The brief stall warnings at 02:10:10 were the result of a combination of control inputs and turbulence (vertical gusts). That is evident in the traces of normal acceleration shown on page 42 of interim report no.3.
The gust velocities that can be derived from the DFDR data were posted
here.
The meteorological analysis by Tim Vasquez points to the possibility of a gust velocity of 23 m/s = 75.5 ft/s = 4527 ft/min based on the atmospheric temperature profile obtained in a radiosonde ascent from Fernando de Noronhas.
Powdery ICE is thus far unknown to me.
Snow?