Halliday & Matthiessen (2014: 364):
But prepositional phrases encroach on the functional ground of adverbial groups, partly by means of phrasal templates such as in a ... way (manner), as in yeah it’s not done in an antagonistic way (instead of .... not done antagonistically); and adverbial groups may serve as Location in time or space. These latter often have as Head an adverb that derives from preposition + noun (for example upstairs, outside, overseas; today, tomorrow).