From Alex to Jeanne, hurricanes in the Atlantic Ocean have been in the news lately, with more storms on the horizon. Scientists at the Naval Research Laboratory in Monterey, California, are part of ongoing hurricane forecasting efforts to make storm predictions more accurate and timely.

As part of NRL's hurricane forecasting research efforts, NRL's Mr. Jeff Hawkins recently participated on a research flight into Hurricane Ivan. He was participating in a scientific field program to gain better understanding about hurricane environment structure via a high-altitude NOAA jet sampling Hurricane Ivan while it was at Category 5 intensity.

The flight took place Thursday, September 9, aboard a NOAA Gulfstream (G-IV) jet. The jet departed Tampa, Florida, flew above the Bahama Islands, turned right between the Dominican Republic and Puerto Rico, flying around Ivan to the east, south, and west of the storm. The jet then flew back north between Cuba and Haiti and returned to Tampa. The NOAA G-IV jet is tasked to obtain atmospheric measurements around hurricanes. This research was done in collaboration with Mr. Jason Dunion working at NOAA's Hurricane Research Division in Miami, Florida. The G-IV flies around the storm using a track that "optimizes" the data distribution selection using forecast model sensitivity output. The sensitivity analysis enables flight planners to focus on those regions where "data collection" will have the biggest potential positive impact (another area of NRL research).

During the flight, scientists collected atmospheric measurements of winds, temperature, moisture and atmospheric pressure from the aircraft altitude of 41,000 feet to the ocean surface using the airborne vertical atmospheric profiling system (AVAPS). Small GPS dropwindsondes, about 2 feet long and 3 to 4 inches in diameter, are "dropped" from the aircraft and float via chute to the ocean surface. This descent typically takes about 20 minutes. Measurements are constantly relayed to the aircraft, quality controlled onboard, and then sent to shore. Such data sets have proven crucial in improving three-day numerical model storm track forecasts by 20 percent or more. The dropwindsonde data sets are used in real-time by NOAA and Navy models, and are also used by the National Hurricane Center, as well as by NRL, to better understand the storm environment, such as wind shear aloft, dry air entrainment, maximum winds near the surface, minimum sea level pressure, and distribution of winds throughout the column.

Mr. Hawkins' particular interest during the research flight was the impact of the Saharan Air Layer (SAL) on tropical cyclones. The SAL comes off west African as a very dry stable air mass that inhibits convection and clouds. SAL air can maintain its properties surprisingly well across the tropical Atlantic journey and can still be very strong in the Caribbean or western Atlantic. SAL air can surround a storm or come into contact with storms and impact their intensity by infusing dry/stable air into the system. The storms can be weakened by this encounter and then can intensify when they break away from SAL air constraints. Scientists are working to understand this process (how frequently, what space and time scales, what directly impacts intensity) using a combined data set from satellite remote sensing (NRL's role), in-situ measurements (the Gulfstream G-IV), and the few land-based radiosondes.

During the flight, Mr. Hawkins measured some very dry SAL air near the eastern Bahamas that agreed well with satellite measurements of total columnar water vapor derived from the Special Sensor Microwave/Imager (SSM/I) space sensor. Mr. Gene Poe, also from NRL's Marine Meteorology Division, led the effort that performed the calibration and validation of these sensors.