TURBULENCE AND SHEARING STRESSES MEASURED OVER THE NORTH ATLANTIC OCEAN BY AN AIRPLANE-ACCELERATION TECHNIQUE

Abstract

A method of measuring the horizontal and vertical turbulent components of the wind has been developed to meet the need for these measurements over the open ocean. An anemometer, a vertical accelerometer, and a gyroscope mounted in an airplane (U. S. Navy PRY-6A) yield simultaneous records of the total airspeed, vertical accelerations, and the attitude of the airplane. From these records, and from airplane-lift theory and the characteristics of the particular type of aircraft, the vertical and horizontal fluctuations of the wind, the root-mean-square deviation velocities, and the shearing stress can be computed. The method allows the phugoid oscillations of the airplane to contribute nearly periodic velocity terms to both components; these must be eliminated by a computational step. A different reduction method can give phugoid-free vertical velocities, but an additional instrument will be required for phugoid-free horizontal velocities. The airplane lift has a response of from 80 to 100 per cent to gusts of radius 20 meters or more, and the method fails to measure reliably gusts larger than 350 m. This limits the contributions to the quantities presented to those by gusts within the 20- to 350-m range. In the lower levels of polar-continental air masses, root-mean-square vertical velocities have been observed to attain 191 centimeters per second over land and 120 cm/sec over water, while stresses reached maxima of 16.6 dyne/cm2 over land and 3.8 dy/cm2 over water. Shearing stresses in the lowest 60 m in the trade winds averaged 0.2 dy/cm2 over seven days, while vertical velocities averaged 26 cm/sec. The influence of atmospheric stability and convective activity upon the turbulent velocities is discussed.