The bone-dry northern coast of Chile and the adjacent Pacific Ocean provided a dramatic geographic backdrop for the VOCALS-REx field project, which focused on the vast decks of marine stratocumulus clouds that prevail offshore. (Photo by Carlye Calvin.)
by Bob Henson
Given that it sprawls across nearly a third of Earth’s surface, it’s no surprise that the Pacific Ocean has much to do with global climate. It’s the planet’s most prolific generator of tropical cyclones and heavy rains. The massive flows of heat and moisture into and out of the Pacific make it a powerful driver of atmospheric circulation and radiative balance.
NCAR and partners spent an unusual amount of time this year on Pacific-based field work. Below are profiles of three major studies, which unfolded in conditions that ranged from the torrential summer rains of Taiwan to the surreal blend of arid land and cloudy skies found along the coast of northern Chile.
Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment
“I was fortunate to experience three very rainy days during my ten-day stay,” says Ed Zipser. Clearly, this scientist wasn’t on holiday. Zipser, a professor at the University of Utah, was hoping for rain—lots of it—as were the other principal investigators at TiMREX and its counterpart, SoWMEX. The project was organized to improve quantitative forecasting of heavy rains during Taiwan’s monsoon, when floods and landslides often cause death and destruction.
From the S-Pol site in southern Taiwan, NCAR’s Wen-Chau Lee served as one of the TiMREX coordinators. (Photos by Carlye Calvin unless otherwise indicated.)
Close to one meter (39 inches) of rain fell during three wet days of Zipser’s visit to NCAR’s S-Pol radar, which was stationed above a rice paddy near Kaohsiung in southern Taiwan. Astounding as it sounds, rain like this isn’t uncommon at the start of Taiwan’s summer monsoon. What was more unusual was the delayed onset of the Mei-Yu front, the boundary that pushes north each spring in fits and starts, eventually bringing the summer rains. This year, the Mei-Yu arrived two weeks late, with weather patterns distorted by several earlier-than-usual tropical cyclones in the region.
Even after its arrival, the monsoon kept researchers guessing. Persistent southerly flow developed in place of the usual west-southwest flow characteristic of the monsoon. Instead of moist winds striking the southwest side of Taiwan’s mountains and producing upslope rain, the prevailing flow focused downpours along the southeast coastal plain. “We were scratching our heads a few times over this,” says NCAR’s Wen-Chau Lee, who organized SoWMEX/TiMREX with Ben Jou (National Taiwan University).
The late arrival of the Mei-Yu didn’t leave TiMREX totally high and dry. “This offered us a great chance to observe the pre-monsoon convection,” says Utah doctoral student Weixin Xu. The hot, humid conditions that preceded the monsoon led to vigorous updrafts atop southern Taiwan’s mountains. The resulting storms, though scattered, grew as much as 3 kilometers (2 miles) taller on radar than their monsoonal counterparts.
Xu has been using data from NASA’s Tropical Rainfall Measuring Mission (TRMM) to examine features of the Mei-Yu front. However, the satellite wasn’t designed to capture some of the most critical aspects of circulation and microphysics. “It’s impossible to see the real face of the Mei-Yu systems from TRMM,” says Xu. Data from the S-Pol radar is filling in many gaps for Xu and other researchers. The radar’s dual polarization means that scientists can assess the presence of rain, hail, and ice crystals in different parts of a storm and track how these features are evolving. “The microphysical information is invaluable,” says Xu.
Two graduate students from Colorado State University, Amanda Anderson and Angela Rowe, came to Taiwan to work with S-Pol in their first field-project experience. “It was a great opportunity to learn more about tropical weather and the struggle of forecasting it,” says Anderson. She was impressed with Taiwan’s relatively dense network of radiosondes and radars and says she learned much in working with the radar operators from NCAR and Taiwan, especially from their efforts to eliminate false data from mountains and other stationary objects: “I now know more about ground clutter and its suppression than I ever thought possible!”
Taking a break from TiMREX radar duties are (left to right) Angela Rowe and Amanda Anderson (Colorado State University); Chong-Chi “Nick” Tong, Pin-Fang
“Penny” Lin, and Jung-Kuang “Radian” Hsiu (National Taiwan University); and Chaun-Chi “Beth” Tu (University of Hawaii). (Photo courtesy Angela Rowe.)
For her doctorate, Rowe is continuing her master’s degree analysis of S-Pol data from the North American Monsoon Experiment (NAME). “I didn’t participate in NAME, so TiMREX gave me an even greater appreciation for monsoon precipitation,” says Rowe. She now hopes to compare and contrast polarimetric data from the two studies.
Robert Fovell, who carries out simulations of thunderstorms and sea breezes at the University of California, Los Angeles (UCLA), pulled himself away from his computer to check out the TiMREX campaign in person. “It was actually my first visit to the field, so I’m sure I received far more than I contributed,” says Fovell. “I thought TiMREX was magnificently organized and orchestrated, and the experience has already helped me design and execute my next modeling study. There’s nothing like seeing for oneself.”
TiMREX participants included (left to right) Chaun-Chi “Beth” Tu (University of Hawaii), Jung-Kuang “Radian” Hsiu (National Taiwan University), Weixin Xu (University of Utah), Robert Rilling (NCAR), and Tammy Weckwerth (NCAR).
THORPEX Pacific Asian Regional Campaign
While many Americans gazed anxiously toward the Atlantic Ocean, watching hurricanes head toward U.S. shores, dozens of researchers kept their eyes on typhoons in the northwest Pacific during T-PARC. University, federal, and military participants from more than 10 countries studied typhoon life cycles, from their formation in the tropics and subtropics (where the Tropical Cyclone Structure–2008 study, led by the Office of Naval Research, unfolded) to their typical demise as they move northward and encounter westerly winds at midlatitudes.
The T-PARC study area spanned much of the Northwest Pacific Ocean. Several T-PARC aircraft sampled Typhoon Sinlaku, shown here just east of Taiwan on 11 September. (MTSAT-1R image courtesy Japan National Institute of Informatics.)
“We’re running this project across nine time zones. In terms of geographic reach, this is by far the biggest project I’ve been involved with in my 25 years at UCAR,” said James Moore midway through the summer phase of T-PARC. Equally impressive is the scope of T-PARC’s mammoth online field catalog, developed by NCAR’s Earth Observing Laboratory. The catalog holds more than a million items, including reports, images, and analyses and predictions from weather forecast models.
On average, the Northwest Pacific is the world’s most prolific breeding ground for tropical cyclones. Unlike the North Atlantic, it stays warm enough to produce typhoons year round, with the official season running from May to November. Many of these typhoons pummel the east Asia coastline from Vietnam to Japan, then recurve to the northeast, pumping huge amounts of moisture and warm air into the North Pacific. This can affect weather patterns downstream across North America in ways that aren’t fully understood or well depicted by computer models.
“The practical question is figuring out which storms will create havoc for subsequent downstream weather prediction and which will not,” says NCAR’s Christopher Davis, one of T-PARC’s principal investigators.
Davis says the study may help show why some hurricanes and typhoons are able to thrive in the face of wind shear (variations in wind speed and direction with height). Wind shear often tilts and tears apart tropical cyclones, but not always.
“We have many simulations from numerical models that suggest that a tropical cyclone can change its structure so as to mitigate the effects of shear, but we have almost no observations to tell us whether these models are correct,” says Davis.
Typhoons and hurricanes that do survive midlatitude shear are a special breed. While no longer tropical cyclones, they often develop an enlarged swath of damaging winds and flooding rains. The metamorphosis of Pacific typhoons can trigger other destructive storms as far afield as the U.S. Pacific Northwest and Alaska.
During T-PARC’s first few weeks, the study region was abnormally tranquil. The Northwest Pacific saw four named storms in August, whereas the average is between six and seven. The pace picked up in September, when T-PARC successfully sampled Typhoon Sinlaku. At one point a Category 4 system, Sinlaku struck Taiwan as a Category 2 and later skirted Japan’s southern coastline. Also studied were Supertyphoon Jangmi and typhoons Nuri and Hagiput.
Flying in and near the typhoons from a base in Guam were a P-3 aircraft from the Naval Research Laboratory (NRL), which carried the Eldora Doppler Radar, and a C-130 operated by the U.S. Air Force. Other aircraft included a Falcon-20 from the German Aerospace Center, flying out of Japan, and the DOTSTAR Astra (Central Weather Bureau of Taiwan). T-PARC also marked the second major deployment of driftsondes, a sensing system created at NCAR and first used on a large scale for a 2006 African project that studied the earliest stages of Atlantic hurricanes.
At the T-PARC operations center, NCAR’s James Moore analyzes Typhoon Nuri, which struck the Philippines on 19 August. (Photo by Bob Henson.)
Dozens of participants in Asia, Oceania, North America, and Europe used a videoconferencing system called Elluminate to take part in daily meetings held at T-PARC’s operations center at the Naval Postgraduate School. They could view briefings, add to the discussion, and even present PowerPoint shows and specialized products from thousands of miles away. “It’s great technology that has really made this expansive project possible,” says Moore.
The importance of T-PARC goes well beyond typhoons, according to David Parsons, chief of the World Weather Research Program. A second T-PARC phase led by NOAA will examine winter storms, and there are several other related experiments. T-PARC is part of THORPEX, a 10-year international study to foster improvements in predicting high-impact weather one day to two weeks in advance.
“T-PARC is focused on scale interaction—from how the large-scale environment interacts with convection to form typhoons, to how these typhoons perturb the flow over the western Pacific to generate damaging weather downstream,” says Parsons. He explains that recurving typhoons can trigger or enhance a train of Rossby waves that propagate around the globe, spawning destructive events that can range from floods and fire-conducive weather to intense winter storms. “By helping us understand typhoons as well as their downstream effects, T-PARC will contribute to better forecasts and help make people less vulnerable to many kinds of damaging weather,” Parsons says.
VAMOS (Variability of the American Monsoon Systems) Ocean Cloud Atmosphere Land Study–Regional Experiment
The NSF/NCAR C-130 prepares for a night flight at VOCALS.
From October 1903 to January 1918, no measurable rain was reported in Arica, Chile. This barren city thus earned a place in the Guinness Book of World Records for the longest dry spell ever recorded. It does rain once in a while in Arica—the city’s annual average is 0.76 millimeters (0.03 inches)—but the scientists and technicians stationed there for VOCALS can vouch for its parchedness.
The coastal region of northern Chile is a classic coastal desert, where cold upwelling waters help keep the atmosphere moist yet stable and virtually rain free. Gigantic shields of stratocumulus clouds cover the adjacent ocean, reflecting vast amounts of sunlight. Dependable as they are, the cloud decks also harbor important variations that play into regional and global climate. VOCALS is aimed at characterizing the interplay of chemistry, microphysics, and meteorology across the southeast Pacific in ways that could improve daily, seasonal, and multiyear predictions.
of open cells, or POCs—such as the dark areas shown
here—produce relatively clear areas within large stratocumulus
decks, where the cells tend to be cloud-filled. However,
VOCALS scientists are finding much complexity within the
POC structure. (Satellite image courtesy Rob Wood.)
Contrasts abound in the VOCALS study area. Some of the world’s largest copper smelters, based in Chile and Peru, spew aerosols (airborne particles) that make their way into some of the world’s
cleanest air. Also in the mix are volcanic emissions, dust from the
arid coastal lands, and dimethyl sulfide (DMS), a compound emitted
in vast quantities by phytoplankton. The total number of aerosols
per cubic centimeter varies wildly across the VOCALS study area,
ranging from as many as 10,000 near the coast to as few as 10 above
the open ocean in regions where drizzle is washing particles out
of the air.
How clouds respond to these variations is one of the many questions being addressed in VOCALS-REx, according to Robert Wood (University of Washington, or UW), one of the organizers of the field campaign. “The clouds help set the surface temperature of the southeastern Pacific,” he says. The sunlight-starved ocean stays cool, and in turn, the cool water helps keep the air from warming enough to produce deep convection that could lead to heavy rain. Instead, there’s a more shallow deck of clouds, organized in a cellular fashion.
In recent years scientists have found areas up to 1,000 km (600 mi) wide where the cells appear to be hollowed out (see image above), with patches of open sky surrounded by rings of drizzle, each typically about 20 km (11 mi) wide. These pockets of open cells, or POCs, appear to be where much of the action takes place in an otherwise monotonous climate regime. The very existence of these cloud processes is something of a mystery, says Wood.
“We’ve found that the aerosol and clouds undergo remarkable changes across the boundaries of the POCs,” he says.
Along these edges, the VOCALS team discovered what it’s calling “boundary drizzle cells,” with updrafts and other features comparable to deeper convection but scaled down to heights of only 1 to 1.5 km. Scientists have theorized that smelter emissions and other human-produced particles are converging and coalescing near the POCs, which scavenge the aerosols and form drizzle droplets around them.
Jaque and Cristian Azocar (Universidad Arturo Prat) launch
Christopher Bretherton, also of UW, found himself surprised by two aspects of the local climate. “POCs depend on having the right meteorology as well as clean conditions. They are favored by a deep trade inversion, which doesn’t occur all the time,” he explains, adding that “it’s not obvious to me why POCs seem to be so incredibly much more efficient than the surrounding air in scavenging aerosol particles from the boundary layer.”
The observing strategy for VOCALS included day and night flights by five aircraft: the NSF/NCAR C-130, the U.S. Department of Energy’s G-1, a Twin Otter from the Center for Interdisciplinary Remotely Piloted Aircraft Studies, a Dornier-228 from the U.K. National Environment Research Council, and a BAe146 from the U.K. Facility for Airborne Atmospheric Measurements. More than 200 radiosonde launches were organized by NCAR on the coast near Iquique and from aboard Peru’s José Olaya research vessel, and other land-based data were collected hundreds of miles farther south at Paposo.
NOAA’s Ronald H. Brown research vessel was studded with six mobile containers for atmospheric sampling as well as other specialized equipment for ocean physics, chemistry, and biology. “This is probably one of the most complex suites of observations ever attempted from a research vessel,” says NOAA’s Christopher Fairall. As expected, the ship found large variability in aerosol and DMS counts. “One minor surprise has been the richness of ocean eddy activity,” Fairall adds.
Aside from its own complexities, the southeast Pacific plays a role in another phenomenon familiar to Americans. It helps steer the circulation along the eastern equatorial Pacific where El Niño and La Niña take shape. These two parts of the Pacific are as challenging as any region on Earth for global climate models to depict, says Roberto Mechoso (UCLA), who is chairing the VOCALS Science Working Group.
“Because the models are not accurate for such an extensive area, the El Niños they produce in the Pacific are questionable as well. We hope our research will get rid of, or at least greatly decrease, these uncertainties,” says Mechoso. Many centers are already involved in VOCALS modeling, and principal scientists are planning to meet next summer in Seattle.
“VOCALS is committed to collaboration among modelers and observationalists,” says Mechoso. “There is a lot to do.”
Roberto Mechoso (University of California, Los Angeles) is coordinating modeling efforts in support of VOCALS. (Image courtesy UCLA.)
Video of the VOCALS Field Project. Click here to view video.