"A useful working definition identifies the atmospheric boundary layer
as the layer of air directly above the Earth's surface in which the
effects of the surface (friction, heating and cooling) are felt
directly." [The Atmospheric Boundary Layer, J.R.Garrett]
I would define the atmospheric boundary layer to be the region
influenced by turbulence generated either mechanically or thermally
at the Earth's surface.
A Large-Eddy Simulation (LES) is a model which uses the differential
equations of motion to simulate turbulent eddies with grid point
spacings small enough to explicitly resolve internal eddy
dynamics. While a global atmospheric model uses grid spacings of
50-500 km and a limited-area (mesoscale) weather model uses grid spacings of 5-50
km, a LES uses grid spacings of 5-50 meters.
Mesoscale features have horizontal diameters of around 50-500 km
and are thus smaller in size than global-scale (synoptic) weather systems
but larger than boundary-layer eddies.
Large-Eddy Simulation example of three-dimensional thermal plumes
Thermals are created and grow downwind over the relatively warm water surface -
but they grow much more rapidly at the downwind edge of the water surface,
where mean motion creates an enviroment more favorable to vertical growth.
(here color indicates height, with red/purple indicating high/low altitude)