Possible Link of Extreme Hourly Precipitation to Urbanization in Coastal South China

The understanding of past changes in sub-daily extreme precipitation at regional and local scales is severely limited, due to lack of long-term high-resolution observations as well as knowledge on complex interactions among mesoscale dynamics, microphysics, and local underlying surface forcing such as urbanization and surface heterogeneity. The Pearl River Delta (PRD) region, located in coastal South China, is exposed to high flood-induced risk accompanied by rapid urbanization since early 1990s. This study investigates changes in extreme hourly precipitation (EXHP) in coastal South China, focusing especially on its possible link to the PRD urbanization through: 1) exploring long-term changes of hourly extremes (defined by the 95^th percentile) using gauge-based hourly precipitation observations at 69 surface stations over 46 years (1971-2016); and 2) analyzing the 120 extreme rainfall events (EXRE) (with at least one record of >60 mm h^-1 hourly rainfall in the PRD region) in 2011-16 using observations collected by densely-distributed automatic weather stations (AWSs) and radar network in coastal South China.

The spatial distributions of the long-term trends in both the occurrence frequency and intensity of EXHP in coastal South China show a distinct urban rain-island feature. Statistically significant positive trends are mostly observed in the PRD region, with stronger positive trends during the rapid urbanization period (1995-2016) than during 1971-2016. Such trends are closely related to a statistically significant increasing trend in occurrence of the short-duration (≤ 6 h), abrupt extreme rainfall events, rather than the longer-duration, continuous rainfall events. In contrast, only a few stations outside of PRD show significant positive trends with the 1995-2016 rates being comparable or smaller than the corresponding 1971-2016 rates.

The 120 EXRE in 2011-16 are categorized into 1) locally developed storms (52.5%), either under the influence of prevailing southwesterly winds in the lower troposphere or accompanied by a synoptic shear line in the planetary boundary layer over South China, namely, the local/SW wind (31.7%) and local/shear line (20.8%) types, respectively, and 2) migratory storms (47.5%) moving from the northwest (28.3%), southwest (10.8%), northeast (5.0%), or south (3.3%) to influence PRD, namely, the migratory-NW, -SW, -NE, or -S types. Rainfall intensification by the PRD urban heat island (UHI) is noticeable in all the six types, although it is less evident for the migratory-SW and -S types, at least partially because that the surface warm center is located over the inland (northern) PRD with lower potential temperatures in the southern PRD. Moreover, without the UHI the local/SW wind type mostly initiates from late night to early morning in close association with enhanced nocturnal southwesterly low-level jets; whilst with weaker southwesterly winds from the midday to evening the UHI appears to play an important role in the initiation and intensification of this EXRE type.

These observational analyses provide a basis for numerical modeling study to help understand the roles of global climate warming and the local urban environment, respectively, on the changes of EXHP in South China. Preliminary results from such modeling study will also be discussed at the meeting.