Raindrop Size Distribution (RSD) Characteristics during the Southwest Monsoon Period in Western Luzon, Philippines

Abstract

The raindrop size distribution (RSD) characteristics over western Luzon during the Southwest Monsoon (SWM) periods from 2020–2022 are examined using a network of OTT PARSIVEL2 disdrometer. The three-year worth of RSD observations was collected from four disdrometer stations – namely, Science Garden, Quezon City (SG; 14.6° N, 121.04° E); La Mesa watershed, Quezon City (LM; 14.7° N, 121.07° E); Clark, Pampanga (CK; 15.18° N, 120.5° E); and Malolos, Bulacan (MS; 14.8° N, 120.8° E). Results show significant RSD differences between MS and CK stations. On average, the RSD over the MS station is dominated by a high concentration of relatively smaller raindrops, whereas large raindrops are more prevalent over the CK station. On the other hand, similar RSD properties and integral rainfall parameter (IRP) values were observed over SG and LM. In all the stations, values of mass-weighted mean diameter (Dm) and normalized intercept parameter (Log10 Nw) are higher in convective than stratiform rainfall. However, convective rainfall over the CK station has the highest values of mass-weighted mean diameter (Dm) during the SWM period. The diurnal variation of RSD also showed higher Dm over CK station, especially in the late afternoon to early evening hours. Radar reflectivity–rain rate (Z-R) relations derived from the RSDs at each station showed a clear difference between stratiform and convective rainfall. The Z-R relations obtained from each station are found to be different from the conventional Marshall and Palmer relation but similar to Rosenfeld’s tropical relation. The shape-slope (μ-Λ) relations derived from the gamma RSD are also shown to be distinct at each location. Despite being influenced by a similar synoptic system during the SWM period, the geographical setting of the stations and orography may be the primary reasons for the observed RSD differences.