Drop size distribution evolution through convective storm life-cycles and a site-specific Z–R relationship for radar QPE over Kolkata, Eastern India

Rain is never just water falling from the sky; it is a collection of countless individual drops, each with its own size, speed, and history. To a meteorologist, the specific mix of these drops—how many tiny ones there are versus how many large ones—holds the key to understanding how storms form, grow, and eventually fade. This mix, known as the drop size distribution, dictates how a weather radar sees the rain. When a radar beam hits a raindrop, it bounces back a signal. The strength of that signal depends heavily on the drop's size, meaning a storm filled with a few massive drops can look just as bright on a radar screen as a storm filled with millions of tiny ones, even if the actual amount of water falling is very different. Because of this, knowing the exact recipe of drop sizes in a specific region is essential for turning a radar's blurry image into an accurate measurement of how much rain is actually hitting the ground. Without this local knowledge, forecasts can be wildly off, leaving communities unprepared for floods or droughts.

In the Gangetic plains of eastern India, where the city of Kolkata sits, the weather is a dramatic mix of intense thunderstorms, monsoon rains, and winter disturbances. For years, scientists have tried to understand the unique character of rain in this region, but the details of how raindrops evolve during a storm's life cycle have remained largely a mystery. A new study by researchers at the India Meteorological Department has finally filled this gap. By placing a specialized instrument on the ground to count raindrops and watching the same storms with a powerful radar tower nearby, the team tracked nine different storm events across four seasons between October 2024 and April 2026. They wanted to see if the size of the raindrops changed in a predictable way as a storm was born, reached its peak, and then died away, and whether these changes meant they needed different formulas to calculate rainfall for each stage of the storm.

The researchers set up their observation post in Kolkata, using a device called a disdrometer to measure the diameter of every raindrop that fell. At the same time, they watched the storms from a nearby S-band Doppler weather radar, which scans the sky to measure the intensity of the rain. They focused on nine specific storms that occurred between October 2024 and April 2026, capturing over a thousand minutes of rain data. As they watched these storms unfold, a clear pattern emerged. When a storm was just beginning to pass over the city, the rain was made up of many small drops, with an average size of about 1.56 millimeters. As the storm matured and the heavy core moved overhead, the drops grew significantly larger, reaching an average size of 2.37 millimeters. This growth happened because the rising air inside the storm allowed the drops to collide and merge, creating bigger, heavier spheres. However, as the storm began to dissipate, the average drop size shrank again to about 1.91 millimeters. This happened because the largest drops fell out of the sky first, leaving behind a mix of smaller ones. The researchers found that this entire life cycle traced a distinct loop, a predictable journey from small to large and back to small, which had never been documented in this specific region before.

One of the most practical questions the team asked was whether they needed to use a different mathematical rule to calculate rainfall for each of these three stages: the beginning, the middle, and the end. In many parts of the world, the rules change depending on the storm's age. However, when the scientists tested the data from Kolkata, they found something surprising. Despite the clear changes in drop size, the relationship between the radar's signal and the actual rain rate remained consistent throughout the storm's life. The statistical tests showed that trying to create separate rules for the developing, mature, and dissipating phases did not improve the accuracy enough to be worth the extra complexity. Instead, the team derived a single, robust formula that works for the entire life cycle of a local convective storm in this region. This new formula, tailored specifically to the rain characteristics of Kolkata, proved to be far superior to the standard, generic rule used globally.

To prove the value of their new formula, the researchers tested it against a real storm event that occurred in April 2026. They compared the rainfall estimates generated by their new local formula against the estimates from the old, generic global formula. The results were striking. The generic formula, which assumes a standard mix of raindrops that does not match the reality of Kolkata, overestimated the rainfall by nearly 240 percent. In contrast, the new site-specific formula reduced that error to just 21 percent. This means that for the first time, radar operators in this part of India have a tool that can tell them with much greater confidence how much rain is actually falling, rather than guessing based on rules designed for a different climate. The study also revealed that the radar itself had a consistent bias, reading the rain intensity as weaker than it actually was, a finding that allows technicians to adjust the machine for better accuracy in the future.

The implications of this work extend beyond just better numbers on a screen. By understanding that the rain in Kolkata follows a specific, predictable pattern of drop sizes, meteorologists can now distinguish between different types of storms more effectively. The study also highlighted a crucial distinction: while this new formula works perfectly for local thunderstorms, it is not suitable for the massive, swirling rainbands of tropical cyclones, which have a completely different structure. This means that for accurate forecasting in Kolkata, weather services now need a two-part approach: one set of rules for the fierce local storms that sweep through the seasons, and a different set for the rare but powerful cyclones that strike from the ocean. This nuanced understanding, built on years of careful observation and the direct comparison of ground-level drops with sky-scanning radar, marks a significant step forward in protecting a densely populated region from the vagaries of the monsoon and the thunderstorms that define its climate.