Why Measuring The Size Of Raindrops From Space May Help Improve Weather Predictions

While weather predictions are getting increasingly accurate, meteorologists are often still unable to gauge the severity of storms. That's because forecasting what the elements have in store for us is a complicated task that is dependent on numerous variables including temperature, humidity, jet streams, and even the size of raindrops!

For, believe it or not, raindrops are not created equal! According to scientists, the size depends on factors like the location of the cloud and also where the raindrops are within the cloud. Smaller drops, only 0.5 - 2 mm wide, are found at the top. As the drops fall through the storm cloud, they collide with each other and grow. Hence, the ones in the middle tend to be slightly larger, measuring about 3 - 5 mm, while the heaviest ones, 4 - 6 mm wide, accumulate at the bottom.

So why is knowing the size of raindrops in a cloud necessary? According to Joe Munchak, a research meteorologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the ratio of the size of raindrops or drop size distribution, is one of the many factors responsible for determining the length and severity of a storm.

The researcher says, "Without knowing the relationship or the ratio of those large drops to the smaller or medium sized drops, we can have a significant error in how much rain we know fell and that can have some big implications for knowing long term accumulations which can help with flash flood predictions.”

However, up until now, accurate information about drop size distribution has only been available for certain isolated areas on Earth. As a result, the weather models used by meteorologists are based on assumptions, which often lead to inaccurate predictions.

That is about to change thanks to the Global Prediction Management (GPM) satellite, a joint effort between NASA and Japanese Aerospace Exploration Agency (JAXA). The satellite, which was launched in 2014, is equipped with a Dual-frequency Precipitation Radar that takes three-dimensional images of raindrops and snowflakes around the world from space.

Besides helping improve our weather models, the data from GPM will also enable researchers to get more insight into the structure of the storm and its behavior. That's because drop size distribution also has an influence on the intensity of the inclement weather. For example, smaller raindrops often evaporate as they fall, cooling the surrounding air. As the colder, heavier air moves towards the ground, it can potentially transform into powerful, damaging winds. Conversely, the same high winds could interfere with the upward flowing warmer air that is fueling the storm and weaken or even completely dissipate it. Munchak says, "GPM measurements will really help predict these complex interactions that depend in part to the drop size distribution."

NASA researchers believe that the information provided by GPM will go a long way in improving our weather models and forecasts! This, in turn, will help cities and towns better prepare for any inclement weather that may be coming their way.

Resources: NASA.gov