Hailstorm Events over a Tropical Maritime Region: Storm Environments

Research on hailstorms has concentrated on mid-latitude weather systems due to their frequency, severity, and relatively good quality of hail reports. However, in recent years, locations in the Maritime Tropics have begun to report hail events. From 2017 to 2023, five hail events were reported in Surabaya, an urban coastal region and the second-biggest metropolitan area in Indonesia. To our knowledge, only a single case study of a hailstorm in Indonesia has been performed to date; a general study of hail events in this region is lacking. Such a study is needed to improve the prediction of hailstorms in Maritime Tropical environments. The goal of this study is to distinguish between the environmental characteristics of hailstorms and non-hail convective events in the Surabaya, Indonesia region, as a first step towards establishing environmental conditions required for hailstorms in Maritime Tropical regions.

Simulations of past storm events are conducted with the Weather Research and Forecasting (WRF) model to examine the storm morphology, precipitation, including hail, and updraft strengths among five hailstorms and five convective storms that did not have corresponding hail reports. Weather radar, sounding data, and hail reports are used to verify the simulated storms. Then, characteristics of the two types of environments, such as instability parameters related to updraft strength, vertical wind shear, moisture content, melting layer height, and local sea-surface temperature, are compared for the hail and non-hail cases. Preliminary results suggest that hail events in Surabaya were mostly single-cell storms, occurring in the afternoon and having a short lifetime (e.g., about 30 minutes to 2 hours). The hailstorms tend to occur in the peak-to-late rainy season (January, February, and March), and the storm forcings were either sea breeze fronts or local lifting due to solar heating interacting with the westerly monsoon flow. Differences in the hail-producing and non-hail-producing storms will be presented, along with commentary comparing the Maritime Tropical hailstorm environments with those typical of continental mid-latitude hailstorm environments in the United States.