A joint mission led by NASA and the Japan Aerospace Exploration Agency Global Precipitation Measurement (GPM) has for the first time revealed 3-D snapshots of raindrops and snowflakes around the world from space.
The size of falling raindrops depends on several factors, including where the cloud producing the drops is located and where the drops originate in the cloud. With the mission's new 3-D data, scientists hope to improve rainfall forecasts, which, in turn, could help people around the world prepare for extreme weather events.
"The drop size distribution is one of many factors that determines how big a storm will grow, how long it will last and how much rain it will ultimately produce," explained Joe Munchak, a research meteorologist at NASA's Goddard Space Flight Center in Greenbelt, Md. "We've never been able to see how water droplet sizes vary globally until now."
Storm clouds house a variety of drop sizes that eventually precipitate as rain or snow. Drops stored near in the cores of clouds tend to be bigger, as they collide more frequently with each other and aggregate as they fall towards Earth's surface. However, droplets from high altitudes or the edges of clouds tend to be smaller because they miss colliding into other or break apart during their decent.
Launched in 2014, GPM carries the first Dual-frequency Precipitation Radar (DPR) to fly in space, as well as a multi-channel GPM Microwave Imager (GMI). While the DPR captures detailed 3-D measurements of rainfall, the GMI uses a set of 13 optimized frequencies to retrieve heavy, moderate and light precipitation measurements at the Earth's surface.
The number of drops and snowflakes of different sizes at various locations within a cloud is referred to as "particle size distribution." The ratio between drop sizes is vital to accurately measuring the total precipitation of a storm.
"Without knowing the relationship or the ratio of those large drops to the smaller or medium sized drops, we can have a big 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," Munchak added.
Previously, researchers relied on assumptions. However, with GPM's revolutionary 3-D snapshots of drop size distribution, scientists can better understand the structure of a storm and how it will behave.
"Drop size distribution influences storm growth by changing the rate of evaporation of rain as it falls through dry air," Munchak said.
For instance, smaller drops tend to evaporate faster, cool the air more quickly, and cause a stronger flow of downward moving air that creates damaging winds. On the other hand, these same downdrafts can also interfere with upward flowing air that fuels the storm, causing it to weaken or dissipate all together.
"GPM measurements will really help predict these complex interactions that depend in part of the drop size distribution," Munchak concluded.
