A forced radially outward how with secondary, buoyancy induced convection has been studied numerically in an axisymmetric geometry, consisting of two differentially heated, horizontal, coaxial, circular plates with a diameter of 25 times their mutual spacing. A forced laminar flow is supplied through the centre of the upper plate. The onset of thermal instability, leading to axisymmetric and three-dimensional rolls, has been determined as a function of the Reynolds, Prandtl and Rayleigh numbers. (C) 2000 Elsevier Science Ltd. All rights reserved.