How do the Convective and Microphysical Characteristics of Extreme Precipitation over the Pearl River Delta at Monsoon Coast Vary with Increasing Rainfall Extremity?

Disagreement in the overlapping ratio between extreme precipitation and extreme convection lingers in literature, and their use of different rain-rate thresholds indicate different rainfall extremity examined and possible changes in relevant physical processes. In this study, the convective and microphysical characteristics of extreme precipitation events and their variations with increasing rainfall extremity over a monsoon coastal area in South China, i.e., the Pearl River Delta (PRD), are examined using 5 years of dual-polarization radar observations. In the original experiment, an Extreme Precipitation Feature (EPF) is defined as a continuous area of composite radar reflectivity greater than 40 dBZ that consists of at least 51 pixels (lower limit of the meso-γ-scale) and contains more than 8 connected pixels (0.5 km²) of extreme rain rate according to radar quantitative precipitation estimation. Two sensitivity experiments are conducted on the definition of EPFs using different thresholds of the strong-echo area and extreme-rain area.

Based on the 95^th, 99^th and 99.9^th percentiles of 1-min rain rates observed by automatic weather stations, the extreme precipitation features (EPFs) are classified into three groups with increasing rainfall extremity (ER1, ER2, ER3): 84 – 126 mm hr^-1, 126 – 186 mm hr^-1, and ≥186 mm hr^-1. The EPFs with maxHt_40dBZ above 9 km, between 6 km and 9 km, below 6 km are then categorized into “intense”, “moderate”, and “weak” convection, respectively. The vertical profiles of dual-polarimetric variables, the retrieved ice water content (IWC), liquid water content (LWC), and raindrop size distribution (RSD) are comparatively examined among the three EPF groups with increasing rainfall extremity. Results obtained from the two sensitivity experiments are consistent with the original experiment and thus support the robustness of major conclusions, which are as follows:

• Common features are observed in the three groups of EPFs despite their rainfall extremity. Most EPFs (about 70–90%) are meso-γ-scale convective elements embedded in non-linear shaped 20-dBZ precipitation regions, of which about 85–65% is on the meso-β-scale and 15–30% on the meso-γ-scale. The EPFs have a wide range of convective intensity from “weak” to “intense” convection with active warm-rain processes, and a major portion of EPFs (about 60–80%) contain moderate-to-intense mixed-phase microphysical processes. Coalescence dominates the liquid-phase processes (accounting for about 70%) and the RSDs feature a mean size larger than the “maritime-like” and a mean population much higher than “continental-like” regime.
• With the increasing rainfall extremity, the convective and microphysical characteristics change to various extents. The horizontal scales of 40-dBZ regions tend to increase significantly. The fractions of EPFs with “intense” and “weak” convection substantially increase (to 2.8–8 times) and decrease (to about 40–50%), respectively. The more extreme rainfall is accompanied by slightly enlarged 40-dBZ and 20-dBZ regions, enhanced mixed-phase processes, larger IWC and LWC (significant at 0.01 confidence level), slight increases (albeit statistically significant at 0.01 confidence level) in the mean size and number concentration of raindrops, and a roughly equal fraction of coalescence in the liquid-phase processes.

The above results based on thousands of EPF samples provide some new insights on the statistical features of extreme precipitation from the convective and microphysical perspectives in the monsoon coastal areas. In particular, this study illustrates a substantial increase with higher rainfall extremity in the overlapping ratio between extreme precipitation and extreme convection, which was unknown previously. Under the influences of the strong East Asian summer monsoon flows and the complex underlying surface (urban cluster, coastal terrain), the local atmosphere over PRD features with abundant moisture, moderate-to-large convective available potential energy, low lifting condensation level, deep warm-cloud layers, and mostly small-to-moderate vertical wind shears, collectively favoring development of both mixed-phase and warm-rain microphysical processes in non-linear shaped convective rainstorms.

In the future, comparisons of warm-sector and frontal heavy rainfall in the dependence of microphysical characteristics on rainfall intensity need further investigation. How the dependence varies with climate regimes also deserves further analysis. Moreover, microphysics of extreme precipitation accumulated over a longer period such as 1 to 6 hrs need examination to complement this and previous studies that have focused on extreme instantaneous rain rates.