]> The bottom layer of the troposphere that is in contact with the surface of the earth. A layer of air tens of meters thick at the bottom of the atmosphere where the variation of vertical turbulent flux with altitude is less than 10% of its magnitude. A layer within the atmosphere bounded below by the surface, and above by a more or less sharp discontinuity in some atmospheric property. Internal boundary layers are associated with the horizontal advection of air across a discontinuity in some property of the surface (e.g., aerodynamic roughness length or surface heat flux) and can be viewed as layers in which the atmosphere is adjusting to new surface properties. See thermal internal boundary layer, mechanical internal boundary layer. (Abbreviated MIBL.) An internal boundary layer caused by advection of air across a discontinuity in surface roughness. When the new surface is rougher than the old one, the MIBL depth grows roughly as the 0.8 power of the ratio of the two roughness lengths. In this example, the MIBL grows to include the whole surface layer. A type of atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix, primarily in the vertical, quantities such as conservative tracer concentrations, potential temperature, and momentum or wind speed. The cool layer of air adjacent to the ground that forms at night. At night under clear skies, radiation to space cools the land surface, which in turn cools the adjacent air through processes of molecular conduction, turbulence, and radiative transfer. This causes a stable boundary layer to form and grow to depths of a few hundreds of meters, depending on the season. Many interacting processes can occur within the statically stable nocturnal boundary layer: patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets. For flow over a hill, the top layer in the boundary layer that accelerates relative to its upstream value due to the Bernoulli effect. The bottom layer of the troposphere that is in contact with the surface of the earth. It is often turbulent and is capped by a statically stable layer of air or temperature inversion. The middle portion of the nocturnal atmospheric boundary layer characterized by weak sporadic turbulence and initially uniformly mixed potential temperature and pollutants remaining from the mixed layer of the previous day. Same as transition layer. The lowest atmospheric layer immediately adjacent to a surface covered with relatively large roughness elements such as stones, vegetation, trees, or buildings. A type of atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix, primarily in the vertical, quantities such as conservative tracer concentrations, potential temperature, and momentum or wind speed. The statically stable layer of air at the top of the atmospheric boundary layer. Because the troposphere is statically stable on the average (i.e., potential temperature increases with height), and because turbulence in the boundary layer causes potential temperatures to become somewhat well mixed there, conservation of heat requires that there be a potential temperature increase (i.e., a temperature step or inversion) at the top of the boundary layer. It is this inversion that separates the boundary layer from the rest of the troposphere by limiting the domain of turbulence. It is also responsible for trapping pollutants near the ground during fair weather. A statically stable layer at the top of the atmospheric boundary layer. An air layer with its base at the ground surface and in which temperature increases with height. These often form at night over land under clear skies and are statically stable. See inversion, lapse rate. A sharp horizontal temperature contrast of a few degrees Celsius within a width of tens of centimeters to a few meters along the trailing edge of a thermal plume in the surface layer of the atmospheric boundary layer. The development of an area in the boundary layer (e.g., often observed on surface or 850-mb charts) where moisture values become higher than in the surrounding region. Moisture pooling typically occurs in an area of low-level convergence during the warm (growing) season, and can have a significant effect on convection initiation and evolution. The three-part change of the atmospheric boundary layer that typically occurs during fair weather over land on sunny days. In the early morning, the mixed layer is shallow, slowly deepening, cool (in a potential temperature sense), and is capped by the remains of the stable boundary layer from the previous night. In mid- to late morning, the top of the mixed layer exhibits rapid rise as heating eliminates the nocturnal inversion, and the mixed layer grows through the residual layer. The third stage in late morning and afternoon is that of a deep (order of 1?2 km) convective boundary layer of relatively constant depth.