View source for Estimated Attenuation

=== '''Overview''' ===
Attenuation of <math>Z_H</math> (<math>A_H</math>) and differential attenuation of <math>Z_{DR}</math> (<math>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>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>K_{DP}</math> for radar frequencies between 5-19 GHz ([http://wiki.lrose.net/index.php/Estimating_Attenuation#References Bringi and Chandrasekar 2001]). Simulations using gamma-shaped drop size distributions show power law relationships between attenuation and <math>K_{DP}</math>. These relationships take the form of <math>A_H=\alpha K_{DP}^{b}</math> and <math>A_{DP}=\beta K_{DP}^{b}</math> for attenuation and differential attenuation, respectively. The coefficients <math>\alpha</math>, <math>\beta</math>, and <math>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 [http://wiki.lrose.net/index.php/Estimating_Attenuation#References Bringi and Chandrasekar (2001)] based on the radar wavelength. The default coefficients can be overwritten in the <math>K_{DP}</math>-specific [http://wiki.lrose.net/index.php/RadxKdp#2.29_.5Bmath.5D.5Cdisplaystyle.7B_K_.7BDP.7D_.7D.5B.2Fmath.5D-specific_parameter_file 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>Z_H</math> and <math>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>K_{DP}</math> [http://wiki.lrose.net/index.php/RadxKdp#1.29_Main_parameter_file parameter file]. 

The coefficients will be set according to ''KDP_specify_coefficients_for_attenuation_correction'' as described above in the <math>K_{DP}</math>-specific [http://wiki.lrose.net/index.php/RadxKdp#2.29_.5Bmath.5D.5Cdisplaystyle.7B_K_.7BDP.7D_.7D.5B.2Fmath.5D-specific_parameter_file RadxKdp parameter file].

====== '''RadxPid''' ======
If you are running RadxPid, which estimates <math>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 [http://wiki.lrose.net/index.php/RadxPid#1.29_Main_parameter_file parameter file]. 

The coefficients will be set according to ''KDP_specify_coefficients_for_attenuation_correction'' as described above in the <math>K_{DP}</math>-specific [http://wiki.lrose.net/index.php/RadxPid#2.29_KDP-specific_parameter_file RadxPid parameter file].

====== '''RadxRate''' ======
If you are running RadxRate, which estimates <math>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 [http://wiki.lrose.net/index.php/RadxRate#1.29_Main_parameter_file parameter file]. 

The coefficients will be set according to ''KDP_specify_coefficients_for_attenuation_correction'' as described above in the <math>K_{DP}</math>-specific [http://wiki.lrose.net/index.php/RadxRate#2.29_KDP-specific_parameter_file RadxRate parameter file].


===== '''References''' =====
Bringi, V. N., and V. Chandrasekar, 2001: Polarimetric Doppler Weather Radar. Principles and Applications. Cambridge University Press, 636 pp. [https://doi.org/10.1017/CBO9780511541094 Book]