Tracking typhoons across the Pacific

A far-flung study examines Pacific cyclones and their U.S. impacts

While many Americans gazed anxiously toward the Atlantic Ocean in recent weeks, watching hurricanes stream our way, several dozen staff from EOL and ESSL/MMM kept their eyes on the Pacific. Stationed from Guam to Boulder, they’re part of a vast project called T-PARC (THORPEX Pacific Asian Regional Campaign). Organized by the World Weather Research Program of the World Meteorological Organization (WMO), T-PARC involves university, federal, and military participants from more than 10 countries.

T-ParcThe T-PARC study area spans much of the Northwest Pacific Ocean. Several T-PARC aircraft sampled Typhoon Sinlaku, shown here just east of Taiwan on September 11. (MTSAT-1R image courtesy Japan National Institute of Informatics.)

T-Parc VideoTechnicians from the French space agency, CNES, launch the sixth driftsonde for T-PARC on the early morning of August 27 from Hawaii. The equipment includes (top to bottom) the balloon, a gondola and ballaster from CNES, and the NCAR driftsonde package, including the dropsonde sensors that descend while the balloon is airborne. (Video courtesy Terry Hock, EOL.)

Jim MooreAt the T-PARC ­operations center, Jim Moore (EOL) analyzes Typhoon Nuri, which struck the Philippines on 19 August. (Photo by Bob Henson.)

EnlargeMichael Montgomery (Naval Postgraduate School) joined EOL’s Michael Bell (center) and Wen-Chau Lee (right) for a T-PARC flight. (Photo by Tammy ­Weckwerth.)

BallonStaff from the French space agency (CNES) prepare for an early-morning T-PARC driftsonde launch on Hawaii’s Big Island. (Photo by Hal Cole.)

The team is studying typhoons across the Northwest Pacific Ocean, from their formation in the tropics and subtropics (where the U.S. Navy–led Tropical Cyclone Structure–2008 study is taking the lead) to their typical demise as they move northward and encounter westerly winds at midlatitudes. The resulting interaction can sometimes affect weather in North America and beyond. The field phase of T-PARC began on August 1 and goes through early November.

“We’re running this project across nine time zones,” says Jim Moore, who’s sharing operational management duties with his EOL colleague Dick Dirks. He adds, “In terms of geographic reach, this is by far the biggest project I’ve been involved with in my 25 years at UCAR.” Equally impressive is the scope of T-PARC’s mammoth online field catalog, developed and maintained by EOL. It will hold more than a million analyses and predictions from weather forecast models by the time the experiment is done.

Typhoons and hurricanes differ in name only: they’re both tropical cyclones, which tend to form in light winds above warm oceans. 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 wreak havoc across the east Asia coastline from Vietnam to Japan.

Typhoons typically recurve to the northeast, pumping huge amounts of moisture and warm air into the North Pacific. This can torque 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 Chris ­Davis (ESSL/MMM), one of T-PARC’s principal investigators. Chris 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 Chris. “T-PARC will hopefully provide these observations.” All quiet on the western front? 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. Only one August system, Nuri, featured winds above hurricane force (119 kilometers, or 74 miles, per hour). However, the T-PARC crew has made the most of the storms it’s seen, with successful flights undertaken from Guam by a P-3 aircraft from the Naval Research Laboratory (NRL) and a C-130 operated by the U.S. Air Force. Other aircraft have flown out of Japan and Taiwan.

T-PARC marks 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. The driftsonde system includes a three-story-tall balloon that lofts a set of several dozen instrument packages (dropsondes) into the stratosphere, where light easterly winds prevail. Once airborne, a driftsonde can move lazily westward for days (see graphic on page 1). Using a satellite-based communications system, scientists release dropsondes that fall by parachute into regions of meteorological interest, ­collecting weather data as they descend. Nine driftsonde systems had been launched by early September from a base on the arid, stark ­southwest end of Hawaii’s Big Island. EOL’s Terry Hock and colleagues, and the French ballooning specialists on hand to carry out the launches, are pleased at the results thus far. “The project is having its ups and downs,” Terry says, “but that’s the nature of the balloon business.”

Even farther away from home, a group of EOL technicians and scientists has been based in Guam during T-PARC. They’re operating the Electra Doppler Radar (ELDORA), built through an NCAR-French collaboration and now installed on and integrated with the NRL P-3. “The radar encountered some problems after it arrived in Guam. However, its performance has been very impressive in the research flights ­conducted so far,” says chief scientist Wen-Chau Lee (EOL).

If there’s a home base for the far-flung T-PARC project, it’s the operations center at the Naval Postgraduate School in Monterey. As with most field projects, the “ops center” plays host to daily weather briefings and flight planning discussions. For T-PARC, dozens of participants in Asia, Oceania, North America, and Europe are taking part in the meetings virtually. Through a videoconferencing system called Elluminate, scientists can 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 Jim Moore.

The importance of T-PARC goes well beyond typhoons, according to Dave Parsons. He’s now on leave from EOL and based in Geneva, Switzerland, where he serves as 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 ten-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,” Dave says. He explains that recurving typhoons can trigger or enhance a train of Rossby waves. These dips in the polar jet stream can persist for days, propagating around the globe and spawning destructive events that can range from floods and fire 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,” Dave says.

This article has been revised to include mention of the Tropical Cyclone Structure 2008 study and to identify the Naval Postgraduate School, rather than the Naval Research Laboratory, as host of the T-PARC operations center.