Extended Abstract (708K)1A.6
Turbulent flux measurements within a hurricane boundary layer from an instrumented aircraft
Jeffrey R. French, NOAA/ARL, Idaho Falls, ID; and P. G. Black
Measurements of sensible and latent heat and momentum flux within the boundary layer were acquired using an instrumented aircraft in two storms during the 2003 hurricane season. A NOAA P3 research aircraft was instrumented with a specially modified NOAA Air Resources Laboratory-designed 'Best Aircraft Turbulence' (BAT) probe to measure simultaneously the three-dimensional wind vector and ambient air temperature to near 1 m spatial resolution. A open-path device measuring infrared absorption provided estimates of high frequency fluctuations in the water vapor field.
Six flights were made, 3 each in Hurricanes Fabian and Isabel during the period when both storms were category 4/5 on the Saffir-Simpson Scale. During each flight, one to three hours of flight time were dedicated to performing 'stepped descents', a pattern designed to measure turbulent fluxes at several levels from the top of the Boundary Layer to 200 feet above the surface. The stepped descents were performed within dry slots between rain bands. Leg lengths were roughly 20 km and flown both parallel and perpendicular to the wind, accounting for changes in wind direction with altitude.
During the six research flights, useful measurements were obtained from the BAT probe in four complete stepped descents. Surface winds at the location of the stepped descents measured from dropsondes and a stepped frequency microwave radiometer (SFMR) ranged from 45 kts on the first flight in Fabian to 55 kts on the second and third flights in Isabel. Preliminary analysis of the data reveal a strong trend for increasing variance in the vertical velocity and temperature field with decreasing altitude in the boundary layer. Similarly, power spectra from w'(theta)' show a well-defined inertial subrange at all levels and increasing power within the inertial subrange with decreasing altitude. The peak wavelength of w'(theta)' fluctuations decreases in increasing altitude. All of these observations are consistent with our understanding of boundary layer dynamics and provide evidence that the instruments were indeed able to capture the dynamic and thermodynamic character of the hurricane boundary layer. Continued analysis will focus on calculation of fluxes at flight levels and extrapolation of surface values.
Session 1A, CBLAST I
Monday, 3 May 2004, 8:30 AM-10:15 AM, Le Jardin Room
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