Halliday & Matthiessen (1999: 217-8):
Circumstantial elements can be realised grammatically either as adverbial groups or as prepositional phrases. These represent circumstances of two different types.Type (1) are qualities ("simple circumstances") — but construed not as qualities of a particular participant but as qualities of a figure as a whole; for example, steadily, extremely loudly, perfectly in it rained steadily all night, they were shouting extremely loudly, it suits your complexion perfectly. Typically such adverbs are derived in the grammar from adjectives, with the added suffix -ly; a few have special forms, like well (from good), and sometimes the same form is both adjective and adverb, e.g. fast in a fast car, she drives fast.The usual function is as circumstance of Manner, with the meaning 'in such a way', 'to such a degree'; and if the manner of doing determines the quality of the outcome there may be very little difference between a circumstance of this kind and a resultative Attribute: cf. don't chop the parsley too fine/too finely. There are some adverbial expressions realising other types of circumstance, e.g. everywhere (Location: space), recently (Location: time); as well as others which might be interpreted differently because of the nature of the quality itself, e.g. pointlessly 'in a pointless manner' or 'for no good reason' (Cause).Sometimes the Manner element is a quality of the process itself, rather than the manner in which it unfolds, for example he was falsely accused/wrongly dismissed; these tend to be bonded rather closely to the Process element in the clause. (The adverbial form also functions as a quality of a quality, like frostily, superficially in a frostily polite receptionist, what he says is superficially correct; here it is not a circumstantial element but is part of the Epithet in the nominal group.