[aerohead]

Beautiful car! As the windshield is relatively flat,I suspect air is being directed over the roof in such a way,that it's velocity,as compared to that at the side of the car,is much higher,and leads to very powerful C-pillar vortices,common in fast-back style cars.And do the the upward rake of the rear quarter panel,I suspect the air is colliding together much as depicted in the graphic.On my CRX,I created a hatch wing that integrated the C-pillars,and provided increasing tumblehome,and increasing lateral radius as it extended towards the tail.Extending into the boattail,this added structure provided a mixing zone,where the roof and side airstreams could meet at nearer the same velocity,virtually eliminating the vortices.As seen from the rear,above the beltline of the car,the outline of the car is semicircular in cross-section.The slope of your Tigra's backlight seems a little "fast",at 15-degrees,and your "bubble"is evidence of minor flow separation,which is no big deal.I can't think of a fix that does not entail lengthening the car,which affects your tailights,and also raises the spectre of creating a blindspot in your rear field of vision,something I've had to live with, with the CRX.Lessening the backlight angle may compromise vision due to diffraction.Extending the rear spoiler straight back to a point where it's upper edge intersects the roofline at 12-degrees,instead of 15,while perhaps introducing a locked vortex above,may allow the flow above to exit at a slower velocity.By not "narrowing" the spoiler,the edges may help to "burst" the powerful C-pillar vortices,and allow easier mixing of the air.Without flow imaging,its a tough nut to crack.