The Characteristics and Evolution of Orographic Snow Clouds under Weak Cold Advection

Abstract

On 25 February 1999, on the western side of the central mountain range of Japan, orographic snow clouds had formed under conditions of weak cold advection due to a winter monsoon after a cyclonic storm. The data from Ka-band Doppler radar, microwave radiometer, hydrometeor videosondes, and 2D Grey imaging probe provided unique datasets that were used to analyze the evolution of meso- and microscale structures, especially ice and supercooled liquid water (SLW) evolutions associated with variations in surrounding conditions. In the present case, four stages were identified in the evolution of the clouds: stratiform (I), transition (II), shallow convective (III), and dissipating (IV). During stage I, substantial blocking of the low-level flow occurred. The echo top was relatively flat and the echo pattern was stratiform with a bright band. The clouds were considered to be almost glaciated, primarily by the deposition growth of ice crystals. The wind speed up the slope gradually increased in the latter half of stage I and reached maximum intensity at stage II. Simultaneously, the LWP increased and the hydrometeors observed at the surface and aloft indicated heavy riming by accretion of supercooled droplets. During stage III, the wind up the slope weakened and the clouds became shallower and more convective. Snowflakes and aggregated snow images detected frequently at the surface suggested that some of the mass in the precipitation was due to depositional growth, which is consistent with the clouds of low LWP [high ice water path (IWP)] during stage III. In stage IV, the clouds dissipated as a relatively warm/dry airflow from the south dominated during stable conditions. These results indicate that SLW was associated primarily with orographic lifting but not with convection. The dominance of SLW was reflected in the enhanced flow up the slope that should increase the upward air motion. The convective clouds showed only small amounts of SLW and an abundance of ice crystals.