INTERCONNECTS

Findings from this year's active Atlantic hurricane season confirm that track forecasts have markedly improved, following computer-modeling advances at the National Oceanic and Atmospheric Administration (NOAA) that include the use of enhanced wind data from parachute-borne instrument packages devised at the National Center for Atmospheric Research (NCAR). The interagency U.S. Weather Research Program (USWRP) is providing support for the model improvements. Preliminary data from NOAA's National Hurricane Center show that the average track error in NHC's 2003 Atlantic forecasts has been the smallest on record. Some of this year's hurricanes, such as Isabel, followed relatively smooth paths that made track forecasts easier. Others, such as Kate and Nicholas, evolved far more erratically. Such variation in storm behavior from year to year is one of several factors that influence forecast quality. However, "the past four years have seen a consistent improvement in our forecast tracks," says NHC deputy director Edward Rappaport. The boost in hurricane track accuracy follows a more steady improvement since the 1960s of 1-2% a year. Since 2000, the NHC forecasts have benefited from major USWRP-supported improvements in the global computer forecast model developed at NOAA's Environmental Modeling Center (EMC), a part of NOAA's National Centers for Environmental Prediction (NCEP). The 48-hour forecasts from NCEP's global model now predict the tracks of Atlantic tropical cyclones about 35% more accurately than they did prior to 2000, Rappaport says. "Improvements to our forecasts are directly linked to advances in the numerical models," says Rappaport. "The models drive the process of making better forecasts. The sustained improvement in performance by the NCEP global model provides confidence to the hurricane forecasters charged with making the official forecast." In the late 1990s, USWRP-supported scientists at EMC upgraded the NCEP model to include a better depiction of the hurricane core circulation, a more detailed description of the hurricane's larger-scale environment through advanced use of satellite data, and improved model physics. Other changes to the NCEP model allow it to more accurately incorporate data from dropsondes, the parachute-borne instrument packages designed at NCAR and deployed by NOAA's high-altitude Gulfstream IV aircraft. Introduced in the mid-1990s, these enhanced dropsondes yield far greater detail on winds in and near hurricanes, including high-altitude steering currents. The dropsonde development was funded by the National Science Foundation (NCAR's primary sponsor), NOAA, and the German Aerospace Center. "These improvements helped lead to an outstanding track forecast of Hurricane Michelle in 2001," says Naomi Surgi, leader of EMC’s advanced hurricane modeling project. While most other forecast models showed that Michelle would strike Florida, the NCEP global model correctly predicted the storm would curve northeast, away from the state. Based on this guidance, unnecessary evacuations were avoided. Despite their progress in track prediction, forecasters have made comparatively little headway in predicting how strong a hurricane will become and how much rain it will produce. Inland rainfall is now one of the main hurricane-related threats to life and limb. For example, more people died from inland flooding related to Tropical Storm Allison in 2001 than died directly from the far more intense Hurricane Andrew in 1992. "The intensity problem and the rainfall problem are now the most significant challenges we face over the next decade," says Surgi. "This is what USWRP will be focusing on." USWRP support is also assisting in the development of the multi-agency Weather Research and Forecasting (WRF) model, to be used by NOAA for public forecasts starting in late 2004. WRF is the successor to a long line of research models developed at NCAR, including the widely used MM5, the latest version of the NCAR/Penn State mesoscale (thunderstorm-scale) model. MM5 has been a mainstay of the research community and has been used in forecasting operations in a number of countries. "WRF will serve both the research and operational communities by providing a direct link between the two," says USWRP chief scientist Robert Gall, who also heads NCAR's meteorology division. "In this way, new forecasting technologies developed by researchers can move quickly into day-to-day forecasting operations." A research-oriented version of WRF, with a resolution as fine as 2.5 miles (4 kilometers), tracked Hurricane Isabel at NCAR this autumn. NCEP is developing a special version of WRF tailored for hurricanes. Called HWRF, it will include air, sea, and land coupling; a nested wave model; and analysis techniques that use airborne data to provide a better initial description of the three-dimensional circulation within the hurricane's core. "In combination with these advances, we expect that HWRF's high resolution and state-of-the-art physics will help improve the skill of hurricane intensity and rainfall outlooks," says Surgi. The HWRF is expected to replace the current operational high-resolution hurricane model at NCEP in 2006. USWRP sponsors include NOAA, NASA, NSF, and the U.S. Department of Defense.