Abstract
A thin upglide cloud (400km north of the surface warm front) and the deeper upglide cloud (200km north of the warm front) associated with the same warm frontal system were investigated. The microphysical structure and precipitation mechanisms associated with these clouds were studied using data collected with a hydrometeor videosonde (HVVIS) and a Doppler radar as well as routine rawinsonde and surface measurements operated by the Japan Meteorological Agency.
The HVYIS observations showed the following microphysical structures which were common to both the clouds. Columnar type crystals predominated throughout the whole cloud layer, and some capped columns and hexagonal plates were also observed below the -17°C level. These crystals were hardly rimed nor aggregated. Only in the melting layer of the deep upglide cloud were wet aggregates of these crystals found. No supercooled cloud droplets were found in the clouds, as expected from the lack of rime on the crystals. However, a low concentration of drizzle drops coexisted with snow crystals in the shallow layer (2 km deep) just above the 0°Clevel. These results demonstrate that the dominant mechanism of precipitaition formation in these clouds was the depositional growth of snow crystals above the warm-frontal surface.
In the thinner cloud, snow crystals rapidly evaporated in a dry layer (R.H. ∼30 %) just below the warm-frontal surface before they melted.
In the deep upglide cloud, the increase of the precipitation rate due to the collection of cloud droplets was less than 10% of the precipitation rate at the surface. Between the 0°C and -10°C levels, low concentrations of drizzle drops were observed and the air was almost water-saturated. In this region, snow crystals grew rapidly by vapor deposition and formed 80% of the total precipitation mass reaching the ground. Such a humid condition was produced by strong updrafts (with a maximum value of r∼30cm/s) associated with strong southerly flow over the frontal surface.
