Main Authors: Jeong-Eun Lee, Sung-Hwa Jung, Soohyun Kwon
• Format: Article
• Language: English
• Published: MDPI AG 2020-12-01
• Series: Remote Sensing
• Subjects: quasi-vertical profile; S-band dual-polarization radar; bright band; melting layer; microphysical process
• Online Access: https://www.mdpi.com/2072-4292/12/24/4061

Bright band (BB) characteristics obtained via dual-polarization weather radars elucidate thermodynamic and microphysical processes within precipitation systems. This study identified BB using morphological features from quasi-vertical profiles (QVPs) of polarimetric observations, and their geometric, thermodynamic, and polarimetric characteristics were statistically examined using nine operational S-band weather radars in South Korea. For comparable analysis among weather radars in the network, the calibration biases in reflectivity (Z_H) and differential reflectivity (Z_DR) were corrected based on self-consistency. The cross-correlation coefficient (ρ_HV) bias in the weak echo regions was corrected using the signal-to-noise ratio (SNR). First, we analyzed the heights of BB_PEAK derived from the Z_H as a function of season and compared the heights of BB_PEAK derived from the Z_H, Z_DR, and ρ_HV. The heights of BB_PEAK were highest in the summer season when the surface temperature was high. However, they showed distinct differences depending on the location (e.g., latitude) within the radar network, even in the same season. The height where the size of melting particles was at a maximum (BB_PEAK from the Z_H) was above that where the oblateness of these particles maximized (BB_PEAK from Z_DR). The height at which the inhomogeneity of hydometeors was at maximum (BB_PEAK from the ρ_HV) was also below that of BB_PEAK from the Z_H. Second, BB thickness and relative position of BB_PEAK were investigated to characterize the geometric structure of the BBs. The BB thickness increased as the Z_H at BB_BOTTOM increased, which indicated that large snowflakes melt more slowly than small snowflakes. The geometrical structure of the BBs was asymmetric, since the melting particles spent more time forming the thin shell of meltwater around them, and they rapidly collapsed to form a raindrop at the final stage of melting. Third, the heights of BB_TOP, BB_PEAK, and BB_BOTTOM were compared with the zero-isotherm heights. The dry-temperature zero-isotherm heights were between BB_TOP and BB_BOTTOM, while the wet-bulb temperature zero-isotherm heights were close to the height of BB_PEAK. Finally, we examined the polarimetric observations to understand the involved microphysical processes. The correlation among Z_H at BB_TOP, BB_PEAK, and BB_BOTTOM was high (>0.94), and the Z_DR at BB_BOTTOM was high when the BB’s intensity was strong. This proved that the size and concentration of snowflakes above the BB influence the size and concentration of raindrops below the BB. There was no depression in the ρ_HV for a weak BB. Finally, the mean profile of the Z_H and Z_DR depended on the Z_H at BB_BOTTOM. In conclusion, the growth process of snowflakes above the BB controls polarimetric observations of BB.