Describe the Problem

During development for issue #2729, I uncovered problems in how the range/azimuth grids are defined based on the configuration file settings. These grids are used by the by the TC-Diag and TC-RMW tools.

1. There is an off-by-one error when defining the azimuths.

This issue is in the TcrmwGrid::set_from_data(const TcrmwData & d) function on this line of code:

# INCORRECT
RLLD.delta_rot_lon = 360.0/(Azimuth_n - 1);

The longitude rotation increment is defined as the number azimuths minus 1, and that - 1 should be removed. While doing development for #2729 and comparing the diagnostic output produced by TC-Diag to the output from the Python TC Diagnostics code, removing the - 1 made the previously inconsistent results much, much closer.

2. There is an off-by-one error when defining the range increment.

Internally, MET computes and stores the maximum range value. Since the ranges always start with a range of 0 km, this line of code is correct. We should divide by n-1 when computing the range delta:

# CORRECT
RLLD.delta_rot_lat = range_max_deg/(Range_n - 1);

One bug is in the TC-RMW source code when defining that maximum range value on this line of code. We are multiplying by n_range when it should be n_range-1.

Another bug is in the TC-Diag source code when defining that maximum range value of this line of code. We are multiplying by n_range when it should be n_range - 1.

3. TC-RMW always defines ranges relative to the radius of maximum wind and never as a fixed number of kilometers.

That same TC-RMW line of code from above contains another bug. Note that we're always multiplying by the radius of maximum winds to define the maximum range. The logic to differentiate between RMW ranges and fixed KM ranges is missing.

4. The RMW grid configuration options are over-specified.

In the TC-RMW config file, specifying n_range, max_range_km, and delta_range_km (or rmw_scale) over-specifies the ranges. Only 2 of the 3 are actually required. After consulting with @KathrynNewman, we determined that max_range_km should be removed, and rmw_scale should be set to a default value of bad data (i.e. NA).

To be consistent with the existing documentation, if rmw_scale is set to something other than NA, its value should take precedence over the delta_range_km setting.

Note that the TC-RMW application code needs to be updated to implement this logic correctly.

Expected Behavior

The longitudinal rotation increment should be be computed simply as 360 divided by the number of azimuths (360.0/Azimuth_n). This differs from the latitudinal increment for which Range_n - 1 really should be used. Since the xml/unit_tc_rmw.xml tests use 180 azimuths, it's easy to overlook this very small difference in results since the 360/180 and 360/179 are very similar numbers.

To Reproduce

I tested this using the simplest setup I could imagine, using only 2 azimuths. With only 2 azimuths, the bug is obvious:
RLLD.delta_rot_lon = 360.0/(Azimuth_n - 1) = 360 / (2 - 1) = 360
... versus ...
RLLD.delta_rot_lon = 360.0/Azimuth_n = 360 / 2 = 180

With this bug in place, rotating a fully 360 degrees should result in identical values being extracted for both azimuths. And indeed, that's exactly the behavior I found.

Describe the steps to reproduce the behavior:
1. Modify internal/test_unit/config/TCRMWConfig_pressure_lev_out by changing:

-n_azimuth      = 180;
+n_azimuth      = 2;

2. Run perl/unit.pl xml/unit_tc_rmw.xml
3. Observe the duplicate output values:

ncdump -v TMP test_output/tc_rmw/tc_rmw_pressure_lev_out.nc
 TMP =
  302.540002441406, 302.540002441406,
  302.615292718035, 302.615292718035,
  302.690582994665, 302.690582994665,
  302.2719353873, 302.2719353873,
  301.782571564577, 301.782571564577,
...

Setting n_azimuth = 2 should extract values from opposite sides of the storm but clearly the same values are being extracted here.

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Define Related Issue(s)

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• METplus, MET, METdataio, METviewer, METexpress, METcalcpy, METplotpy
  No impacts, but will change the results of existing METplus use cases.
• Remove the TC_RMW_MAX_RANGE_KM configuration option  METplus#2513 to remove the tc_rmw_max_range_km config option from the TC-RMW wrapper.
• Update Truth: For dtcenter/MET#2840 METplus#2526 to update the METplus truth data due to TC-RMW and TC-Diag diffs.

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