EPIC simulations of bright companions to Neptune's great dark spots

We present a three-dimensional model of the bright companion clouds associa ted with Neptune's Great Dark Spots (GDSs). Our results support the hypothe sis that the bright companions of the southern-hemisphere GDS discovered in 1989 and of the northern-hemisphere GDS discovered in 1994 are methane clo uds that form at or just below the tropopause and that they are caused by l ifting in a manner analogous to the formation of orographic clouds. We vary the vertical position of the GDS and find that the companion cloud is a ro bust feature except when the anticyclone itself does not survive. When a GD S is started with its top in the stratosphere it drifts much too rapidly to ward the equator and quickly disperses. On the other hand, if its top is we ll below the tropopause there is a tendency for the companion clouds to be too large. Hence the top of a GDS is probably at the tropopause. Along an o pen streamline that threads a bright companion, the typical pressure and te mperature drops are about 3 mb and 1 R, respectively, corresponding to a li ft of about half a kilometer or 4% of the pressure scale height, and the re lative horizontal wind speed is about 45 m s(-1) eastward through the cloud . (C) 2001 Academic Press.