Hurricane season in the North Atlantic Ocean officially began on June 1, though the season typically peaks in September. Through August 1, 2011, four tropical storms had developed in the Atlantic basin, none of them reaching hurricane strength. Nonetheless, some hurricane watchers expect this to be an above-average season.
Earth Observatory caught up recently with Scott A. Braun, a research meteorologist at NASA’s Goddard Space Flight Center. Scott’s specialty is using computer models to recreate the components of hurricanes, including winds and rainfall, in order to better understand the detailed inner workings of storms. He has been an active participant in several NASA-led research programs, including the recent Genesis and Rapid Intensification Processes experiment. He is also leading a new effort to use two unmanned aircraft to study the formation and intensification of hurricanes. We asked Scott for his take on the 2011 Atlantic hurricane season.
As we head into the height of the season, what are your expectations? Or do you stay away from seasonal predictions?
We do not perform hurricane predictions at NASA since we are a research agency, rather than an operational one. NASA’s role is to develop new technologies or to use current ones to better measure the characteristics of hurricanes and the conditions that produce them. We make that information available to operational agencies so they can use it to develop their own forecasts.
Hurricane research at NASA is mostly focused on the physical processes that underlie hurricane formation and intensification, as well as the processes that affect rainfall. Some research focuses on how hurricane activity is related to larger-scale climate processes and how it might change in a future climate. But in general, we do not try to produce forecasts of seasonal activity or of individual storms. That is the role of the National Oceanic and Atmospheric Administration (NOAA).
We seem to be between La Niña and El Niño events, how might that affect the season?
While there may be considerable variability from event to event, in general Atlantic hurricane activity is greater during La Niña years and lower in El Niño years. La Niña and El Niño affect ocean temperatures in the Pacific, but they also change the large-scale atmospheric circulation in a way that impacts the Atlantic. In an El Niño year, the air masses in the western Atlantic experience greater overall sinking, which suppresses cloud development. El Niño also increases upper level winds and deep-layer vertical wind shear, which both act to prevent or weaken storms. In La Niña events, the large-scale sinking of air is reduced and the upper-level winds tend to be weaker, so conditions are more favorable for storm development.
How might the recent volcanic eruptions in Iceland, Chile, and Eritrea affect the formation of hurricanes?
I am not aware of any evidence to suggest that volcanic eruptions have a direct effect on tropical cyclone activity.
2011 has been a crazy year for weather in North America, and many people have been pointing to warming temperatures and other aspects of global change. What do you say to people who ask if climate change is affecting the weather?
Although severe weather is expected to increase as a result of global warming, it is difficult to attribute the events of a given year to global warming or climate change. Weather is the result of a complex combination of atmospheric circulation patterns, including La Niña and El Niño, the North Atlantic Oscillation, and others. These patterns can combine to favor severe weather in some years and suppress it in others. In order to attribute extreme weather to climate change, we would need to see a trend showing that storms have been increasing over some long period of time, rather than an increase in any particular year. Future studies will be needed to determine whether this year’s weather is part of such a trend.
What is the most interesting thing that has happened to you or that you have observed while studying hurricanes?
Last year, I got to fly through the eye of a category 4 hurricane during the GRIP field experiment. While I had been on hurricane flights during previous campaigns, I had never been through a clear eye of a major hurricane. During GRIP, I got to lead two DC-8 flights into Hurricane Earl on August 29 and 30, 2010. On the first day, as the storm intensified from category 1 to 2, the eye was still fairly cloudy at upper levels and there wasn’t much of a view. On the 30th, Earl intensified to a category 4 storm and developed a clear eye, offering a fantastic view of the eye and eye wall.
Through GRIP, I also was involved with the first-ever use of high-altitude, unmanned flights over hurricanes. We successfully flew the Global Hawk aircraft over storms from an altitude of about 62,000 feet (19,000 meters). The long duration of the aircraft—about three times that of conventional aircraft—allows for much longer sampling of the intensity and structure of storms. Using that experience, I am now leading a new project called the Hurricane and Severe Storm Sentinel that will use two Global Hawks to sample both the environment of storms and their inner-core structures.
What are the biggest unanswered questions about hurricanes?
The biggest questions are related to storm formation and intensification. While we know the conditions that favor development, we still do not know the exact mechanisms by which a tropical disturbance becomes a tropical storm or hurricane. We do not know whether storm formation is determined more by the environment of the disturbance or by processes inside the disturbance (such as thunderstorm activity). Once a storm forms, its rate of intensification can be very hard to predict. Again, it is unclear if intensification is related to the inner workings of the storm or characteristics of the wider environment.
What will it take to answer those questions?
We need better satellite and aircraft observations of storm environments and their inner workings. Then those observations need to be blended into computer simulations to investigate which physical processes are most important and how they work. If we can better understand these processes and make them more predictable, then we can develop and improve techniques for hurricane prediction.
Are there any interesting NASA-funded efforts to study hurricanes?
In 2010, NASA conducted the Genesis and Rapid Intensification Processes (GRIP) experiment, in coordination with some NOAA and National Science Foundation experiments. NASA used its DC-8 and WB-57 aircraft, as well as the Global Hawk, to sample a number of storm systems. We obtained excellent data on the formation of Hurricane Karl and Tropical Storm Matthew, as well as observations of the rapid intensification of Karl and Hurricane Earl. Research using these data is underway.
While there are no planned NASA hurricane flights in 2011, NASA will be conducting the Hurricane and Severe Storm Sentinel (HS3) field campaign from 2012 to 2014 to obtain a more comprehensive data set on storm formation and intensification. HS3 will use two Global Hawk aircraft: one to sample the storm environment and another to sample the inner cores of hurricanes.
Unlike GRIP—where the Global Hawk flew from NASA’s Dryden Flight Research Center in California—HS3 flights will take off and land from NASA’s Wallops Flight Facility in Virginia. With flights based on the East Coast, the aircraft will be able to fly up to five hours in storms off the western coast of Africa and as much as 20 hours around storms off the East Coast of the United States. The long duration of the Global Hawk flights (up to 25-30 hours), plus specially designed instruments, should greatly increase observing capabilities and our ability to address the major science questions.
