A stationary snowband caused by land-sea thermal contrast in the prevailing alongcoast wind during the cold-air outbreak

During the cold-air outbreak, a stationary snowband is occasionally observed over the Sea of Japan along the coasts of Japan. The snowband brings heavy snowfall in the coastal areas, because it is maintained stationarily for a long time. When the snowband is formed, a southerly offshore wind is present in the coastal areas. Between the southerly and the westerly monsoon, a convergence zone is formed, which causes the snowband. Therefore, the southerly is a key factor for the formation of the snowband. Formation mechanisms of the southerly offshore wind are, however, not clear. In order to clarify the formation mechanisms of the stationary snowband causing heavy snowfall, the formation mechanism of the southerly offshore wind is studied using a Doppler radar and a high-resolution numerical model.

We first examined occurrence frequency of the stationary snowband along Hokuriku coast, which is located on the Sea of Japan side in central Japan. Eight cases were found for 6 winters from December 2003 to March 2009. The wind direction of the ordinary winter monsoon is northwest. On the other hand, in all the cases, strong west or west-southwest winds prevailed in the lower atmosphere in the Hokuriku District. Since the direction of the Hokuriku coastline is southwest, the stationary snowband along the Hokuriku coast occurs in the prevailing alongcoast wind. Composite fields show that an upper-level trough is located over the Korean Peninsula and a low-pressure area is present in the northern Sea of Japan at the sea level. This environment field provides the alongcoast wind predominant in the Hokuriku District.

We performed a numerical simulation, using a cloud resolving model CReSS (the Cloud Resolving Storm Simulator), for the case of 24 - 26 January 2009. The simulation well reproduced the precipitation amount and the location of the snowband in comparison with the observation of the Japan Meteorological Agency (JMA) radar. In the lower atmosphere, the convergence zone is formed between the environmental alongcoast wind and the southerly offshore wind. The convergence zone was also observed by a polarimetric Doppler radar of Nagoya University.

We performed sensitivity experiments for orography, roughness of land surface and thermal properties of the surface. The experiments showed that the southerly offshore wind is mainly formed by the effect of thermal contrast between the land and sea. The roughness difference between the land and sea and deflection by orography somewhat contributed to an intensification of the southerly offshore wind.

In order to clarify the dynamics of the southerly wind, we examined forces acting on air parcels arriving at the south of the convergence zone by a back trajectory analysis. The trajectories and the forces exerted on the air parcels were calculated using the result of the simulation. The northward component of the pressure gradient force increased as the air parcels approached the convergence zone along the trajectories over the land. The northward increase of the pressure gradient force was caused by a mesoscale high over the land. The meso-high is generated by the land-sea thermal contrast. The northward pressure gradient force accelerates the air parcels toward the north. This causes the southerly wind in the coastal region. The shift of wind direction over the land due to difference of roughness between the land and sea is small compared to that due to the pressure gradient force associated with thermal contrast between the land and sea.