Ahh, the subtleties of the English language.

Webster_1913's definition is somewhat incomplete, even though a recent version of the same dictionary has the same definition. I will attempt to remedy this.

Ameliorate is used exclusively to describe the effects of an action rendering an intolerable situation tolerable. A good synonym would be "ease".

Thus, one would not say: "My $500 winning lottery ticket ameliorated my bank account, which had plenty of money in it already."

One might say, however: "My $500 winning lottery ticket ameliorated a situation with my landlord, who was ready to evict me for nonpayment."

However, do *not* use "ameliorate" if an action removes the problem entirely. In that case, use "remedy" or "fix".