Dual-polarization configuration file for attenuation correction of Z

The distinct parameter lines are listed and described below, with their default settings for C-band radars.

The first section of the configuration file sets the parameters used to calculate the actual attenuation at each range bin.

#=================================================#
#Attenuation correction
# Algorithm selection. Available options:
# DP - propagational depolarization based attenuation
#      (dBZ_real(r) = dBZ_measured(r) + alpha PhiDP(r)).
# IDPC - iterative DP constrained attenuation correction.
#        Single iteration with DP corrected dBZ(r) as
#        an initial dBZ(r).
algorithm = IDPC

This setting selects of the dual-polarimetric attenuation corrections, based on the iterative constrained process or only the first guess field. The recommended setting for all operational systems is IDPC.

# DP attenuation constant for horizontal polarization alpha_h = 0.105

This setting sets the value of the constant used within Equation 4 (see Iterative PhiDP Correction) when computing the total horizontal reflectivity attenuation along the radial. The 0.08 value is the recommended global setting.

This value is slightly variable with temperature. The following table shows the estimated alpha_h correction coefficients for several wavelengths and mean temperatures provided by Jameson 1991.

Estimated alpha_h Correction Coefficients for Different wavelengths and Mean Temperatures

Temperature (°C)	S-band (2.80 GHz)	C-band (5.48 GHz)	X-band (9.34 GHz)
0	0.026	0.097	0.248
10	0.019	0.081	0.241
20	0.015	0.066	0.230
30	0.012	0.053	0.213
40	0.009	0.043	0.195

# DP attenuation constant for vertical polarization
alpha_v = 0.065

This setting sets the value of the constant used in Equation 4 when computing the total vertical reflectivity attenuation along the radial. The 0.105 value is the recommended setting. The value is slightly variable with temperature.

# The melting layer (aka bright band) above sea level.
# No attenuation is added in the part of the ray above ML
melting_layer = 1300.0

This setting sets the height of the 0° C level above the surface reference height in IRIS. This sets a limit in the correction algorithm to not add accumulate additional attenuation when the beam is above this height. This value is only used within the IRIS/Reingest process. RVP10 uses the melting level values specified in setup memory location.

# The above value for melting layer is used only if melting
# layer altitude is not set for processed data set or if
# the following flag is set to 'yes'
force_conf_melting_layer = no

When re-ingesting data into IRIS and performing dual-polarization attenuation correction, setting this question to yes overwrites the melting level height originally stored with the data. This allows users to correct wrong heights or add a height value to the archived data, which may not have had the variable stored.

# The b parameter from the reflectivity based attenuation
# equation: A(r) = a Z(r)^b
b_exp = 0.78

This sets the value of the exponent within the summation terms seen in equation 5. The recommended value is 0.78 for C-band radars, and 0.46 for X-band radars.¹

The PhiDP section of the configuration file sets the parameters used to condition and unfold PhiDP for input to the attenuation correction, either the LSQ or Cubic Spline² techniques.

# PHIDP processing parameters

# PHIDP processing algorithm type (in IRIS reingest, use
# dspx for RVP):
# LSQ - Weighted least square fit to a linear function
#       FIR filter coefficients are used as weights. Use
#       fir_width to control width of the filter.
# SPLINES - Smoothing and numerical derivatives calculation using cubic splines algorithm
PhiDP_alg = SPLINES

This parameter allows you to choose SPLINES or LSQ method during IRIS/Reingest processing. In RVP10 processing, a similar question is in the dspx mp menu.

# Smoothing factor for the first, non-adaptive, stage of
# cubic splines processing (in IRIS reingest, use
# dspx for RVP)
standard_smoothing_factor = 0.1

For IRIS/Reingest, this parameter sets the smoothing factor during the first pass through the Angular PhiDP data when performing cubic spline processing. In RVP10 processing, a similar question is in the dspx mp menu.

# Smoothing factor for the second, adaptive, part of
# cubic splines processing (in IRIS reingest, use
# dspx for RVP)
adaptive_smoothing_factor = 1.1

For IRIS/Reingest, this parameter sets the adaptive smoothing factor during the second pass through the Angular PhiDP data when performing cubic spline processing. This factor will be adjusted with a weighting function. In RVP10 processing, a similar question is in the dspx mp menu.

# The width, in km, of FIR filter used as weights in
# LSQ processing. (in IRIS reingest, use
# dspx for RVP)
fir_width = 3.0

For IRIS/Reingest attenuation correction, the impulse response of the FIR filter is configured in kilometer widths. In RVP10 processing, these parameters are set in the dspx mp menu. This variable also sets the range window for calculating K_dp.

The data thresholding parameters are used to reduce the amount of data being considered during the attenuation correction process. This is performed to reduce the CPU load on the computer.

# =========================================================
# Data thresholding parameters (for processing mask)

# The shortest distance, in km, at which polarimetric
# measurements, such as PHIDP, RHOHV, ZDR, etc, are
meaningful.
recovery_range = 1.0

Close to the radar site the dual polarimetric data values may not be representative of the intrinsic values due to near-field antenna issues. This setting specifies an initial range limit to ignore during the attenuation correction processing.

# Lowest allowed/meaningful SNR
snr_lim = 3.0

With very weak signals the PhiDP measurements become very noisy. A signal to noise ratio threshold may be set. This flags range bins, having SNR below this value, to not be included into the calculations.

# Lowest allowed/meaningful RHOHV
rhohv_lim = 0.8

Hydrometeor particles typically have RhoHV values greater than 0.85. The setting creates a threshold to not allow data from range bins having RhoHV values below the criteria to enter the calculations.

The weather classification algorithm parameters are used in creating the rain versus no weather classifications. See Cubic spline fit.

# =========================================================
# The weather classification algorithm parameters. (for the
# build in weather mask)

# Number of sequentially "good" bins needed to transition
# from "no weather" to "rain".
# Note: It is also the width of the gate used to compute
# STD of PHIDP, and mean value of RHOHV. [bins]
long_gate = 15

This question defines the width of a sliding window to calculate the average RhoHV and standard deviation of PhiDP at specific range bins. The window width is defined in range bin units.

# Number of sequentially "bad" bins needed to transition
# to "no weather". [bins]
short_gate = 10

When the range bin is currently in a rain segment, but the average RhoHV and PhiDP standard deviation are below the thresholds, this setting states how many consecutive range bins must be below the criteria to exit the rain segment.

# Lowest allowed mean RHOHV at the transition to "rain"
rhohv_enter_r = 0.85
# Highest standard deviation of PhiDP at the transition
# to "rain" [deg]
PhiDP_std_enter_r = 7

These 2 criteria are the definitions when the range bin is considered to be the start or a rain segment. Both criteria must be met to define the start of a rain segment.

# Low mean RHOHV that triggers exit from "rain"
rhohv_exit_r = 0.70

# High standard deviation of PhiDP that triggers exit
# from "rain" [deg]
PhiDP_std_exit_r = 15.0

This sets the thresholds when the range bin is thought to be bad and should be in the No Weather category. When either of the actual values are below the average RhoHV or above the standard deviation of PhiDP, the bin is flagged as bad. When a consecutive number of [short gate] bins are flagged as bad the rain segment transitions to No Weather.

# Lowest allowed SNR in "hail" region
# Note: Should be higher then snr_lim
# snr_exit_h = 8.0
# Lowest allowed RHOHV in "hail" region.
# Note: Should be lower then rhohv_lim_r.
# rhohv_exit_h = 0.6

# Standard deviation of PhiDP that triggers exit from "hail"
# Note: Should be higher then PhiDP_std_exit_r
# PhiDP_std_exit_h = 20.0

As hail is uniquely different than the presumed rain and No Weather classification, there is a different amount of attenuation than with liquid hydrometeors. In the future, the attenuation correction algorithm will include a different computation to calculate the amount of attenuation within the hail segment. For now, the parameters to distinguish a hail segment are place holders for future development.

This section allows you to specify parameter quantities per site. This is especially useful in the case of IRIS/Reingest receiving data files from multiple radar sites. To use this parameter, state [SITENAME] as a header line and repeat any of the name-value pairs from within this configuration file that must be unique to that site.

# ========================================================
# per-site sections can be used to add per-site variation
# of the parameters set in the general section.
[wes-grad]
alpha_h = 0.1