Hailstorm events over a maritime tropical region: a study of environments and microphysics of Surabaya hail events, Indonesia

Sari, Fitria Puspita

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https://hdl.handle.net/2142/132767 Copy

Description

Title

Hailstorm events over a maritime tropical region: a study of environments and microphysics of Surabaya hail events, Indonesia

Author(s)

Sari, Fitria Puspita

Issue Date

2025-11-24

Director of Research (if dissertation) or Advisor (if thesis)

Trapp, Robert J

Doctoral Committee Chair(s)

Trapp, Robert J

Committee Member(s)

• Nesbitt, Stephen W
• Riemer, Nicole
• Kristovich, David A

Department of Study

Climate Meteorology & Atm Sci

Discipline

Atmospheric Sciences

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Hailstorms
• Hail
• Maritime Tropics
• Indonesia
• Environmental
• Microphysics
• NetCAPE

Abstract

In recent years, several hail events have been observed in the Maritime Tropics (mT), including five occurrences in the urban area of Surabaya, Indonesia. Past studies of mT hailstorms have been limited to individual case studies and often have restricted accessibility to a general international audience, leading to an under-representation of hail research in this region within global hail studies, even though this type of severe convective storm (SCS) is among the costliest weather events. Therefore, this study is presented to explore in more depth the hailstorm environments in mT regions and their implications for cloud microphysical processes related to hail production. Using real-case simulations with the Weather Research and Forecasting (WRF) Model, we reproduced ten thunderstorms over Surabaya: five documented hail events and five additional no-hail storms for comparison. We found that common environmental predictors of hail used for mid-latitude SCS events, such as Convective Available Potential Energy (CAPE) and deep-layer bulk shear, were less useful for this region. Consequently, we proposed a new metric for predicting hail environments in maritime tropical areas, namely NetCAPE, which incorporates water-loading correction and accounts for the net values of CAPE and CIN across several atmospheric layers within the defined hail growth zone (HGZ), from freezing level up to –20°C. The median low-level NetCAPE for the five hail events was over 40% greater within various depths of the HGZ. When combined with lower near-surface (surface to 925 hPa) relative humidity and lower melting level heights, it effectively differentiated hail from no-hail cases. We also classified the hail environments based on this metric into three main types, the most common of which are single-cell, pulse-type, and short-lived storms. We further tested the NetCAPE variable—particularly the value from the surface to the 0°C level, termed NetCAPE0C—to understand why this variable effectively predicts hail environments in these specific mT regions. We conducted idealized simulations using the Cloud Model 1 (CM1) for typical single-cell and pulse-type hailstorms in this environment. In general, we found that as the NetCAPE0C in the simulated environment increased, the maximum hail size near the ground also increased. Our results further show that NetCAPE0C is effective because the layer extending from the surface to the 0°C isotherm encompasses the region where supercooled liquid water content (SLWC) is maximized—typically concentrated near the –5°C isotherm. NetCAPE0C also controls the updraft width and depth, especially in the earlier stages of storm development, where larger values promote quicker raindrop formation and thus accelerate riming and hail growth. In these simulated pulse storms, we also learned that hail growth in this mT region is generally constrained by three mechanisms: (1) the rapid warm-rain process, which, under weaker updrafts, leads to earlier precipitation fallout and reduces the chance for raindrops to be lifted higher and later freeze as hail embryos; (2) an imbalance between updraft strength, abundant SLWC, and a shallow HGZ, which confines most SLWC near the freezing level instead of higher in the HGZ, thereby reducing the vertical distance available for riming and limiting hail size; and (3) relatively simple hail trajectories, which primarily undergo wet growth with abundant shedding while descending just above the 0°C level. Finally, the results of this study reveal the distinct environmental and microphysical characteristics of hailstorm development specific to mT regions, which differ markedly from those of mid-latitude supercell-type hailstorms. Nonetheless, future work should aim to conduct broader studies, including a climatology of hail events across different areas within mT regions, to assess potential local influences of topography and land use. This would also allow testing whether the proposed NetCAPE metric is robust in predicting hail events across a wider range of storm types and regional environments. Such climatological analyses are expected not only to identify more general and robust environmental predictors but also to investigate how future climate warming, land-use change, and aerosol variations might affect hail occurrence and size in these regions. This effort would ultimately strengthen the understanding and generality of mT hailstorm behavior and address questions that could not be fully answered in the present study.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132767

Copyright and License Information

Copyright © 2025 by Fitria P. Sari. All rights reserved.

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