Development Process of Convective Cells in Coastal and Inland Areas Within a Snow Streak: a Case Study Using X-Band Polarimetric Radar in the Ishikari Plain, Japan During a Winter Monsoon Surge

Abstract Snow streaks associated with winter monsoon surges frequently produce coastal snowfall over the Ishikari Plain, Hokkaido. However, the processes responsible for inland precipitation containing graupel remain less well understood than those in coastal areas. This study investigated a snow streak from the coast to approximately 40 km inland using two X-band polarimetric radars, dual-Doppler wind retrievals, and hydrometeor classification. In the coastal region, a convective cell with radar reflectivity exceeding 35 dBZ made landfall within approximately 10 km of the coast, accompanied by an updraft stronger than 2 m s⁻¹. Positive $$\\:{Z}_{DR}$$ and relatively high $$\\:{\\rho\\:}_{hv}$$ were consistent with a substantial contribution from graupel-type particles. The cell weakened rapidly inland. Farther downstream, ice-crystal-type particles may have been present at altitudes of 2–3 km. In the inland region 20–40 km from the coast, weak updrafts of 1 m s⁻¹ developed at altitudes of 2–4 km under relatively humid lower-tropospheric conditions. As radar reflectivity increased to approximately 30 dBZ, the contribution of graupel-type particles increased below approximately 2 km, where $$\\:{Z}_{DR}$$ was near zero or negative and $$\\:{\\rho\\:}_{hv}$$ approached unity. These observations suggest that riming in weak inland updrafts may have promoted rapid particle growth and inland graupel formation.