I'd like to brew a black lager. I don't have a temperature control setup yet, so accepting that my fermentation will be around 68-70 F, what ale yeasts would come the closest to mimicking the clean and crisp characteristics of a lager, or is this possible? Would adjusting pitching rate/aeration help?
The best choice is Kolsch Yeast.
Only downside is 68-70 ambient temps. Those yeasts need to ferment around 65, so try to get the temp down a little more. Put your carboy in a water bath and drop in a few frozen water bottles each day. That should get you down to the mid-60's.
Also, after you bottle, you'll need to let the beer carb up, then "lager" it in your fridge, in maybe a 12-pack at a time, for a few weeks before drinking it. The beer will change a lot in that time. Kolsch's can seem a bit fruity when young, but it drops out to a nice crispness later.
I do a nice clone of Negra Modela with Kolsch yeasts. Here's the recipe: http://hopville.com/recipe/203557/vienna-lager-recipes/negra-kolschero
I'll put in my two cents.
I would say the best analog of the clean, crisp taste of a (pils) lager would be a California Common.
Wyeast's WY2112 is actually listed as a lager yeast, but has a much higher temperature profile than standard lager yeasts. I do not consider WY2112 a lager yeast, but there it is.
WLP001 is White Labs California Common yeast, considered by them to be an ale yeast.
Both strains produce clean, clear beers that accentuate hop flavor. Hoppy characteristics are more known for the style of California Commons. Either yeast can be used in other recipes and styles and still produce a clean, crisp beer.
Graham's suggestion of Kolsch yeasts is also a good one, those yeasts do nice imitation lager as well.
The downside is that with all of these yeasts, Kolsch or California Common, you will need a temperature at most in the mid-60s. Yeast of any variety simply produce a lot of esters at higher temperatures.
Good Luck, hope this helps!
There are a number of ale yeasts that stay clean, but you'll be hard pressed to achieve that at those temps. And then keep in mind that you HAVE to cold condition the beer after fermentation to get anywhere near a lager. I don't find kolsch (WAY too fruity IMO) or CA common yeasts clean enough for pseudo lagers. My go to yeast for that is WY1007. WY1056/WLP001 would be next with US05 after that. But in order to do what you need to do, you really have to ferment them under 65 and 60 is better. When they're done, you need to give them a month or 2 around 35 to really condition them and make them crisp. Unfortunately, adjusting pitching or aeration won't do you much good for this. There's just really no way around it.
I've had good luck with WLP008 East Coast Ale yeast for pseudo lagers. Your attenuation will be a little lower but very clean to my pallet. Works well at 72°F (my brew room temp). It does require a little time to mature in the bottle / keg. Young it has a slight sour taste. To my knowledge Wyeast doesn't have a competitor for the East Coast Ale strain.
It's hard to find right now, but Charlie Papazian's Cry Havoc yeast from White Labs made just an excellent maibock at about 68F for me. A long, slow, cool aging period was at least partly responsible for it, but the yeast lives up to its purported ability to ferment at ale temperatures.
I detected none of the berry or apple esters described here. That may be because maibock is less restrained than, say, a pilsener. In any case, if you can find it, give it a try.
A little FWIW: After 25+ yr.s of lager brewing (roughly 96% lager to ale ratio) my main lagering unit; circa 1946 died. I decided to tip-toe back to ales as my mainstay. I was excited about clean lager like beer without the decoctions and long fermentations. First yeasts that met my fermentation environment were Calif. Common and Cry Havoc. For approx. 8 barrels, I split the wort between these two for side by side analysis.
I was struck by the similarities and am convinced if not siblings, these yeast are no further removed than cousins. I found both to extremely flexible, temperature wise and their behavior, appearance and final results so close, the average Joe can't taste the difference.
I do not have my notes in front of me but I recall: Both take a really long time to completely ferment out. Yeah, I could've cut it short and had fine beer; but: given lengthy time in primary (many weeks to months) really provides an ultra clean, clear beer. Really close to lagering times!
Of course a lot depends on all the key elements to brewing; water, malt bill, mash and boil techniques, hopping; etc, ad nauseam.
But, it is temperature flexible, though most mine w/these strains started warm and ended on the cool end of the range. If you are patient, I have let these go for many months on the warm end and had clean lager-ish characteristics.
That being said, given your temperature constraints, why not make a clean ale with London Ale or some other old standby?