5:15 PM - 7:15 PM
[AAS10-P04] Analysis of the Graupel formation process in inland Ishikari Plain using X-band dual-polarization radar: Band-Shaped Snow Cloud on December 22, 2023
Keywords:Graupel Particle, Dual-Polarization Radar, Ishikari Plain
In the winter months of the Ishikari Plain, localized snowfall is frequently caused by band-shaped clouds aligned with the seasonal winds, as well as snowfall clouds formed along the discontinuity line between northeast to east winds originating from inland Hokkaido and the prevailing seasonal winds. Studies on the distribution of snowfall particles in the Ishikari Plain have reported several cases based on the concept of size sorting, where the proportion of graupel particles is higher in coastal areas (within 15 km from the coast) and lower in inland areas (more than 15 km from the coast) (Harimaya and Sato, 1992; Harimaya and Kanemura, 1995; Harimaya et al., 1999).
In contrast, the band-shaped snowfall cloud observed between 0800 and 0900 JST on December 22, 2023, which was the focus of this study, showed regions with peak average reflectivity (25–30 dBZ) both in the coastal and inland areas, suggesting the generation of graupel particles even in the inland region (Miki et al., 2024). Therefore, the study primarily focused on the formation process of high-reflectivity snowfall regions using X-band dual-polarization radar (X-MP) for hydrometeor classification (HC) and dual-Doppler analysis. The analysis was conducted using the X-MP systems installed in Ishikari and Kitahiroshima by the Ministry of Land, Infrastructure, Transport, and Tourism. HC methods based on Kouketsu et al. (2015) were applied, utilizing X-MP data and temperature and humidity initial values from the Japan Meteorological Agency's Local Forecast Model (LFM) to investigate the distribution of snowfall particles within the cloud. In addition, dual-Doppler analysis using two X-MP radars (Protat and Zawadzki, 1999) was conducted to analyze the vertical flow and horizontal divergence fields within the cloud. By analyzing the time variations in three-dimensional reflectivity at five-minute intervals, it was confirmed that precipitation particles with high reflectivity, which had been advected from the sea, were falling in the coastal region.
This region was primarily classified as DG (Dry Graupel) in the HC, corresponding to areas with strong updrafts (approximately 2 m/s). In contrast, no distinct cellular structure was observed in the inland area. In this region, dual-Doppler analysis revealed weak updrafts (approximately 1 m/s) at around 2 km altitude, and the HC results showed that the DG distribution expanded from around 2 km altitude to the surface. The DG distribution reached the surface within a range of 25–35 km from the coast, correlating with the region of maximum reflectivity in the inland area. Additionally, near the surface within the region of maximum reflectivity, the correlation coefficient between polarizations (ρhv) increased ahead of the expansion of the DG distribution, reaching values above approximately 0.995. These results suggest that in the inland region, particles such as snow crystals generated by the strong updrafts in the coastal region were advected by the horizontal wind and, under weak updrafts at altitudes above 2 km in the inland region, grew into high-reflectivity particles classified as DG, primarily through the cloud particle capture process. Furthermore, the change in ρhv approaching nearly 1 before the expansion of the DG distribution indicates a gradual increase in the proportion of DG particles, becoming the dominant particle type. These findings suggest that as snowfall clouds redevelop in the inland region, there is a possibility of localized increases in snowfall accumulation.
In contrast, the band-shaped snowfall cloud observed between 0800 and 0900 JST on December 22, 2023, which was the focus of this study, showed regions with peak average reflectivity (25–30 dBZ) both in the coastal and inland areas, suggesting the generation of graupel particles even in the inland region (Miki et al., 2024). Therefore, the study primarily focused on the formation process of high-reflectivity snowfall regions using X-band dual-polarization radar (X-MP) for hydrometeor classification (HC) and dual-Doppler analysis. The analysis was conducted using the X-MP systems installed in Ishikari and Kitahiroshima by the Ministry of Land, Infrastructure, Transport, and Tourism. HC methods based on Kouketsu et al. (2015) were applied, utilizing X-MP data and temperature and humidity initial values from the Japan Meteorological Agency's Local Forecast Model (LFM) to investigate the distribution of snowfall particles within the cloud. In addition, dual-Doppler analysis using two X-MP radars (Protat and Zawadzki, 1999) was conducted to analyze the vertical flow and horizontal divergence fields within the cloud. By analyzing the time variations in three-dimensional reflectivity at five-minute intervals, it was confirmed that precipitation particles with high reflectivity, which had been advected from the sea, were falling in the coastal region.
This region was primarily classified as DG (Dry Graupel) in the HC, corresponding to areas with strong updrafts (approximately 2 m/s). In contrast, no distinct cellular structure was observed in the inland area. In this region, dual-Doppler analysis revealed weak updrafts (approximately 1 m/s) at around 2 km altitude, and the HC results showed that the DG distribution expanded from around 2 km altitude to the surface. The DG distribution reached the surface within a range of 25–35 km from the coast, correlating with the region of maximum reflectivity in the inland area. Additionally, near the surface within the region of maximum reflectivity, the correlation coefficient between polarizations (ρhv) increased ahead of the expansion of the DG distribution, reaching values above approximately 0.995. These results suggest that in the inland region, particles such as snow crystals generated by the strong updrafts in the coastal region were advected by the horizontal wind and, under weak updrafts at altitudes above 2 km in the inland region, grew into high-reflectivity particles classified as DG, primarily through the cloud particle capture process. Furthermore, the change in ρhv approaching nearly 1 before the expansion of the DG distribution indicates a gradual increase in the proportion of DG particles, becoming the dominant particle type. These findings suggest that as snowfall clouds redevelop in the inland region, there is a possibility of localized increases in snowfall accumulation.