backfilling profile gaps for migration with low sd_vvp using airspeed

[adokter]

For C-band OPERA radars profiles sometimes show gaps that are associated with low sd_vvp (below the default sd_vvp_treshold = 2 m/s). We propose to backfill those gaps by considering airspeed. This could be done with a function that does the following

input : a vp or vpts object

parameters:

• airspeed_min: the minimum airspeed associated with a height bin to be considered birds. default tbd by @YohanSassi
• sd_vvp_min: the minimum sd_vvp value for a height bin to be considered for backfilling, by default 0, or a different optimal value tbd by @YohanSassi.
• vid_min: an optional minimum Vertically Integrated Density for a profile to be considered for backfilling. This will help with backfilling only cases with high migration. Experience is that gaps / holes never cover the full altitude profile, only part of it. Therefore selection on a minimum VID is feasible and will help eliminating cases without bird migration (reducing risk of backfilling precipitation).
• airspeed: name of the vpts column containing the airspeed, default "airspeed".

Function name: TBD, e.g. backfill_by_airspeed()

Backfilling will then occur according to this rule:

If airspeed > airspeed_min && sd_vvp_min < sd_vvp < sd_vvp_threshold then dens = eta / rcs

Another common airspeed filter used in the field is removing altitude bins with low airspeeds (typically lower than 5). We propose this should not be part of this specific function, but it could be implemented as an equivalent to sd_vvp_threshold(), e.g. airspeed_threshold(). Then airspeed_threshold(my_vpts) <- 5 would remove all altitude bins with airspeed below 5 m/s.

[YohanSassi]

From the tests we made it appears that the optimal values are:

• airspeed_min = 7
• sd_vvp_min = 1.6
• vid_min = 1

I think you forgot the vid_min parameter when describing the backfilling process, it should be:
If airspeed >= airspeed_min && sd_vvp_min <= sd_vvp <= sd_vvp_threshold && vid >= vid_min then dens = eta / rcs

The function to remove all altitude bins with airspeed below a specific value e.g., airspeed_threshold(my_vpts) <- 5 will be useful.