which wind component should the hot film voltages be calibrated against?

[garyjg]

I have been presuming the wind component perpendicular to the hot film should be derived from the tilt corrected coordinates, which means rotating the $u, v$ coordinates in the geo-gravity dataset by the azimuth offset back to sonic instrument coordinates, so $u$ is positive into and along the sonic boom, perpendicular to the hot film.

My question is this: Should just the $u$ component be used, the wind component in the horizontal plane and perpendicular to the hotfilm, or should the wind speed be $|(u,w)| $, since $w$ is also perpendicular to the hot film?

Note no survey of the hot films themselves were done, and they were replaced multiple times, so we are just assuming they were horizontal. Of course, eyeballing horizontal is not very accurate, especially if eyeballing against the sonic boom which itself may not have been level. So maybe we may as well just use the $u$ (or $|(u,w)| $) measured by the sonic in instrument coordinates, since that may be closest to the perpendicular wind speed seen by the hot film.

[garyjg]

@ChristophRoden mentioned the following in an email:

https://journals.ametsoc.org/view/journals/atot/27/1/2009jtecha1320_1.xml
https://journals.ametsoc.org/view/journals/atot/28/1/2010jtecha1399_1.xml

The first ref discusses calibration extensively : "As described previously, the probe calibration was made in the plane of the probe only, requiring the out-of-plane velocity component to be relatively small. These requirements are automatically fulfilled for a horizontally oriented X hot film, because the mean vertical velocity in the lower ABL is usually small and only the fluctuating normal velocity is dominant."

The quote makes me wonder if we have been describing the hotfilm orientation correctly and consistently. I'm pretty sure we rotated them so they lied in the horizontal plane, meaning they were aligned with sonic v and orthogonal to u and w, and the probe itself was as "flat" and as level as possible. However, the quote implies that a "horizontally oriented" probe measures horizontal velocity, so the film itself is aligned vertically. Maybe there is a convention for "horizontal" and "vertical" probes that I don't know about?

Likewise, the quote refers to minimizing the out-of-plane velocity, but currently I am not planning to do anything about that. If during a hot film calibration period the wind velocity includes a significant v component, then the calibration may be compromised, but I'm thinking we should leave it up to the user whether to use data during those periods.