Measuring Precipitation, One Drop at Time

By Richard Merritt

Rain is never appreciated during a picnic and fog is aggravating to motorists, but those small droplets of water may actually hold the key on a small scale what is happening, or could happen, on a much larger scale.

While oceans, lakes, rivers or rain are overt examples of how water supports all life on earth, they are not always the whole story. There are, for example, many scientists who seek to better understand the larger picture by measuring and examining the tiny pictures behind them.

Two Duke Ph.D. students of Ana Barros, professor of civil and environmental engineering at Duke’s Pratt School of Engineering, recently concluded field research projects this spring and summer -- one in the U.S. Midwest, the other in Peru – gathering information on these drops of water.

One of the students, Anna Wilson, hopes that her exacting measurements of rain drops can ultimately lead to more accurate climate models and rainfall retrieval algorithms from space-based sensors, while Jessica Erlingis wants to discern any relationship between rainfall and fog on the biodiversity of plants and animals along the Andes Mountains.

“The reason why we care so much about getting raindrop size distributions right is that we still do not know ho how to describe these processes in climate or weather models, and the differences between actual and predicted rainfall using different approximations can be up to 100 percent,” Barros said.  “Therefore this is a fundamental climate problem to be resolved.”

Disdrometers and Spiders

Though considered a dry period for the Midwest, Wilson was excited that there were several hydrological “events” during her three-and-a-half week field experience in Oklahoma. During this time, a vast array of sensors, detectors, and other ground-based meteorological equipment collected vital information about the events.

The group of scientists and students from across the country and Canada even had two specially designed aircraft ready to make aerial measurements of weather phenomena, while satellites gathered data from the heavens. The goal was to accumulate as much detailed information as possible about single events to be analyzed and used to help construct models based on actual data, as well as to improve the algorithms used by scientists to measure rainfall from satellites.

Amidst all these scientists and technologies, Wilson manned a disdrometer, a device that provides detailed measurements of raindrop size distribution, shape and velocity. It can also measure the angle with which the drops hit the ground. It is basically a box lined with mirrors and cameras that captures and records individual drops as they enter an opening on the top.

“Getting this kind of information is very important because there really aren’t a lot of high-density ground measurement data available,” Wilson said, adding that one of the more interesting challenges of tending to these devices was keeping them clear of spiders, which apparently find the disdrometer’s interior an inviting place to hang out.

“It was great to have the experience of working with so many scientists using such a variety of equipment,” Wilson said. “I learned so much just being able to talk and interact with these specialists. Not only were we gathering all these important data, but personally, just learning about how these different types of sensors work should make my contribution that much more meaningful as we plan for a similar project in western North Carolina in 2013.”

Wilson was referring to a project she is helping to organize, which will focus on the hydrology of the Smoky Mountains. “I’m interested in learning about how light rainfall and especially fog contributes to the overall water ‘budget’ of the mountains,” she said.

The data collected this spring is now being analyzed by Wilson and scientists from such universities as McGill, University of Utah, Colorado State University, University of Illinois Urbana-Champaign and University of Alabama Huntsville. Wilson was assisting the Alabama team.

The entire effort is a collaborative effort between research universities, the U.S. Department of Energy Atmospheric Radiation Measurement Climate Research Facility and the National Aeronautics and Space Administration’s Global Precipitation Measurement Ground Validation (GV) program.

A Water Inventory of the Andes

As a larger effort to better understand the effects of rainfall on the biodiversity along the Andes Mountains, a team of scientists headed to Peru to take a detailed water “inventory” of a portion of the eastern slope of that South American mountain chain.

Specifically, they were interested in learning more about the interaction of the complex orography of the Andes with the formation of rainfall, clouds, and fog as well as climate change impacts in one of the world’s biodiversity hotspots. During this project, the scientists will be installing special sensors at various elevations to measure precipitation along the eastern slopes of the Andes.

Erlingis and Barros joined a team of scientists from Wake Forest University installing the sensors in the Sacred Valley area, which is near the Inca capitol of Cusco and ancient city of Machu Picchu.

“Our ultimate goal is characterize the rainfall along the Andes and note any correlations with the biodiversity of the area,” Erlingis said. “This data can serve as a baseline to follow the effects of climate change along the Andean range.”

In addition to amount of water, whether in rain or fog form, future sensors will be installed to collect data on particles in the air around which water droplets can form. These particles can be organic, like pollen or spores, or inorganic, like dust or sea salt. Such aerosols, Erlingis pointed out, can be important seeds for the formation of clouds.

“One of the main challenges we face in installing and maintaining these sensors is the difficulty of the terrain,” Erlingis said. “Once we can get to a location with all our equipment and tools, which in no mean feat, we have to install towers that rise above the vegetation canopies to make sure we are getting accurate samples.”

Another issue is the occurrence of landslides, which is fairly common on the mountain slopes.

To date, the highest gauge has been placed at an elevation of 3,800 meters. The system of sensors and gauges will be maintained by graduate students from Wake Forest and members of the Amazon Conservation Association, who is working with the university scientists.

“As a meteorologist, I find all this work so exotic, and so challenging,” she said.

While Barros said that there are dramatic and potentially dangerous events, such as storms, which can cause mudslides or floods in mountainous regions, the importance of lighter rainfall should not be dismissed.

“In the Appalachians, for example, light rainfall accounts for about 60 percent of the annual rainfall, and therefore freshwater availability,” Barros said. “Based on what we experienced in the Andes, this is also likely the case there.  The fact that light rainfall is such a predominant component of the water cycle is especially important in the context of climate change. Light rainfall is supposed to decrease substantially in the future, thereby posing a major threat to environmental sustainability and freshwater availability.