Comparison of Drop Size Distributions During Two Stages of An Extreme Rainfall Event over Shanghai Associated with Typhoon Fitow (2013)

Typhoon Fitow (2013) caused heavy rainfall in Shanghai after it made landfall on the border between Zhejiang and Fujiang provinces in China. The precipitation in Shanghai was caused by the passage of rainbands during two stages. The first stage was associated with Fitow’s outer rainbands, which were embedded beneath a widespread stratiform cloud (Stage I), and the second was caused by a coastal front-like rainband as Fitow dissipated (Stage II). This study examines the evolution of raindrop size distributions (DSDs) measured by an optical particle size velocity (PARSIVEL) disdrometer in Shanghai.

The DSD analysis shows that the total rainfall duration and amount was similar in each stage, but the convective rainfall associated with the outer rainbands in Stage I had higher drop concentration, greater rainfall rate and water content, whereas the coastal front-like convective rainfall in Stage II had a longer duration and larger mass-weighted mean diameter, resembling a continental convective rain. As a result of the higher concentration of midsize drops, the Z–R relationship coefficient was lower for the convective rain in Stage I, similar to the climatic value for eastern China during the summer, implying that the rainfall recorded in typhoon systems is more intense than that in hurricane and frontal systems for a given radar reflectivity. Based on the backward trajectories of air particles, the different rain formation mechanisms are discussed to account for the different microphysical characteristics of the two stages.