Clouds over the tropical island of Dominica puzzle mountain meteorologists. (Alison Nugent)
Roseau, Dominica (TDN)
It’s raining over Dominica, but cloud physics can’t explain why. Clouds are more important to the environment than you might think. Not only do they provide sun protection for fair-skinned people and rain for crops, they also buffer solar radiation, insulating the globe from heat. That means understanding how and why clouds form is crucial to predicting, modeling, and even controlling climate change. “We want to predict the future, but it’s hard to predict these clouds right now,” says atmospheric scientist Campbell Watson, who studies the formation of fluffy cumulus clouds on this tiny tropical island. In contrast to other leafy islands in the region, rain pours down like showerheads for six hours a day on Dominica, with only brief flashes of blue sky. The weather changes so quickly that even on a cool, windless morning, it can start pouring by tea time. No textbook, study, or climate model can explain why. There are more mountains than people here, and the water is as clear as the local white rum. Located in the Caribbean Sea about 600 kilometers (370 miles) northeast of Venezuela (not to be confused with the much larger Dominican Republic to the northwest), the Dominican Republic is famous for a few things: bananas, the first British Caribbean colony run by freed black slaves, Jean Rhys’s novel Wide Sargasso Sea, and rain. Lots of it. While the islanders seem content with their inexplicable rainfall—they use the downpours as hydropower—climate scientists are eager to solve the puzzle. The persistence and severity of the Dominican downpours offer a chance to better understand what’s going on 2,000 meters above our heads. “It’s in the Caribbean, there are no islands, and there’s no upwind or downwind terrain,” says Alison Nugent, who earned her doctorate on the project and is now an assistant professor at the University of Hawaii at Manoa. “So you know that everything that happens on the island is because of the island, and nothing else.” This makes the Dominican Republic easier to study than a mountain range in the middle of the mainland.
To further investigate this phenomenon, Yale University launched the Dominican Experiment (DOMEX) from 2010 to 2015, a project funded by the U.S. National Science Foundation. Led by Professor Indiana Jones Ron Smith, an expert in orographic precipitation research, this group of scientists is one of the few in the world to combine mountain meteorology and cloud physics to answer this cloud puzzle. Their findings could impact our future understanding of climate change. Why does it rain? To figure out what exactly is going on over Dominica, we first need to quickly review why and how rain falls. Typically, for a cloud to rain, the tiny water droplets that make up the cloud need to fuse together through a process of collision and coalescence to form raindrops. Once a drop has collected enough water and becomes heavy enough, gravity pulls it toward the ground. The drop collects more water on its way down and continues to grow through a process called accretion. When this happens in large quantities, it starts to rain. In addition to this process, there are two subtypes of rain clouds: cold clouds and warm clouds. Cold clouds have areas that sit above the freezing point of the atmosphere where some of the water turns into ice crystals that love to attract water vapor.
Once the ice crystals grow large enough, they also fall from the clouds, melting on their way down as rain.
This is a photo of a strange cloud formation over Dominica. (Alison Nugent)
But warm clouds are below freezing, which means there are no helpful ice crystals to aid the fusion process. “How warm clouds produce rain is a hot topic in cloud physics right now,” Watson says with a laugh. “People have been speculating for years, but not everyone agrees.” The problem with warm clouds is that droplets of similar size bounce off each other rather than fuse together. So you need to create a large number of droplets of different sizes in the cloud — called a cloud droplet spectrum — so that the larger droplets absorb the smaller ones when they collide. There are several credible theories about how warm clouds produce this broad spectrum, most of which take up to three hours to form. In the Dominican Republic, however, the process takes just 10 to 12 minutes. In the time it takes you to cook pasta, a cloud can form in the Dominican mountains, create a droplet spectrum large enough to start pouring down, and then quickly dissipate. “None of the current theories about warm clouds can explain why this process happens so quickly, and why the rain in the Dominican Republic is so fast and intense,” says Watson, who is currently working with IBM Research and its related business, The Weather Company. “As we look at the data, it feeds back to us. We’re trying to combine observations and modeling to understand a very complex process.” To get the data, the team flies a specially equipped aircraft through the clouds to get all kinds of atmospheric information. The twin-engine turboprop King Air, owned and operated by the University of Wyoming and an NSF facility, is equipped with instruments that measure in situ physical properties such as air movement, turbulence, temperature and pressure, as well as properties of aerosols, clouds and rain, such as number density and size. It is also equipped with cloud radar and lidar. In the sped-up time-lapse footage below, the aircraft can be seen flying through the clouds, taking measurements over Dominica. The instrument at the bottom left of the frame is a gust sonde, which “extends forward in front of the aircraft like the horn of a narwhal,” Nugent said. “It measures the three components of velocity very accurately, which is a difficult thing to do when your reference frame is flying through the air at 250 miles per hour.” Many of the instruments on board measure at a frequency of 25 hertz – that’s 25 data points per second.
Climate Change Models and the Future of Clouds If atmospheric scientists can figure out why Dominican clouds produce so much rainfall so quickly, they will be able to use their findings to contribute to a range of analyses, one of which is more accurate models of climate change predictions. “If you think about the climate system as a whole, there are thousands of tiny puzzle pieces that need to fit together,” Watson said. “Clouds are particularly important because they affect radiation, which leads to warming.” That’s because clouds can both protect us from the sun’s rays by reflecting them, but also allow some of them to pass through the clouds to the Earth’s surface. This delicate balance will have a huge impact on long-term global warming. The number density and size of cloud droplets help determine the proportion of solar radiation that is either reflected back into space or allowed to pass through the clouds to the Earth’s surface and then bounce back into the clouds to be absorbed and re-emitted, just like the greenhouse effect. In this way, more clouds can be both beneficial (blocking solar radiation) and harmful (blocking radiation from reflecting between clouds and Earth). “It’s still very difficult to predict how cloud properties will change in the future, and whether these changes will have a warming or cooling effect on the Earth,” Watson said. “We know that some clouds have a greater cooling effect on the Earth, but it’s a challenge because there’s still a lot we don’t understand about the composition of clouds around the world.” “Precipitation is very, very important for life on Earth,” Nugent added. “Where and how much precipitation falls determines climate zones, like rainforests and deserts, and where humans, plants and animals can thrive. Clouds are the gatekeepers of precipitation.” The importance of aerosols Clouds can save us or they can destroy us, but there’s a second factor to why they’re important to modern research: “We live in an era of climate change, which creates a unique opportunity to observe changes around us as if they were controlled experiments,” Nugent says. One of the team’s surprising findings was that on windless days, the air above Dominica was filled with tiny aerosols floating in the air. Like the spray in a can of hair spray, aerosols are so small that they hang suspended in the air. Some aerosols are naturally occurring, like pollen, spores, smoke, sea salt, and dust, while others come from human activities, like the CFCs that created the ozone hole over Antarctica. The aerosols over Dominica are naturally occurring and appear to greatly affect the cloud droplet spectrum. Aerosols are important because they are a necessary ingredient for cloud formation. “One of the main conclusions from the Dominica study is that precipitation on the island depends strongly on wind speed, and in some cases aerosols play a role as well,” Nugent says. “The reliance on aerosols at lower wind speeds was unexpected to us, and it raises questions about the impact of aerosols in other situations, especially when aerosols may change with climate change.” Nugent is currently working to determine the impact of aerosols on precipitation in Hawaii to try to answer some of the unanswered questions in the Dominican case. Watson and Nugent agree that understanding rainfall in the Dominican Republic and how it may change with climate change could be important for future adaptation.
We are still unsure how global warming will affect clouds, and how clouds in turn will affect global warming, but we are learning day by day.
