Clouds, aerosol particles, and Research Flight 10 over the southeast Pacific

The familiar click of the alarm, whistling, ukulele, violin, … “Oh well in five years’ time we could be walking around the zoo…” With the lyrics from Noah and The Whale’s “5 Years Time” the alarm I’d set six hours ago sounds off, and I wake up in an unfamiliar hotel in Arica, Chile. It’s dark outside, clock reading 12:01am, and I’m lucky if I caught five hours of sleep. It’s three hours until my first research flight on the C-130, a retired military transport aircraft that has been outfitted with various instruments to make measurements of the atmosphere. So shaking off the drowsiness, I shower and prepare for my flight.

NSF/NCAR C-130 (aka Hercules) outfitted with instruments to measure the atmosphere (C. Terai)

NSF/NCAR C-130 (aka Hercules) outfitted with instruments to measure the atmosphere (C. Terai)

I am participating in the VOCALS Regional Experiment, a month long field campaign tasked to study the regional climate of the southeast Pacific. As part of the field campaign, the C-130 will be taking off from Arica and flying west over the southeast Pacific Ocean, weaving through, below, and above the clouds while making atmospheric measurements. For my graduate studies I’ll be using the aircraft measurements to characterize the Texas-sized cloud deck that stretches westward off the coast of Chile. Known as stratocumulus, this cloud type resembles a clumpy white blanket when seen from space. I’ll be looking at how the stratocumulus is affected by pollution particles from the cities like Santiago that dot the Chilean coast. To get at this question, I will be analyzing measurements of particulate concentrations, cloud thicknesses, and rain rates.

MODIS visible image of the VOCALS region taken on 2008 Nov. 6. (credits: NASA/GSFC Rapid Response)

MODIS visible image of the VOCALS region taken on 2008 Nov. 6, a couple hours after the RF10 flight. The outline of the South American coast is partially visible on the ride side of the image. (credits: NASA/GSFC Rapid Response)

Clouds can warm or cool the Earth, depending on their height and thickness. Because the marine stratocumulus off the coast of Chile lies low within the atmosphere, it acts like a white T-shirt and cools the southeast Pacific region. In this area, an outstanding question has been whether the pollution aerosol particles from coastal cities add to regional cooling by interacting with the cloud deck and changing its abundance and reflective properties. Globally speaking, understanding how tiny aerosol particles affect cloud properties remains one of the largest challenges to quantifying human influence on the climate.

Aerosol particles can influence cloud properties, because they provide the sites on which water vapor condenses to form cloud drops. There are various sources of aerosol particles, whether ocean sea spray or human combustion, but the concentration of particles in polluted urban areas can easily be 30 times higher compared to concentrations found in remote environments over the open ocean. Over the southeast Pacific, large scale shifts in the wind patterns can vary the concentrations over the ocean by a factor of ten, depending on where the wind is blowing from.

Clouds can react to increases in aerosols in various ways, but much attention has been focused on two particular pathways. First, increasing the number of cloud droplets by increasing the number of aerosols can make the cloud droplets smaller, if we assume that the same amount of cloud water condenses in the polluted and clean case.  This increases the surface area of the cloud droplets and makes for a more reflective cloud, implying a stronger cooling from there being more aerosols. Another consequence of smaller cloud droplets is that they become less efficient at colliding and coalescing. If they are less effective at colliding and coalescing, rainfall is delayed or suppressed.

One of the hypotheses being tested with the C-130 measurements is that the suppression of rain from clouds in polluted environments leads to longer-lasting, thicker clouds. There are many steps for this hypothesized mechanism to work, so I’ll be using the aircraft measurements to test a couple of the intermediate steps. One of them being, “Does increased number of aerosol particles suppress rain in the clouds?”  If the answer is yes, it’ll be one step towards confirming that more aerosol particles lead to longer-lasting clouds, and hence more cooling.

When I board the C-130 at 2am the morning of my flight, I don’t know the answer to that question yet. Instead, I learn that I have a window seat. Score! One of the reasons I’d gotten into this field of study was that I enjoyed gazing at clouds whenever I had the chance to fly. One thing I failed to realize though was that with a takeoff of 3am and a flight path, taking us out west 1500 kilometers, we’d be flying under the cover of night for 5 of the 8 ½ flight hours. So instead of looking out the window into the darkness, I attend to a blinking light: my job is to make sure the light keeps on blinking and to let the other scientists know if it stops. I was only a month into my graduate studies, so the other scientists hadn’t trusted me with too much responsibility.

Sunrise taken from the C-130 during RF10 during VOCALS-REx (C. Terai)

Photo of stratocumulus and sunrise taken from the C-130 during RF10 during VOCALS-REx (C. Terai)

The highlight of the flight, of course, was when the sun finally rose. This happened on the return leg of the flight, when we headed back east, straight into the rising sun. I saw the clouds and felt very small. It’s one thing to see the clouds stretch for thousands of kilometers in a satellite image; it’s another to sit in a plane and see an endless sea of clouds. It stretched to the left and right of the plane, as far as the eye could see. A patchwork of soft yellow, orange sunlight and blue, gray shadows.

The large range of scales over which atmospheric processes occur, from the condensation of water vapor on particles smaller than the width of the hair to the large-scale effect of high pressure systems on that Texas-sized cloud field, makes this science question difficult to tackle. That is why current efforts to better understand how aerosols affect clouds have involved a range of observations, from aircraft to satellite measurements, and a range of models, from large-scale climate models to smaller-scale but highly complex cloud-resolving models.

For my part, I’ve spent more than five years’ time since that research flight exploring a zoo of measurements that we collected on that flight and other such flights. What I’ve eventually discovered from analyzing the aircraft data is that increasing the aerosol concentrations does indeed correlate with suppressed rain. In other words, more pollution suppresses rain in these clouds. I’ve also found that the thickness of the cloud changes the extent to which the rain is suppressed. This would imply that the clouds stick around a lot longer because of the pollution from coastal cities, like Santiago. And it also might mean a cooler southeast Pacific due to the pollution, but other studies are currently being conducted to examine the extent to which this is the case. What I’ll always remember from that flight though is the view of the rising sun over the sea of stratocumulus.

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Chris Terai

I am a PhD candidate in Atmospheric Sciences at the University of Washington. I like clouds and am studying them for my PhD dissertation, specifically on how aerosol particles affect cloud properties and the precipitation that falls from them.
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