Simultaneous Radar, Aircraft and Meteorological Investigations of Clear Air Turbulence

Kenneth M. Glover Air Force Cambridge Research Laboratories, Bedford, Mass.

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Roland J. Boucher Air Force Cambridge Research Laboratories, Bedford, Mass.

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Hans Ottersten Air Force Cambridge Research Laboratories, Bedford, Mass.

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Kenneth R. Hardy Air Force Cambridge Research Laboratories, Bedford, Mass.

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Abstract

The results of simultaneous studies of clear air turbulence (CAT) in the lower 15 km of the atmosphere by multi-wavelength radar, jet aircraft and special rawinsondes at the JAFNA radar facility at Wallops Island, Va., are reported. The most important finding is that for the particular aircraft and velocity used in these experiments, every clear air radar echo above 3 km is associated with aircraft reports of at least some perceptible degree of turbulence. Between the altitudes of 3 and 6 km, all CAT is detected by the radars; however, the ability of the radars to detect weak CAT decreases with increasing altitude and only the more intense turbulence is detected above 12 km. The indications are that strong CAT at high altitudes in the free atmosphere is generally associated with zones of increased refractive index variability and enhanced radar backscattering. Therefore, if radars of extreme sensitivity are employed, the useful range for CAT detection may be extended considerably and may possibly satisfy the requirements of an operational ground-based CAT detecting radar system. The vertical vector wind shear appears to be the most significant meteorological factor in specifying turbulent regions. A wind shear criterion ≥ 0.8 × 10−2 sec−1 applied to rawinsonde data specifies the presence or absence of turbulence correctly in 77% of all cases, including 100% of the cases involving CAT greater than light.

Abstract

The results of simultaneous studies of clear air turbulence (CAT) in the lower 15 km of the atmosphere by multi-wavelength radar, jet aircraft and special rawinsondes at the JAFNA radar facility at Wallops Island, Va., are reported. The most important finding is that for the particular aircraft and velocity used in these experiments, every clear air radar echo above 3 km is associated with aircraft reports of at least some perceptible degree of turbulence. Between the altitudes of 3 and 6 km, all CAT is detected by the radars; however, the ability of the radars to detect weak CAT decreases with increasing altitude and only the more intense turbulence is detected above 12 km. The indications are that strong CAT at high altitudes in the free atmosphere is generally associated with zones of increased refractive index variability and enhanced radar backscattering. Therefore, if radars of extreme sensitivity are employed, the useful range for CAT detection may be extended considerably and may possibly satisfy the requirements of an operational ground-based CAT detecting radar system. The vertical vector wind shear appears to be the most significant meteorological factor in specifying turbulent regions. A wind shear criterion ≥ 0.8 × 10−2 sec−1 applied to rawinsonde data specifies the presence or absence of turbulence correctly in 77% of all cases, including 100% of the cases involving CAT greater than light.

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