2I suspect there's no answer to your question. You're asking how native speakers would normally refer to the capital of Mongolia in an informal setting, but this situation just doesn't arise often enough for there to be a rule for it. If the capital is Ulaanbaatar then speakers will refer to it as Ulaanbaatar. It would help if you give details for the context, there's a big difference in writing style between writing a letter to a friend and writing in shorthand over instant messenger programs. – Mark – 2015-06-11T14:58:30.227

That's hard. If the typical spelling is 2 distinct words, I would use UB, but if it is usually one word, then I might use Ul or possibly still UB because of where the sylables lie in the word. I don't think I would use Ub, unless I know that this was the correct abbreviation. This one probably requures very specialized knowledge to answer 100% correctly. – Michael Dorgan – 2015-06-11T17:47:59.780

I don't think it's Ub - If I see Ub, I might think you're referring to an element's name. – M.A.R. – 2015-06-11T18:06:56.687

1In the US, we use initials sometimes for those city names with multiple words, like LA or D.C., and sometimes not, like New Orleans and Baton Rouge. If you abbreviate Ulan Bator, very few people will know what you mean unless you define it first. If you're the one defining it, you can choose whatever makes sense to you. Only states in the US have "official" abbreviations for the postal system. – ColleenV – 2015-06-11T18:56:29.903