MESOSCALE SHALLOW CONVECTION IN THE ATMOSPHERE

This paper is a review of the observational, experimental, theoretical , and numerical studies of mesoscale shallow convection (MSG) ih the a tmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length s cale of a few to a few tens of kilometers, and takes distinctive planf orms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MC C), comprising three-dimensional cells. MSC is characterized by its sh ape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For ce lls the horizontal extent is their diameter, whereas for rolls it is t heir spacing. Rolls usually align along or at angles of up to 10 degre es from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rol ls may occur over both water surface and land surfaces. Mesoscale conv ective cells may be divided into two types: open and closed. Open-cell circulation has downward motion and clear sky in the cell center, sur rounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 1 0 to 40 km and aspect ratios of 5 to 50, and both occur in a convectiv e layer with a depth of about 1 to 3 km. Both the magnitude and direct ion of horizontal wind in the convective layer change little with heig ht. MSC results from a complex and incompletely understood mix of proc esses. These processes are outlined, and their interplay is examined t hrough a review of theoretical and laboratory analyses and numerical m odeling of MSG.