The development and intensification of multiple misocyclones in shallow cumulus convection over the warm ocean during winter cold outbreak

Recent observational studies of cold-air outbreaks in a coastal region in northern Japan in winter have revealed that there are unexpectedly frequent occurrences of microscale vortices (e.g., misocyclones and waterspouts) embedded in winter convective storms that develop over the Japan Sea. These vortices in some instances evolve into well-defined tornadoes. One of the unique features of winter-season convection over the Japan Sea is that the environmental condition for convection is very unstable owing to high sea surface temperature (SST) relative to cold-air outbreak from Siberia (i.e., winter monsoon). Because of this unstable state in addition to cold outbreaks, cumulus convection actively develops over the warm sea, which leads to heavy snowfall and severe lightening over the coastal regions to the Japan Sea. Another unique feature is that the cloud-top height is generally lower than that of a common cumulonimbus cloud whose top reaches the tropopause; therefore, the measure of stability indices appears to indicate less unstable: e.g., convective available potential energy (CAPE) amounts at most to some hundreds. The observations indicate that multiple misocyclones are generated within the convective systems that are organized by shallow cumulus convection over the Japan Sea. The present study investigates the development and intensification of multiple misocyclones in a banded convective system during the winter cold outbreak by conducting very-high-resolution simulations with the Weather Research and Forecasting (WRF)-Advanced Research WRF (ARW) model. The high-resolution simulation incorporates a high-resolution topography dataset in order to better represent the terrain features and hence to realistically represent surface wind variability that is affected by complex topography. A case on 1-2 December 2007 over the Shonai Plains, Yamagata Prefecture, Japan is chosen for the present analysis. The Doppler Radar observation indicated that a banded convective system had four misocyclones, one of which evolved into a 0-Fujita-scale tornado.

The control simulation is performed over the four nested domains with the innermost domain having a horizontal grid spacing of 80 m. The control simulation well represents well-defined multiple vortices (with the vorticity of about 0.1 s^-1 and the size of 500 m) embedded in a precipitating convective band. These microscale vortices are generated at the leading edge of a weak cold-air outflow from the banded convective system. At the outflow boundary, the horizontal wind shear is significantly large. The vortices extend vertically up to the 1.5-km level and are seen to be enhanced by the convective updrafts that develop at the leading edge of cold outflow. Although the height of the cloud top is only at most 5 km and CAPE is relatively low (at most a few hundreds), the convective updrafts exceeding 10 m/s are sufficiently strong to enhance the vortices. Considering that the low-level vertical shear as well as the cold outflow is weak, the source of the vortices seems to be not in the vertical shear but in the horizontal shear. This intensified horizontal shear as well as strong convective updrafts plays a major role in enhancing the vortices. In addition to the enhancement of convective updrafts, the vortices themselves provide low-level convergence within the horizontal shear zone, which further intensifies the strength of the vortices. Although the amount of CAPE is not so large as compared to the environments for severe convective storms in other midlatitude regions, the instability is concentrated in the lowest levels; the surface layer exhibits an absolutely unstable state even for dry convection. The strong updrafts and multiple vortices in shallow convection are a unique feature of the convection over the Japan Sea where high SST enhances surface heat fluxes to produce an extremely unstable state in the lower atmosphere.

A sensitivity simulation with SST uniformly decreased by 5 K is also performed in order to examine the effects of warm SST. Although the overall features of the synoptic-scale and mesoscale meteorological disturbances are less affected by the SST change, the strength of convective updrafts within the mesoscale precipitating convective band is significantly reduced. The reduced convective updrafts lead to the reduction in the number and strength of microscale vortices. Therefore, high SST provides an environment favorable for the development and enhancement of convective clouds and hence the associated microscale vortices. However, even decreasing SST, the lowest atmosphere indicates an unstable state, and therefore strong convection occurs, in spite of less degree of organization, to produce occasionally intense vortex in the horizontal shear zone. In other words, high SST is favorable for the organization of convection and the generation of multiple vortices within the convective system. The warm sea surface is regarded as a unique feature for the generation of microscale vortices such as misocyclones and waterspouts in the coastal regions of Japan during the winter-monsoon season.

From the comparison of surface winds simulated by the WRF model and observed by dense surface observation network, the present 80-m grid simulation captures observed high wind speeds due to the strong vortices, which indicates that resolving microscale disturbances is important for the representation of strong wind events in numerical weather prediction models.