Estimated Attenuation

Overview

Attenuation of [math]\displaystyle{ Z_H }[/math] ([math]\displaystyle{ A_H }[/math]) and differential attenuation of [math]\displaystyle{ Z_{DR} }[/math] ([math]\displaystyle{ A_{DP} }[/math]) occur whenever scattering occurs, but the effects are most noticeable for radars with shorter wavelengths and when precipitation is intense. In the LROSE applications, attenuation and differential attenuation are both estimated from [math]\displaystyle{ K_{DP} }[/math]. The attenuation-corrected fields can be used by both RadxPid and RadxRate.

Background

Attenuation occurs due to absorption and scattering by particles in the radar volume and in Rayleigh-scattering regimes, the attenuation is proportional to the liquid water content and can be shown to be nearly linearly related to [math]\displaystyle{ K_{DP} }[/math] for radar frequencies between 5-19 GHz (Bringi and Chandrasekar 2001). Simulations using gamma-shaped drop size distributions show power law relationships between attenuation and [math]\displaystyle{ K_{DP} }[/math]. These relationships take the form of [math]\displaystyle{ A_H=\alpha K_{DP}^{b} }[/math] and [math]\displaystyle{ A_{DP}=\beta K_{DP}^{b} }[/math] for attenuation and differential attenuation, respectively. The coefficients [math]\displaystyle{ \alpha }[/math], [math]\displaystyle{ \beta }[/math], and [math]\displaystyle{ b }[/math] vary based on radar wavelength.

Attenuation Correction in LROSE

The default setting in LROSE is to used the temperature-averaged coefficients detailed in Table 7.1 (page 494) of Bringi and Chandrasekar (2001) based on the radar wavelength. The default coefficients can be overwritten in the [math]\displaystyle{ K_{DP} }[/math]-specific parameter file, by setting KDP_specify_coefficients_for_attenuation_correction = TRUE. The coefficients can then be specified in KDP_dbz_attenuation_coefficient, KDP_dbz_attenuation_exponent, KDP_zdr_attenuation_coefficient, and KDP_zdr_attenuation_exponent.

The specific usage of attenuation-corrected [math]\displaystyle{ Z_H }[/math] and [math]\displaystyle{ Z_{DR} }[/math] depends on the LROSE application.

RadxKdp

If you are running RadxKdp, the attenuation-corrected fields can be saved to the output files by adding them to the 'Specifying Output Fields' section in output_fields in the main [math]\displaystyle{ K_{DP} }[/math] parameter file.

The coefficients will be set according to KDP_specify_coefficients_for_attenuation_correction as described above in the [math]\displaystyle{ K_{DP} }[/math]-specific RadxKdp parameter file.

RadxPid

If you are running RadxPid, which estimates [math]\displaystyle{ K_{DP} }[/math] first, the attenuation-corrected fields can be used to determine the PID by setting PID_use_attenuation_corrected_fields = TRUE in the main PID parameter file.

The coefficients will be set according to KDP_specify_coefficients_for_attenuation_correction as described above in the [math]\displaystyle{ K_{DP} }[/math]-specific RadxPid parameter file.

RadxRate

If you are running RadxRate, which estimates [math]\displaystyle{ K_{DP} }[/math] and then determines the PID first, the attenuation-corrected fields can be used to determine the PID by setting PID_use_attenuation_corrected_fields = TRUE and to estimate the precipitation rate by setting RATE_use_attenuation_corrected_fields = TRUE in the main Rate parameter file.

The coefficients will be set according to KDP_specify_coefficients_for_attenuation_correction as described above in the [math]\displaystyle{ K_{DP} }[/math]-specific RadxRate parameter file.

References

Bringi, V. N., and V. Chandrasekar, 2001: Polarimetric Doppler Weather Radar. Principles and Applications. Cambridge University Press, 636 pp. Book