The LI-7500... Riding With "Miss Piggy"

Photo courtesy of N.O.A.A.
Photo courtesy of Frank Marks
Hurricane Isabel, Sept. 16, 2003

As another hurricane season approches, few of us will soon forget the devastation wrought by such recent storms with now-familiar names like Isabel, Fabian, Rita, and of course, Katrina. Anyone in the path of one of these killer storms can appreciate the work that a number of hurricane researchers perform; work that helps forecasters predict hurricane intensity, and when and where the hurricane will make landfall.

A variety of specially equipped National Oceanographic and Atmospheric Association (NOAA) aircraft are used in hurricane forecasting, as well as for other meteorological research. These "hurricane hunters" include two Orion WP-3D turboprop aircraft, one of which is affectionately known as Miss Piggy (see "For More Information" at the end of this article for an explanation of where the Muppet insignia originated). The P-3s have been NOAAs primary airborne platforms for meteorological research since the late 1970s. NOAA pilots routinely fly the P-3s at low altitudes (1500 – 10000 ft.), directly through the eyewall of a hurricane, amidst rain, hail, strong winds, and violent updrafts and downdrafts. Data collected during these roughly ten-hour missions are fed into computer models that predict hurricane intensity and landfall.

As part of the Office of Naval Research (ONR)-sponsored Coupled Boundary Layers Air-Sea Transfer (CBLAST) experiment, Dr. William M. Drennan of the Rosenstiel School of Marine and Atmospheric Science at the University of Miami (FL) and colleagues at NOAA outfitted a P-3 to carry out direct turbulent flux measurements in the high wind boundary layer of a hurricane. The CBLAST experiment took place in the Atlantic ocean during the 2002-2004 hurricane seasons. Six flights were made during the 2003 hurricane season alone; three into Hurricane Fabian, and three more into Hurricane Isabel. Hurricanes draw their energy supply from the ocean. Fluxes of sensible heat and water vapor enrich the atmospheric boundary layer; as warm, moist air is transported into the hurricane’s inner region or rainbands, it fuels the release of latent heat that drives the storm. An understanding of these air-sea fluxes is needed to improve model predictions of hurricane intensity and path.

The LI-7500 (bottom of photo) was mounted in an
aluminum enclosure and strapped down.

A need for fast response humidity measurements was identified early in the CBLAST experiment. Closed path Infrared Gas Analyzers (IRGAs) were unsuited for the hurricane environment, where rain and sea spray could enter the optical path and cause damage to the instrument, or alternatively, cause the instrument’s calibration to be adversely affected. Dr. Drennan chose the LI-COR LI-7500 Open Path CO2/H2O Analyzer to avoid these problems. Because the struts on the LI-7500 are not designed for the stresses encountered during aircraft deployment, Dr. Drennan mounted the LI-7500 in an aluminum enclosure, with the LI-7500’s sensor head strapped down. The enclosure was insulated and maintained at a constant 40 °C temperature. Air flow was directed through an insert placed directly in the LI-7500 sensor head optical path. The LI-7500 was calibrated in this enclosure using the LI-COR LI-610 Portable Dew Point Generator. Dr. Drennan found that the sea spray or rain had little effect on the LI-7500 measurements despite the aircraft passing through regions of heavy rain. After successfully deploying the LI-7500 aboard Miss Piggy, a second LI-7500 was later installed in Kermit, another P-3 Hurricane Hunter.

The LI-7500 is designed to make fast, precise measurements of in situ densities of CO2 and H2O in turbulent air structures. The LI-7500 features low power consumption, no internal or external tubing, a sealed sensor head that allows operation in harsh environments (-25 to 50 °C), and a variety of data output options, including Digital-to-Analog (DAC) Converters, updated at 300 Hz. For more information about the LI-7500, visit www.licor.com and click on the Gas Analyzers link.

For More Information:
LI-7500 Open Path CO2/H2O Gas Analyzer
http://celebrating200years.noaa.gov/magazine/muppets/welcome.html
www.aoml.noaa.gov/hrd/aircraft.html
www.publicaffairs.noaa.gov/grounders/hurricanehunters.html
cblast.ecs.umass.edu/cblast/overview.php
www.whoi.edu/science/AOPE/dept/CBLASTmain.html
www.noaa.gov
www.aoc.noaa.gov

References:
Drennan, William M. et.al., 2007. Turbulent Fluxes in the Hurricane Boundary Layer, II: Latent Heat Fluxes.
/J. Atmos. Sci./ 64:1103-1115.