Spatiotemporal Variability and Recent Anomalies of ERA5-Derived Precipitation Extremes over North Sumatra, 2011-2025

The Weather's Hidden Rhythm

Imagine the weather not just as a single number on a thermometer or a total bucket of rain for the year, but as a complex, living story. In the world of meteorology, scientists have long known that "average" can be a trickster. A year might have the exact same amount of rain as the year before, but the way that rain falls could be completely different. Instead of gentle, steady showers, the sky might dump everything in a few terrifying, violent bursts, or it might hold back for weeks, leaving the earth parched. This is the realm of "precipitation extremes"—the study of the most intense, the longest, and the wildest swings in rain.

Why does this matter? Because our world is built on the assumption that rain will behave somewhat predictably. But in tropical places like Indonesia, where the ocean, mountains, and islands dance together to create the weather, things are getting stranger. Scientists use special tools to measure these wild swings, looking at things like the "wettest single day," the "longest dry spell," or the "total rain from the heaviest storms." By tracking these specific characters in the weather story, researchers can spot changes that a simple "total rainfall" number would completely hide. This is crucial for keeping cities safe, farming healthy, and understanding how our changing climate is reshaping the water cycle.

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The Rain Detective's Report: North Sumatra's Wild Ride (2011–2025)

In this study, two meteorology detectives, Putu Bagus Paramanandana and Adi Mulsandi, decided to investigate the rain story of North Sumatra, a province in Indonesia known for its lush landscapes and complex geography. They didn't just look at the rain; they looked at the personality of the rain. Using a super-advanced computer model called ERA5 (think of it as a giant, high-definition weather time machine that fills in the gaps where real rain gauges are missing), they analyzed daily rain data from 2011 to 2025.

Their mission was to answer three big questions: Where does the rain fall hardest? How has the "mood" of the rain changed recently? And which years were the most chaotic?

To do this, they didn't just count the rain. They used six different "lenses" to view the weather:

1. Total Rain: The whole bucket.
2. The "Super-Heavy" Days (R99p): The rain that falls on the top 1% of the wettest days.
3. The "One-Day King" (Rx1day): The single wettest day of the year.
4. The "Heavy Rain Count" (R10mm): How many days had at least 10mm of rain.
5. The "Wet Spell" (CWD): The longest streak of rainy days.
6. The "Dry Spell" (CDD): The longest streak of dry days.

The Plot Twist: It's Not About the Total
The first thing the detectives found was that North Sumatra is a patchwork quilt. The rain doesn't fall evenly. The western and southern parts of the mainland are generally wetter, while the east is often drier. But here's the kicker: the places with the most total rain aren't always the places with the most extreme rain.

Imagine two friends, Alex and Sam. Alex gets 100 inches of rain a year, but it's spread out evenly. Sam also gets 100 inches, but 90 of them fall in three massive, terrifying storms. They have the same "total," but their lives are very different. The study found that in North Sumatra, the "Sam" effect is real. Some areas get moderate total rain but have huge contributions from those super-heavy days, while other super-wet areas don't necessarily have the most extreme single-day bursts.

The Recent Drama: From Drought to Deluge
The real story, however, is in the recent years. The researchers compared the years 2019 through 2025 against a "normal" baseline from 2011 to 2020.

• 2019: The Quiet Year. This year was a bit of a downer. The "Super-Heavy" days and the "One-Day King" were both weaker than usual. It was a year where the rain was less extreme, but also less intense.
• 2020–2022: The Mixed Bag. The weather started to swing back and forth, with some areas getting wetter and others staying dry.
• 2023–2025: The Party (and the Chaos). This is where things got wild. Starting in 2023, the northern and eastern parts of the province started seeing a massive uptick in extreme rain.
  • 2024 was a standout for the "Super-Heavy" days (R99p), with the northern and eastern sectors experiencing deviations that were four standard deviations away from the normal baseline.
  • 2025 took the crown for the "One-Day King" (Rx1day), with the northern mainland seeing single-day rainfalls that also approached four standard deviations from the norm.

The "Chaos Score"
To figure out which year was the most "out of whack," the team created a "Chaos Score." This score didn't care if the rain was too wet or too dry; it just measured how far the year strayed from the normal pattern across all six lenses.

• The Winner: 2023 took the top spot with a score of 1.300.
• The Runner-Up: 2024 was right behind with 1.295.
• Third Place: 2025 came in at 1.186.
• The Calmest: 2020 was the most "normal" year, with a score of only 0.662.

Interestingly, 2019 ranked fourth (score 1.053). Even though it was a "dry" year with less extreme rain, it was still considered highly anomalous because it was so far from the average. This proves that a year doesn't have to be a flood to be a "weird" year; it just has to be different.

The Mystery of the Dry Spells
The detectives also looked at how long the dry spells lasted (CDD) versus how often heavy rain happened (R10mm). They found that the length of dry spells was much more unpredictable than the frequency of heavy rain. In the north and east, the "dry spell" length was swinging wildly from year to year, while the number of heavy rain days stayed relatively steady in its own local hotspots. This means that while the heavy rain might be predictable in where it happens, the length of the droughts is getting more erratic.

The Verdict
The study concludes that you cannot understand North Sumatra's weather by just looking at the total rain for the year. The rain is changing its style. We are seeing a shift where the northern and eastern parts of the province are experiencing more intense, concentrated bursts of rain and more unpredictable dry spells.

The authors are careful to note that while their computer model (ERA5) shows these patterns clearly, it's not a perfect crystal ball. The model is great at showing the general shape of the rain, but it might miss the tiny, local details caused by steep mountains or specific wind patterns. They suggest that before we start building flood defenses based on this, we need to double-check these findings with real-world rain gauges and satellite data.

But one thing is clear: the rain in North Sumatra is getting more dramatic. The years 2023, 2024, and 2025 weren't just wetter; they were a completely different kind of weather story than the decade before, with the north and east leading the charge in this new, intense chapter.