JPL
Careers
Education
Science & Technology
JPL Logo
JPL Logo
Earth
.

NASA Solves a Drizzle Riddle

Jul 25, 2017
Drizzle over land.
Credit: Wikimedia Commons contributor GerritR, CC BY-SA 4.0

What makes clouds drizzle? A new study overturns a common assumption about raindrop size.

A new NASA study shows that updrafts are more important than previously understood in determining what makes clouds produce drizzle instead of full-sized raindrops, overturning a common assumption.

The study offers a pathway for improving accuracy in weather and climate models' treatments of rainfall -- recognized as one of the greater challenges in improving short term weather forecasts and long-term climate projections.

The research by scientists at NASA's Jet Propulsion Laboratory in Pasadena, California; UCLA; and the University of Tokyo found that low-lying clouds over the ocean produce more drizzle droplets than the same type of cloud over land. The results are published online in the Quarterly Journal of the Royal Meteorological Society.

Water droplets in clouds initially form on microscopic airborne particles, or aerosols. Scientists have been studying the role of aerosols in clouds and rain for decades. There are more aerosols over land than over the ocean, and scientists had thought the additional aerosols would tend to form more drizzle over land as well. The new study shows that the presence of aerosols alone can't explain where drizzle occurs.

To understand what else plays a role, research team leader Hanii Takahashi of the JPL and UCLA Joint Institute for Regional Earth System Science and Engineering looked at updrafts -- plumes of warm air rising from the solar-heated Earth. Within tall thunderclouds, strong updrafts play a role in rain formation. In low-lying clouds, however, updrafts are known to be much weaker, and they haven't received much scientific attention in connection with rain.

"There was a previous hypothesis that updrafts could be important," Takahashi said. "But the hypothesis had never been tested, and I wasn't sure if updrafts were strong enough to affect the size of rain droplets."

Existing measurement systems struggle to monitor updraft velocities directly. To infer these velocities, Takahashi's team combined measurements from NASA's CloudSat and Aqua satellites and other sources with ground-level radar data from a U.S. Department of Energy observing site in the Azores.

They found that the updrafts in low-lying clouds over land, while weaker than updrafts in tall thunderclouds, were still strong enough to keep drizzle droplets aloft. As the droplets floated within clouds, they continued to grow until the updrafts couldn't hold them up any longer. Then they fell as full-sized raindrops.

In similar clouds that formed over the ocean, updrafts were even weaker than over land. As a result, droplets fell out of the clouds as drizzle, before they had the opportunity to grow into full-sized raindrops. This helps explain the preponderance of drizzle over the ocean.

This finding gives new insight into the basic atmospheric process of rain formation, something that's helpful in both weather forecasting and climate modeling. Takahashi hopes it will help her fellow climate modelers look beyond aerosols in their assumptions about low-lying clouds. These clouds have a strong effect on projections of Earth's future surface temperatures. In most models, the assumptions currently used to obtain realistic surface temperatures result in an unrealistically drizzly world.

"If we make updraft velocities more realistic in the models, we might get both more realistic drizzle and more realistic surface temperature projections as a result," she said.

The size of water

Airborne water vapor molecules condense on aerosol particles called cloud condensation nuclei and grow into droplets of different sizes. Here are some relevant diameters:

-- A typical cloud condensation nucleus is 0.0002 millimeters, or mm (about 1,000 times bigger than a water molecule).

-- A typical cloud droplet is around 0.02 mm (100 times bigger than the cloud condensation nucleus). Cloud droplets don't have enough mass to fall.

-- A typical drizzle droplet is 0.5 mm (25 times bigger than a cloud droplet). Drizzle is just heavy enough to fall.

-- A typical raindrop is about 2 mm (100 times bigger than a cloud droplet and 4 times bigger than drizzle).

News Media Contact

Written by Carol Rasmussen

Alan Buis

818-354-0474

alan.buis@jpl.nasa.gov

2017-198

Related News

Climate Change .

NASA-Built Instrument Will Help to Spot Greenhouse Gas Super-Emitters

Earth .

NASA Satellites Detect Signs of Volcanic Unrest Years Before Eruptions

Technology .

POINTER: Seeing Through Walls to Help Locate Firefighters

Climate Change .

After COVID-19 Delay, Delta-X Field Campaign Begins in Louisiana

Asteroids and Comets .

NASA Analysis: Earth Is Safe From Asteroid Apophis for 100-Plus Years

Earth .

Major Earth Satellite to Track Disasters, Effects of Climate Change

Asteroids and Comets .

Asteroid 2001 FO32 Will Safely Pass by Earth March 21

Earth .

NASA Data Powers New USDA Soil Moisture Portal

Weather .

A Pioneering NASA Mini Weather Satellite Ends Its Mission

Climate Change .

NASA Satellites Help Quantify Forests’ Impacts on the Global Carbon Budget

Explore More

Image .

Preparing WATSON for Borehole Descent

Image .

Fluorescence Map of a Greenland Borehole

Image .

WATSON's Field Test in Greenland

Image .

Suez Canal Crisis

Image .

Namibia Dunes

Topic .

Earth

Image .

Pacaya and Fuego Volcanoes, Guatemala

Image .

Mt. Etna--February 26, 2021

Image .

Mt. Etna February 2021

Image .

Tumbiana Stromatolite

About JPL
Who We Are
Executive Council
Directors of JPL
JPL History
Documentary Series
Virtual Tour
Annual Reports
Missions
All
Current
Past
Future
News
All
Earth
Mars
Solar System
Universe
Technology
Galleries
Images
Videos
Audio
Podcasts
Infographics
Visions of the Future
Slice of History
Engage
JPL and the Community
Lecture Series
Public Tours
Events
Team Competitions
JPL Speakers Bureau
Topics
Solar System
Mars
Earth
Climate Change
Stars and Galaxies
Exoplanets
Technology
JPL Life
For Media
Contacts and Information
Press Kits
More
Asteroid Watch
Robotics at JPL
Subscribe to Newsletter
Universe Newsletter
Social Media
RSS
Get the Latest from JPL
Follow Us

JPL is a federally funded research and development center managed for NASA by Caltech.

More from JPL
Careers Education Science & Technology Acquisitions JPL Store
Careers
Education
Science & Technology
Acquisitions
JPL Store
Related NASA Sites
Basics of Spaceflight
Climate Kids
Earth / Global Climate Change
Exoplanet Exploration
Mars Exploration
Solar System Exploration
Space Place
NASA's Eyes Visualization Project
Voyager Interstellar Mission
NASA
Caltech
Privacy
Image Policy
FAQ
Feedback
Site Manager: Veronica McGregor
Site Editors: Tony Greicius, Randal Jackson, Naomi Hartono