Clouds and thermals in the convective boundary layer can exist in a variety of forms. These range from cellular convection to well defined cloud streets or rolls. Also, such convective motions typically have a well defined horizontal scale which ranges from approximately the depth of the mixed layer upwards. Such organization and scale selection can occur in both dry and moist conditions. However, neither the horizontal scale selection mechanism, nor the transition from cellular convection to roll convection are well understood.

The convective motions in the mixed layer generate gravity waves which exist primarily in the stable atmosphere aloft. These waves also have horizontal scales which are well defined and close to that of the boundary layer thermals. Previous studies have shown that these gravity waves are intricately coupled to the convective motions, and that there is possibly a resonant effect which determines the horizontal scale.

Using a numerical cloud model, it will be shown that the gravity waves generated by the boundary layer thermals feedback on the boundary layer and set the horizontal scale. The mechanism governing the scale selection will be discussed using linear gravity wave theory. This mechanism is also used to discuss the transition from cellular to roll convection. In principle, these arguments can be used to determine the characteristics of boundary layer convection given the background flow conditions.