Estimating the Orientation and Spacing of Midlatitude Linear Convective Boundary Layer Features: Cloud Streets

Linear features in a clear convective boundary layer (CBL) over the North Atlantic Ocean were studied during a weak cold air outbreak using a down-looking airborne lidar. Sequential lidar profiles were placed together and color coded to provide images of aerosol and molecular scattering from below the aircraft to the ocean surface, over a 36-km segment of a flight track approximately 150 km off the coast of southern Virginia. The aircraft flew on a path approximately perpendicular to the expected orientation of cloud streets if they had formed. The lidar image clearly shows randomly sized convective cells in the CBL, grouping under the crests of a gravity wave in the stable troposphere. It is suggested that the wave develops as energetic convective cells in the CBL penetrate into the stable layer aloft and act as obstructions to the relative flow. An analytic study, published in 1965, demonstrates that vertical disturbances on the top of the CBL adjust to be in resonance with a horizontal gravity wave in the free troposphere. The results of the study along with an interpretation of the lidar images have led to the development of a simple conceptual model that is used to estimate the spacing and orientation of long linear convective features in the midlatitude CBL. In addition, the conceptual model can explain the change in cloud street patterns with increasing fetch, seen in satellite images. Comparisons with observations from this study and five other midlatitude field programs show good agreement. A suggestion for future research is presented.