Refractive Index Structure Parameters: Time-Dependent Calculations Using a Numerical Boundary-Layer Model

Stephen D. Burk Naval Environmental Prediction Research Facility, Monterey, CA 93940

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Abstract

A second-moment turbulence closure model is used to investigate the make-up of the refractive structure parameter Cn2 for acoustic, optical and microwave radiation. For these different forms of radiation, Wesely (1976) develops functions describing the dependencies of Cn2 on temperature and moisture structure parameters. Expressions are developed here which relate the temperature and moisture structure parameters to model ensemble-averaged turbulence variables. This permits model evaluation of the Wesely functions throughout the planetary boundary layer.

Three numerical experiments are conducted. Two deal with the marine planetary boundary layer (MPBL) and the final one involves an overland simulation. In the MPBL cases, moisture fluctuations play a dominant role in determining microwave Cn2 and significantly affect acoustic and optical Cn2 values. In fact, in one MPBL experiment, the near-surface acoustic and optical Cn2 values are primarily determined by turbulent moisture perturbations. The overland simulation shows large diurnal variations in structure parameters. Moisture fluctuations are dominant aloft in determining microwave Cn2, but during the afternoon near the surface, temperature perturbations make a comparable contribution to microwave Cn2. Acoustic and optical Cn2 are determined primarily by temperature fluctuations except near the inversion, near sunrise and near sunset.

Abstract

A second-moment turbulence closure model is used to investigate the make-up of the refractive structure parameter Cn2 for acoustic, optical and microwave radiation. For these different forms of radiation, Wesely (1976) develops functions describing the dependencies of Cn2 on temperature and moisture structure parameters. Expressions are developed here which relate the temperature and moisture structure parameters to model ensemble-averaged turbulence variables. This permits model evaluation of the Wesely functions throughout the planetary boundary layer.

Three numerical experiments are conducted. Two deal with the marine planetary boundary layer (MPBL) and the final one involves an overland simulation. In the MPBL cases, moisture fluctuations play a dominant role in determining microwave Cn2 and significantly affect acoustic and optical Cn2 values. In fact, in one MPBL experiment, the near-surface acoustic and optical Cn2 values are primarily determined by turbulent moisture perturbations. The overland simulation shows large diurnal variations in structure parameters. Moisture fluctuations are dominant aloft in determining microwave Cn2, but during the afternoon near the surface, temperature perturbations make a comparable contribution to microwave Cn2. Acoustic and optical Cn2 are determined primarily by temperature fluctuations except near the inversion, near sunrise and near sunset.

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