Evolutions of low-level misocyclones observed within a convective snowband during a cold-air outbreak over the Japan Sea area

High-resolution Doppler data of low-level misocyclones were collected within a convective snowband by two X-band Doppler radars on 31 December 2007 during a cold-air outbreak over the Japan Sea area. The misocyclones developed along the low-level convergence line with across-front cyclonic shear within the transversal snowband. Low-level reflectivity field showed a staircase pattern, likely due to the distortion of the snowband near individual misocyclones.

In association with the passage of right side of the misocyclone, surface wind gust was observed. The strongest ground-relative winds were observed on the right side of the misocyclone relative to its motion, where translation and rotation effects are in the same direction. The comparison between the surface wind variations and the radar-observed misocyclone suggested that the surface circulation was almost the same size and intensity as that observed several hundred meters above ground level. It also suggested that the misocyclone tilted leeward with increasing height in its dissipating stage.

An interesting evolution of the low-level misocyclone was observed. The misocyclone was first confined to the low level (~500 m AGL). As the reflectivity aloft increased, the misocyclone propagated upward to at least more than half of the depth of the convective snowband (up to ~2.5 km AGL). The low-level misocyclone rapidly contracted and intensified soon after its upward propagation, suggesting the stretching of vertical vorticity. The intensification of the rear inflow within the snowband was observed almost concurrently with the low-level misocyclone contraction. The gradual dissipation of the misocyclone was observed after the convergence line became highly occluded and the snowband made landfall, losing heat and moisture supply from the warm sea surface. The observed evolution process of the misocyclone is compared with the previous observations and numerical simulations of misocyclones and nonsupercell tornadoes in much deeper convective storms.