Black flecks in ATM data

[benj]

Some (generally high-brightness) level 1 ATM data contains scattered black flecks (can be quite numerous). Example screenshot (from EUCAARI day 136):

Looking at the raw data with qcdisplay, the flecks aren't present, but there are short horizontal white lines. These seem to be less numerous than the black flecks and may or may not be related. Example screenshot (same flight):

Probably not worth doing anything about, since ATM is dying and due for replacement. First noticed as part of #115