Abstract
This paper analyzes the behavior of the precipitation cores (PCOs) in three cumulonimbus clouds that caused localized heavy rainfall on 18 August 2011 in the Kanto region, Japan, and their relationship with temporal variations in surface rainfall intensity. The 3D structure of the cumulonimbus clouds was observed at 2-min intervals using a research X-band dual-polarization radar. A PCO was defined as a 3D contiguous region that contained one local maximum of horizontal radar reflectivity Zh, and the PCOs were automatically detected using adaptive thresholds. Subjective tracking procedures using the PCO dataset, which was based on observations made every 2 min, identified 15 PCOs during the total lifespan of the three cumulonimbus clouds.
The PCOs generally descended toward the ground after their appearance aloft. The average appearance height of the PCOs of 5.25 km above sea level (ASL) was slightly above the ambient 0°C level (5.1 km ASL). The duration of each PCO was roughly proportional to its appearance height.
Of the 12 temporal peaks at maximum surface rainfall intensity (> 10 mm h-1) recorded from the three cumulonimbus clouds, 10 were associated with the descent of PCOs. In each cumulonimbus cloud, the first PCO was detected 10-12 min before rainfall heavier than 10 mm h-1 was recorded. These results indicate that the behavior of PCOs is closely related to the onset of strong surface rainfall and subsequent fluctuations in surface rainfall intensity.
