Q:

Rainbows prior to the Mabul

Did the rainbow exist as we know today, prior to the Mabul? Was it the same, different, or it did not exist at all prior to the Mabul? (sources please) 

A:

According to Rav Saadya Gaon (commentary to Genesis 9:13) the rainbow was not a new phenomenon. Rather it was imbued with new meaning as the sign of the covenant.
Similarly, Ramban, writes (commentary to Genesis 9:12) that although the simple reading of the verses indicates it was a new phenomenon, we are forced to accept the conclusion of the Greeks that rainbows are a natural phenomenon based on light shining on the sky, as this phenomenon can be observed when light shines on a container of water. He notes that a preexisting phenomenon can serve as a sign for future events, as in the mound of rocks (Genesis 31:52).
Additionally, R. Isaac Arama writes in his Akedat Yitshak (Parshat Noah: Sha'ar 14) that Ramban who is well known as one the greatest believers, nevertheless concluded, on the basis of Greek arguments that the rainbow is a natural, and hence preexisting phenomenon. He uses this to demonstrate the lesson that the Torah is tarnished when one provides forced explanations for it, and Ramban shows us the proper path.
Similarly, R. Eliezer Ashkenazi cites Ramban in his Torah commentary Ma'asei Hashem (Ma'asei B'reshit: ch. 28) that the rainbow was a preexisting phenomenon. He explains that specific meteorological conditions are necessary to sustain it. After the deluge, that specific combination of meteorological factors may be deliberately chosen by God to convey a message, whereas before they would have been random. Rashbats writes pretty much the same thing in his commentary to Avot (5:6).
Radak (Commentary to Genesis 9:13) also agrees with Rav Saadya Gaon that rainbows were a preexisting phenomenon; the result of rays of light shining towards clouds, but explains that the phenomenon was exaggerated by greater sunlight after the great deluge, leading to more dramatic rainbows.
The Maor Ha'afelah (Parashat Noah Parashat HaKeshet) quoted by the Midrash HaBeiur (Genesis 9:13) also agrees with Rav Saadya Gaon.
Ibn Ezra (ibid) references Rav Saadya, but disagrees and explains that it was a new phenomenon. However, he writes (ibid: 14) that if one accepts the Greek explanation of the rainbow, then one would have to say that after the deluge the sun's intensity increased. He seems to imply that the less intense sun was insufficient to produce rainbows at all. 
Similarly, R. Isaac Abravanel writes (commentary to Parshat Noah ch. 8: question 15) that Ibn Ezra is correct, and that it was a new phenomenon.
R. Bahya ben Asher (Commentary to Genesis 9:13) suggests both possibilities; that it was a preexisting phenomenon, or that is was a new phenomenon.
Ran compromises between the two approaches, in his Torah commentary (9:12-17) He writes that the phenomenon of rainbows wasn't new, but their placement: in clouds or the sky, rather than exclusively in empty sky, was new.
Similarly, R. Isaac Caro (the uncle of R. Yosef Caro, author of the Shulhan Arukh) repeats this compromise of Ran in his Torah commentary Tol'dot Yitshak (9:13); that the rainbows moved from the empty sky to clouds.

To summarize:

Did the rainbow exist as we know today, prior to the Mabul?

Yes. According to Rav Sa'adya Gaon, Ramban, Ralbag, Akedat Yitshak, Maor Ha'afelah, the Midrash HaBeiur Rashbats, R. Eliezer Ashkenazi, and one possibility of R. Bahya.

Did it exist in a different form before the deluge?

Yes. According to Radak, Ran, and R. Isaac Caro.

Did it not exist at all prior to the Mabul?

Yes. According to Ibn Ezra, R. Isaac Abravanel, and one possibility of R. Bahya.

Lastly, the Mishna (Avot 5:12) states that the rainbow was created during the twilight following the sixth day of creation. Which of the above approaches this supports, can be debated.

A:

Rav Hirsch writes on Noach 9:12

It is by no means necessary to assume that hitherto there had been
  no rainbow and to place it in connection with the atmospheric changes
  which occurred after the Flood. Just as Hashem showed Avraham the
  starry heavens and said, כה יהיה זרעך, as He showed Moshe and Aharon
  the new moon, and with the words החדש הזה לכם consecrated this monthly
  phenomenon - which, of course, had been in existence since the
  beginning of the world - to be a sign of physical and moral
  rejuvenation for Israel; as existing seasons and dates became memorials
  for historical and instructive remembrance for Israel, so the rainbow
  can also have been a well-known phenomen in the sky, and now became designated by
  Hashem to be a sign of His covenant with Man and
  the world. The נתתי can therefore quite literally be: My bow, I have
  long ago placed it in the clouds, henceforth it shall serve as a
  sign of the covenant between Me and you.

This means that it is quite possible (as many meforshim say) that the atmosphere had been saturated until now, and as a result rainbows had not yet been a "normal" weather phenomenon. As an example, in Bereishis, we see that the garden was watered by a mist rather than falling rain. Now that the water had all fallen to cause the Mabul, it became a normal occurrance. However, as Rav Hirsch points out, this does not have to be true and the pasuk can be learned either way.

A:

Ramban (Chapter 9, Verse 12) argues that the rainbow existed before the Mabul. However, only after the Mabul did Hashem select it to serve as a sign of His covenant to not cause another Mabul.  The rainbow is a reversed bow.  One who wishes to indicate peaceful intentions reverses his weapon away from the person he is greeting and points the weapon towards himself.  So too, the rainbow faces away from humanity and towards the heavens.  This demonstrates that Hashem will not use the weapon of water to destroy the Earth.
Malbim (Chapter 9, Verse 13) argues that the rainbow did not exist before the Mabul.  In the pre-Mabul world it rained infrequently.  But when it occurred the rain fell in a deluge. There was no opportunity for a rainbow to form.  Only after the vigor of the material world was diminished did it become possible for a rainbow to form.  The rainbow is the perfect symbol of Hashem’s commitment to not causes another Mabul.  It represents the cessation of rain and the reduced vigor of the material world.  It communicates the reason that another Mabul will not be necessary.

