RR NDB;
Where do you see a drop in temperature? "The background noise changed rapidly around 2 h 09 min 46." (p.73). The ice crystals have the same temperature as the air that carries them. Several UAS reports* notice a sudden rise in temperature, which is to be expected because the rising air in which the ice crystals are formed derives its buoyancy from being warmer than its surroundings. See also
Tim Vasquez' temperature profile at FL 350.
(*)From Interim #2 p.52:
The recordings of total or static temperatures(9) show increases of ten to
twenty degrees during the event, which sometimes began before noticing
any speed anomalies, except in a case where the increase was lower;