Cloud bands in the earth's atmosphere: Observations and Theory

Abstract

It is now well known that parallel cloud bands are widespread in the earth's atmosphere. Observations from manned and unmanned spacecraft and from high-altitude aircraft in connection with soundings from ships and ground stations have shed light on their origin. These and a special investigation of tropical cloudstreets during the BOMEX Project suggest the following typical characteristics of convective cloudstreets: Length = 20 to 500 km; spacing = 2 to 8 km; layer height = 0.8 to 2 km; width-to-height ratio = 2 to 4; wind structure: little change of direction with height; vertical gradient of wind shear (profile curvature) = 10?7 to 10?6 cm?1 sec?1; alignment: along the mean wind of the convective layer. On the theoretical side, linear wind shear is known to favor convective “streeting”. The present theory investigates the effect of the observed profile curvature neglecting linear shear effects. It shows that the curvature itself enforces alignment of ccnvective cells with the flow direction. Inertial forces arising from the vorticity field counteract buoyancy forces. Their ratio as expressed in a modified Froude numer determines the value of the critical Rayleigh number responsible for the onset of convection. In a flowing medium this number is raised, often by several orders of magnitude, over that of a resting medium for all convective modes, except the longitudinal mode. Some three-dimensional computer presentations illustrate these results. A quantitative application of the simplified theory to actual atmospheric conditions is attempted. It indieates that in strong flows heated from below longitudinal rolls may double their amplitude in a matter of 10 minutes while transverse rolls decay at a similar rate with symmetric cells having nearly neutral stability. The relations of this concept to other hypotheses and to the Goertler/Taylor rolls are discussed. Finally it is speculated that the formation of wind streaks on water surfaces may be related to a similar mechanism. DOI: 10.1111/j.2153-3490.1971.tb00585.x