This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 曂 is uncertain. It combines 日 (sun) with 黄 (yellow), but whether this is a semantic compound or a phono-semantic construction is unclear.
With 日 (sun) beside 黄 (yellow), imagine a bright yellow sun filling the sky, suggesting clarity or brightness, though the actual meaning is uncertain.
For コウ, picture a bright yellow sun shining over a calm cove: cove -> コウ, and the sun 日 makes the yellow 黄 light clear.