Filed difference on wind speed averaging in METAR/SPECI IWXXM messages not indicated

[mgoberfield]

ICAO Annex 3, Appendix 3 4.1.3.1, (20th Edition) states that METAR and SPECI wind speed and direction shall be 10-minute averages under nominal conditions (e.g., no sudden wind shifts, frontal passage, etc). However, US reporting standards for METAR and SPECI specify that 2-minute averaged values shall be used instead.

Currently NGITWS in performing the METAR/SPECI TAC->IWXXM translation, the 2-minute averaged wind direction and speed values in US METAR/SPECI reports are placed in <iwxxm:windDirection> and <iwxxm:windSpeed> elements that imply conformance with ICAO Annex 3 reporting standards. This is not correct.

Four options for resolution:

1. No change to current practices in generating and reporting wind direction and speeds even when automated observing systems are producing IWXXM METAR/SPECI messages natively and the TAC form is no longer a reporting standard. Consumers of these messages would need to be aware of the difference in averaging periods as it's not explicitly indicated in the IWXXM message itself.

2. Provide a 10-minute average wind direction and speed. Obviously, this cannot be obtained from the TAC form at this time.

3. a) Change US IWXXM METAR/SPECI messages to set <iwxxm:surfaceWind>'s nilReason to withheld.
   b) Create a new complex type for <iwxxm:MeteorologicalAerodromeObservation> extension element that explicitly indicates that the wind direction and speed are 2-minute averages.

4. Implement options 2 and 3b so that both 10- and 2-minute averages are available in IWXXM METAR and SPECI messages.

Requesting an action on how to proceed. Options 1 and 2 require no change to the UML model.

[mgoberfield]

Mark Zettlemoyer in a 2 June 2021 e-mail suggested the addition to option 1: Add plaintext remark to indicate that wind direction and speed is a 2-minute average.