A Composite Analysis of Summer Afternoon Heavy Precipitation Events in the Taipei Basin Under Weak Synoptic Forcing and PM2.5 Changes

Abstract

Taipei, Taiwan is a city affected by various types of precipitation. However, one of the most impactful short-term sources of precipitation is afternoon thunderstorms. Understanding the trends of these afternoon thunderstorms and the extent to which PM2.5 impacts precipitation totals is imperative for improving forecasting and developing urban infrastructure that can deal with said totals.

The present study looks at the top 5% heavy rain (HR) events in the Taipei Basin during the summer months of June, July, and August from 2005 to 2015. The goal was to find out whether changes in the PM2.5 concentration over the Taipei Basin has had any impact on HR events in the summer months. The total PM2.5 concentration during the same period was analyzed and compared to various characteristics of the HR events. Heavy rain events were defined as having a total rainfall of at least 81.24 mm/day and occurred in a weak synoptic environment. 55 days met the criteria. The rainfall during each day was then analyzed by its temporal and spatial distribution. Rainfall trends were compared to PM2.5 trends. Spatial and temporal analyses of PM2.5 trends for the summer season and each of the HR days were used for comparison. A background summer mean was computed for the decade (2005-2015) and was used as a constant background state value to compare the daily concentration fluctuations. It was found that in only some events the PM2.5 concentrations responded actively to precipitation. Overall concentrations on HR days did not vary significantly from the summer mean.

Further, three rainfall case studies were selected: a Mei-Yu event, a single HR day, and three consecutive HR days. The day before the HR event and the day after were included in the analysis for each case study. The case studies only provided insight into the effect that rainfall has on PM2.5 concentrations but not into the effect that PM2.5 may have on rainfall.

NTU WRF-ARW was used to simulate various PM2.5 concentrations (clean, average, dirty) and see if precipitation spatial and temporal characteristics varied. Changes in aerosol concentrations affected precipitation. The changes were reflected by changes in the timing (initiation and duration) and distribution (location of heaviest rainfall) of the rain. PM2.5 likely affected the microphysics of the storm cloud development, changing the initial conditions for the storm itself. While the exact way that PM2.5 influences HR development is unknown, it is clear that increased concentrations affect rainfall rates in the Taipei Basin.