Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Friday the 13th Hailstorm in the province of Bulacan, Philippines (13 August 2021): A Case Study

Generich H. Capuli

TL;DR

The paper addresses the occurrence of a large hail event in Bulacan, Philippines, by reconstructing the pre-convective environment and storm evolution using ERA5 reanalysis, HIMAWARI-8 satellite data, and PAGASA lightning observations. It employs environmental sounding analyses, ECAPE and LCL-EL shear diagnostics, and multi-sensor storm-tracking to explain how a transition from a capped to an uncapped, moist, high-instability profile supported a hail-producing updraft. Key contributions include integrating ERA5, HIMAWARI-8 Daytime Convective RGB, PLDN lightning data, and citizen hail reports to characterize an easterly severe-weather setup and overshooting-top signatures linked to hail up to several centimeters. The findings have practical implications for hail nowcasting and risk assessment in data-limited tropical regions of the Philippines, illustrating how upslope flows and convective interaction can drive severe hail in the absence of radar.

Abstract

This case study presents a thorough investigation of the environmental setup that led to the hail-producing severe storm that impacted the municipality of Norzagaray and City of San Jose Del Monte, including other nearby areas, in the province of Bulacan on the afternoon of August 13, 2021. During this period, 2-5 cm and potentially as large as $\sim$8 cm diameter hail was reported over these locations of Bulacan. For this purpose, the combination of HIMAWARI-8 AHI, PLDN and its flash counts, and meteorological indices; synoptic, thermodynamic, and kinematic indices, calculated from the ERA5 reanalysis are utilized to understand the nature of the hail event. In the morning, the pre-convective environment was comprised by a warm inversion layer that inhibited storm initiation, until the arrival of ample moisture and convective heating in the afternoon. By the afternoon, model sounding analysis revealed that the environment transitioned into uncapped profile with steep low-level lapse rate owing to warm, moist south-westerly wind flow from the Manila Bay in the lower troposphere and north-easterlies aloft crossing the SMMR induced by a weak low-pressure system located in the eastern Philippine Sea, with minimal turning on the wind profile. This promoted low-level convergence within the area of interest and build up of instability. The updraft associated with convectively unstable atmosphere, sufficient cloud-layer bulk shear, and storm nudging at its maturing phase countered entrainment-driven dilution and aided the growth of ice crystals by rapid collection of supercooled cloud liquid particles, which ultimately led to formation of hailstones.

Friday the 13th Hailstorm in the province of Bulacan, Philippines (13 August 2021): A Case Study

TL;DR

The paper addresses the occurrence of a large hail event in Bulacan, Philippines, by reconstructing the pre-convective environment and storm evolution using ERA5 reanalysis, HIMAWARI-8 satellite data, and PAGASA lightning observations. It employs environmental sounding analyses, ECAPE and LCL-EL shear diagnostics, and multi-sensor storm-tracking to explain how a transition from a capped to an uncapped, moist, high-instability profile supported a hail-producing updraft. Key contributions include integrating ERA5, HIMAWARI-8 Daytime Convective RGB, PLDN lightning data, and citizen hail reports to characterize an easterly severe-weather setup and overshooting-top signatures linked to hail up to several centimeters. The findings have practical implications for hail nowcasting and risk assessment in data-limited tropical regions of the Philippines, illustrating how upslope flows and convective interaction can drive severe hail in the absence of radar.

Abstract

This case study presents a thorough investigation of the environmental setup that led to the hail-producing severe storm that impacted the municipality of Norzagaray and City of San Jose Del Monte, including other nearby areas, in the province of Bulacan on the afternoon of August 13, 2021. During this period, 2-5 cm and potentially as large as 8 cm diameter hail was reported over these locations of Bulacan. For this purpose, the combination of HIMAWARI-8 AHI, PLDN and its flash counts, and meteorological indices; synoptic, thermodynamic, and kinematic indices, calculated from the ERA5 reanalysis are utilized to understand the nature of the hail event. In the morning, the pre-convective environment was comprised by a warm inversion layer that inhibited storm initiation, until the arrival of ample moisture and convective heating in the afternoon. By the afternoon, model sounding analysis revealed that the environment transitioned into uncapped profile with steep low-level lapse rate owing to warm, moist south-westerly wind flow from the Manila Bay in the lower troposphere and north-easterlies aloft crossing the SMMR induced by a weak low-pressure system located in the eastern Philippine Sea, with minimal turning on the wind profile. This promoted low-level convergence within the area of interest and build up of instability. The updraft associated with convectively unstable atmosphere, sufficient cloud-layer bulk shear, and storm nudging at its maturing phase countered entrainment-driven dilution and aided the growth of ice crystals by rapid collection of supercooled cloud liquid particles, which ultimately led to formation of hailstones.

Paper Structure

This paper contains 11 sections, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Materials and Methods
  3. Environmental Reanalysis Dataset
  4. Vertical Profile
  5. HIMAWARI-8 Satellite Data
  6. PAGASA Lightning Network
  7. Results
  8. Overview of the Synoptic Setup
  9. Mesoscale Environment
  10. Storm evolution from Satellite Perspective
  11. Conclusion

Figures (9)

  • Figure 1: Various citizen hail reports in the municipalities of Norzagaray, San Jose Del Monte, and Sta. Maria, Bulacan. Citizen hail samples are included.
  • Figure 2: HIMAWARI-8 AHI scans at 07 UTC; (a) Band 03 0.64 µm with Cloud-layer Shear Vectors (kt) and 300-hPa Geopotential heights (blue; dam), and (b) Daytime Convective RGB with Cloud-layer Shear Magnitudes $>$ 50 kts. The area of interest is demarcated in yellow box.
  • Figure 3: 07 UTC ERA5 Vertical Cross Section of Cloud Ice (shaded) and Liquid Mixing Ratios (dashed; g kg$^{-1}$). The FZL; denoted as the 0 $^{\circ}$C isotherm, and HGZ is also identified as blue dotted lines.
  • Figure 4: Synoptic environment of the Philippine archipelago before (06 UTC), during (07 UTC), and after (08 UTC) the event. (a) 500-hPa Geopotential heights (blue; dam) and winds (kt). Contours are wind speeds $>$ 5 kt. (b) 850-hPa Geopotential heights (blue; dam), temperature (red; $^{\circ}$C), and winds (kt). Contours are wind speeds $>$ 5 kt. (c) Mean Sea Level Pressure (MSLP; red; hPa), Vertical Integrated Moisture Flux Convergence within the first 3 km layer (VIMFC; Kg m$^{-2}$ s$^{-1}$), and its wind components (quivers).
  • Figure 5: 00 UTC Tanay, Rizal Proximity and 06-08 UTC ERA5 Model Sounding Profiles. Legends are included in the upper-left corner of the figures. Annotations to the thermodynamic profiles include the SBLCL, PBL, MULFC, FRZ, and MUEL.
  • ...and 4 more figures