Describe the Problem

Development for issue #2402 flagged unexpected differences in the ascii2nc output for AIRNOW observations:

file1: /data/output/met_test_truth/ascii2nc/airnow/HourlyData_20220312.nc
ERROR: NetCDF headers differ:

Investigating this difference further, I was able to narrow it down and locate a bug in the AirnowLocations::lookupLatLonElev() member function. Running the same command multiple times yielded inconsistent results.

Expected Behavior

All AIRNOW data for which the station location is defined should appear in the output. Running the same command multiple times should produce consistent results.

Environment

Difference flagged via GHA. Reproduced on my Mac Laptop.

To Reproduce

Describe the steps to reproduce the behavior. Replicate the behavior by running a very stripped down run of ascii2nc.

1. Use data for a single station:

cat data_for_124000050314.dat
03/12/22|00:00|124000050314|Québec - Charlesbour|-5|OZONE|PPB|39|Canada-Quebec1
03/12/22|00:00|124000050314|Québec - Charlesbour|-5|PM2.5|UG/M3|5.2|Canada-Quebec1

2. Define the airnow station file for only that station:

cat airnow_monitoring_site_locations_v2_124000050314.txt
StationID|AQSID|FullAQSID|Parameter|MonitorType|SiteCode|SiteName|Status|AgencyID|AgencyName|EPARegion|Latitude|Longitude|Elevation|GMTOffset|CountryFIPS|CBSA_ID|CBSA_Name|StateAQSCode|StateAbbreviation|CountyAQSCode|CountyName
124000050314|000050314|124000050314|O3|Permanent|0314|Québec - Charlesbourg|Active|QC1|Canada-Quebec1|CA|46.861500|-71.257200||-5.00|CA||||||
124000050314|000050314|124000050314|PM2.5|Permanent|0314|Québec - Charlesbourg|Active|QC1|Canada-Quebec1|CA|46.861500|-71.257200||-5.00|CA||||||

3. Set $MET_AIRNOW_STATIONS env var:

MET_AIRNOW_STATIONS=./airnow_monitoring_site_locations_v2_124000050314.txt

4. Run ascii2nc for MET version 11.0.0:

./ascii2nc data_for_124000050314.dat data_for_124000050314.nc

5. Observe inconsistent results when run several times:

DEBUG 2: Finished processing 2 observations for 1 headers.

versus

DEBUG 2: Finished processing 0 observations for 0 headers.

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