The most appropriate single word to describe sunrise(dawn) glow
Consulting the Oxford English Dictionary reveals that while the first definition for 'gloaming' refers specifically to sunset
a. Evening twilight.
the OED also admits of a second meaning
b. Said occasionally of morning twilight.
from which we can also note that 'twilight' itself is not reserved to sunset, as confirmed by the OED definition:
- The light diffused by the reflection of the sun's rays from the atmosphere before sunrise, and after sunset; the period during which this prevails between daylight and darkness.
'Crepuscular' is also related to the ambiguous 'twilight'
- Of or pertaining to twilight.
but also specifically to morning
b. esp. Resembling or likened to the morning twilight as preceding the full light of day; characterized by (as yet) imperfect enlightenment.
So certainly there is nothing in any of those definitions to prevent you using them to describe morning light. Indeed, even 'effulgent' which you claim as a quality more pertinent to sunset is defined as:
Shining forth brilliantly; sending forth intense light; resplendent, radiant
Which cannot be more applicable to the going down of the sun than its rising.
However, I think I do understand what you are getting at. The growing dawn light is perceived as perhaps paler and cooler than sunset, and you seem to be looking for a term relating to the sky itself rather than general light levels, as such I would suggest that 'Brightening' may be appropriate:
Brightening n. 1. The action of making or becoming bright; illumination.
While the term is not exclusive to this meaning, it is used and understood in context, as per this example from Photoreview.com.au
The 'golden hours' for landscape photography occur between pre-dawn brightening in the sky and about an hour-and-a-half after sunrise in the morning
and this from a random blog
I crack the window. I blink, keeping my eyes closed too long. There's a brightening in the sky. I step on it. I arrive home with the dawn, relieved.
*all definitions per OED