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RadxPid

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RadxPid produces three-dimensional fields of the NCAR particle identification (PID).

Overview

RadxPid ingests polarimetric data to estimate the dominant hydrometeor type using NCAR’s Particle Identification algorithm. First, KDP is calculated from PHIDP using the same KDP-specific parameter file as RadxKdp. Next, the polarimetric radar data and calculated KDP are combined with sounding data to determine the most likely dominant hydrometeor. The user decides whether or not the algorithm will use the raw Zh and Zdr data from the radar or correct for attenuation. Finally, the resulting three-dimensional PID categories are sent to an output file.


Scientific Background

For more information on the fuzzy logic algorithm used to estimate the NCAR PID, please refer to this page.

Information about how attenuation is estimated from [math]\displaystyle{ K_{DP} }[/math] for use in determining the PID can be found here.

Information on how to convert soundings into the Spdb format can be found here.


Prerequesites

The following items are required:

  • Polarimetric radar data (any Radx supported format, e.g., cfradial)
  • PID fuzzy logic thresholds file
  • Sounding data (either from observations or a numerical model)
  • Three parameter files to run RadxPid, calculate KDP, and PID.


PID thresholds file

These thresholds describe the membership functions for each combination of the PID category and polarimetric field as well as the variable weights for each particle type. The file also contains a place to enter a sounding, if the radar files cover a narrow time period; otherwise, the sounding will be overwritten by data provided by the user, which is explained below. The particle types are most sensitive to temperature, Zh, Zdr, and the standard deviations of Zdr and PhiDP.


Set this to a file suitable for the radar transmit mode and wavelength:


Wavelength Transmit mode thresholds_file_example
S-band Simultaneous pid_thresholds.sband.shv
S-band Alternating pid_thresholds.sband.alt
C-band Simultaneous pid_thresholds.cband.shv
X-band Simultaneous pid_thresholds.xband.shv

Sounding data

If sounding data varies in time, sounding data in a non-gridded data format (SPDB) will need to be ingested. SPDB data can come from either observations or numerical simulations (e.g., RAP or HRRR). For help converting soundings to this data format, please see the documentation here.

Parameter files

RadxPid uses three separate parameter files. The first includes the basic parameters related to data location and field names, which is similar to the parameter files for the other applications. The second includes the parameters that set the filtering length, method, and relevant coefficients necessary to calculate KDP and estimate attenuation. The third specifies filter parameters and relevant information related to the soundings used by the PID.

1) Main parameter file

Ensure file is up to date

To obtain the default parameter file, use the following command:

/path/to/lrose/install/bin/RadxPid -print_params > param_file_name

If you already have a parameter file and simply want to check for (and add) updated parameters while retaining current parameters, use the following command:

/path/to/lrose/install/bin/RadxPid -params orig_param_file_name -print_params > new_param_file_name

Important parameters

Input params

  • input_dir: directory containing radar data (if not specified on the command line and if mode = REALTIME)
  • mode: determines if the program waits for new files (REALTIME), moves through start and end times specified on the command line (ARCHIVE), or moves through list of files specified on the command line (FILELIST)

Input field information

  • SNR_available: determines if SNR data is in the file or needs to be calculated from DBZ
  • VARIABLE_field_name: tells RadxPid the polarimetric variable names in the ingested cfradial files
  • LDR_available: determines if LDR data is in the file

Computing KDP

  • KDP_params_file_path: path to the KDP-specific parameter file

Computing PID

  • PID_params_file_path: file path for the PID-specific parameter file
  • PID_use_attenuation_corrected_fields: determines whether PID uses Z and ZDR fields that are corrected for attenuation

Writing the output files

  • output_dir: files will be written to this directory
  • output_format: select the preferred file type (usually CFRADIAL)

2) KDP-specific parameter file

Ensure file is up to date

To obtain the default parameter file, use the following command:

/path/to/lrose/install/bin/RadxPid -print_params_kdp > param_file_name

If you already have a parameter file and simply want to check for (and add) updated parameters while retaining current parameters, use the following command:

/path/to/lrose/install/bin/RadxPid -params_kdp orig_param_file_name -print_params_kdp > new_param_file_name

Important parameters

Unfolding and filtering

  • KDP_fir_filter_len: Filter length applied to PHIDP

Handling phase shift on backscatter (PSOB)

  • KDP_psob_method: Method to handle PSOB

3) PID-specific parameter file

Ensure file is up to date

To obtain the default parameter file, use the following command:

/path/to/lrose/install/bin/RadxPid -print_params_pid > param_file_name

If you already have a parameter file and simply want to check for (and add) updated parameters while retaining current parameters, use the following command:

/path/to/lrose/install/bin/RadxPid -params_pid orig_param_file_name -print_params_pid > new_param_file_name

Important parameters

NCAR PID Method

  • pid_thresholds_file_path: file path for the PID thresholds file

Sounding input for PID temperatures

  • use_soundings_from_spdb: tells RadxPid whether to override the sounding in the pid thresholds file with SPDB data
  • PID_sounding_spdb_url: path to SPDB sounding data

Running RadxPid

To check all command line options for RadxPid, including debugging options and file paths, type the following command into a terminal.

/path/to/lrose/install/bin/RadxPid -h

Once your parameter file is complete, use a command similar to the following to run the application:

/path/to/lrose/install/bin/RadxPid -params param_file_name

The user can also specify the location of cfradial files in the following manner:

/path/to/lrose/install/bin/RadxPid -f /path/to/cfradial/files/ -params param_file_name

Other relevant items

Additional parameters to edit

Caution: this is not an exhaustive list. We urge each user to read through the entire parameter file carefully.

Main parameter file

Specifying copy-through fields

  • copy_selected_input_fields_to_output: determines whether original fields are copied to the output file
KDP-specific parameter file

Precip-induced attenuation correction for DBZ and ZDR

  • KDP_specify_coefficients_for_attenuation_correction: the user can either set the coefficients themselves or use the default coefficients based on the radar wavelength
PID-specific parameter file

Computing PID

  • PID_snr_threshold: mininmum SNR required for the PID to be calculated
  • PID_min_valid_interest: mininimum interest value required in order for a PID category to be accepted
  • PID_apply_median_filter_to_VARIABLE: determines whether RadxPid applies a filter to specific polarimetric variables before running the PID
  • PID_ngates_for_sdev: sets the number of gates used to calculate the standard deviations of ZDR and PHIDP

Sounding input for PID temperatures

  • sounding_search_time_margin_secs: the maximum allowable time difference between the radar files and sounding data
  • sounding_location_name: directs RadxPid to the appropriate sounding
  • sounding_required_pressure_range_hpa: sets the minimum pressure range required for a valid sounding
  • sounding_required_height_range_m: sets the minimum altitude range required for a valid sounding