Andrea, I stitched NEF in Photomerge of CC2015, saved to 16bit TIFF and saved "for the web". The overall dark gray tone of the clouds displayed here looks pretty much true to the real clouds I observed. Your version looks a bit too pleasant.
Keith, yes, I should have included the earth horizon stuff more for better composition as pano. Also, I should have used s tripod and the pano head. But it was dawn and the lighting condition changes so fast, so I decided to hand-hold the camera.
Originally I was just trying to capture this beautifully elaborate shape of the clouds for my personal record, but decided to share here because of its interesting look.
Thanks, anyway, for your comments and advices.