ACCURATE CALIBRATION OF DOPPLER WINDS FOR THEIR USE IN THE COMPUTATION OF MESOSCALE WIND FIELDS

TETSUYA FUJITA Department of the Geophysical Sciences, The University of Chicago, Chicago, Ill.

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Abstract

Doppler winds measured by an instrumented aircraft are of great value in determining the wind field accompanying large-scale atmospheric disturbances. When they are utilized in interpreting and computing the wind fields of so-called mesoscale disturbances with their horizontal dimensions of a few to a few hundred miles, slight errors in the vector quantities forming the navigation triangle result in fictitious winds which differ considerably from the real winds. In the first part of this paper, the wind velocity errors due to the backscattering water particles illuminated by Doppler beams, designated as wet, beams, are discussed. The influence of wet beams upon Doppler winds was calculated theoretically under various conditions to allow an estimate of maximum wind velocity error. Following the solution of wet-beam cases, theoretical consideration was given to the fluctuation of the measured winds caused by the constant errors in the true air speed, the aircraft heading, the Doppler ground speed, and the Doppler drift angle. For the purpose of investigating whether this type of error occurs or not, test flights were made over Florida and Oklahoma along a number of loops with varying diameters. Results of the evaluation revealed that the error in aircraft heading is of least importance and that the other errors can be determined and corrected with a high degree of accuracy provided only a few specific loops are flown during each mission. It has become feasible to calculate both divergence and vorticity associated with mesoscale disturbances from the calibrated Doppler winds measured along well-designed flight tracks. It is expected that the basic research presented in this paper will stimulate the use of Doppler wind systems in the determination of the detailed structure of winds accompanying mesoscale meteorological systems.

Abstract

Doppler winds measured by an instrumented aircraft are of great value in determining the wind field accompanying large-scale atmospheric disturbances. When they are utilized in interpreting and computing the wind fields of so-called mesoscale disturbances with their horizontal dimensions of a few to a few hundred miles, slight errors in the vector quantities forming the navigation triangle result in fictitious winds which differ considerably from the real winds. In the first part of this paper, the wind velocity errors due to the backscattering water particles illuminated by Doppler beams, designated as wet, beams, are discussed. The influence of wet beams upon Doppler winds was calculated theoretically under various conditions to allow an estimate of maximum wind velocity error. Following the solution of wet-beam cases, theoretical consideration was given to the fluctuation of the measured winds caused by the constant errors in the true air speed, the aircraft heading, the Doppler ground speed, and the Doppler drift angle. For the purpose of investigating whether this type of error occurs or not, test flights were made over Florida and Oklahoma along a number of loops with varying diameters. Results of the evaluation revealed that the error in aircraft heading is of least importance and that the other errors can be determined and corrected with a high degree of accuracy provided only a few specific loops are flown during each mission. It has become feasible to calculate both divergence and vorticity associated with mesoscale disturbances from the calibrated Doppler winds measured along well-designed flight tracks. It is expected that the basic research presented in this paper will stimulate the use of Doppler wind systems in the determination of the detailed structure of winds accompanying mesoscale meteorological systems.

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