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The Effects of Nonuniform Beam Filling on Vertical Rainfall Velocity Measurements with a Spaceborne Doppler Radar

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  • 1 Dipartimento di Elettronica e Telecomunicazioni, Università di Firenze, Florence, Italy
  • | 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
  • | 3 Dipartimento di Elettronica e Telecomunicazioni, Università di Firenze, Florence, Italy
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Abstract

Information on the global distribution of vertical velocity of precipitating particles is needed in estimating latent heat fluxes, and therefore in the general study of energy transportation phenomena in the atmosphere. Such information is not currently available, but it can potentially be obtained by a spaceborne Doppler precipitation radar. In this paper, the expected performance for this type of Doppler radar for measuring vertical rainfall velocity is investigated. Although the high relative speed of the instrument with respect to the rainfall droplets contributes significantly to the spreading of the Doppler spectrum, accurate estimates of the average vertical velocity can be obtained when the rainfall intensity does not vary significantly within the resolution volume of the instrument. Such a result can be inferred through theoretical calculations and is confirmed by analyzing the Doppler spectra simulated using data gathered by the NASA/Jet Propulsion Laboratory (JPL) airborne rain radar in the Kwajelein Experiment (KWAJEX) and in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). When significant variation in rain rate is present within the radar's field of view in the along-track direction, the Doppler shift caused by the radial component of the satellite motion is weighted differently in different portions of resolution cell. The error caused by this nonuniform beam-filling (NUBF) effect may dominate any other contribution. Under this condition, the shape, average value, and width of the Doppler spectrum do not directly correlate with the vertical velocity of the precipitating particles. Further analysis of the reflectivity pattern is required in order to correct for the NUBF-induced error.

Corresponding author address: Eastwood Im, Mail Stop 300-227, Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109. Email: eastwood.im@jpl.nasa.gov

Abstract

Information on the global distribution of vertical velocity of precipitating particles is needed in estimating latent heat fluxes, and therefore in the general study of energy transportation phenomena in the atmosphere. Such information is not currently available, but it can potentially be obtained by a spaceborne Doppler precipitation radar. In this paper, the expected performance for this type of Doppler radar for measuring vertical rainfall velocity is investigated. Although the high relative speed of the instrument with respect to the rainfall droplets contributes significantly to the spreading of the Doppler spectrum, accurate estimates of the average vertical velocity can be obtained when the rainfall intensity does not vary significantly within the resolution volume of the instrument. Such a result can be inferred through theoretical calculations and is confirmed by analyzing the Doppler spectra simulated using data gathered by the NASA/Jet Propulsion Laboratory (JPL) airborne rain radar in the Kwajelein Experiment (KWAJEX) and in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). When significant variation in rain rate is present within the radar's field of view in the along-track direction, the Doppler shift caused by the radial component of the satellite motion is weighted differently in different portions of resolution cell. The error caused by this nonuniform beam-filling (NUBF) effect may dominate any other contribution. Under this condition, the shape, average value, and width of the Doppler spectrum do not directly correlate with the vertical velocity of the precipitating particles. Further analysis of the reflectivity pattern is required in order to correct for the NUBF-induced error.

Corresponding author address: Eastwood Im, Mail Stop 300-227, Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109. Email: eastwood.im@jpl.nasa.gov

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