Summer cover crop species suggestions for cool, dry and high elevation climates.

I'm interested in building soil in a very large plot where i will alternate between cool and warm season cover crops. I plan on broadcasting the summer mix into the standing cool season mix in early summer then crimp down the cool season mix on top of the seed. My problem however is that I am having trouble coming up with a diverse summer mix for my area. I live in a higher elevation area with cool dry summers. I've experimented with some species that are supposed to be very drought tolerant like sorghum, lablab and cowpeas, but these did not do well at all and I think it is because they require hot weather which we just don't get too much of here. I also tried sunflowers and buckwheat which both did very well here so they will be included in this years mix. I'm just wondering what are some other summer annual cover crops that I could add to my mix that would do well in my area.  I'd like to have several forbs, grasses and legumes if possible and I'm willing to experiment so any suggestions are welcome. Thanks!