The term submeso, like many geophysical terms, does not have precise physical meaning. Googling the term “submeso” yields more than 5700 entries, most of them in oceanic studies and atmospheric urban studies, although a wide variety of other geophysical areas are included. In general, the term submeso covers a range of scales between the largest turbulent scales and the smallest traditional mesoscales (about 2 km horizontal scale in the atmosphere). The small end of the mesoscale range in the ocean is often referred to as the submeso scale (see Fox-Kemper et al., 2008 and references therein), sometimes reserved for motions that are thinner than the boundary-layer depth. Mestayer and Anquetin (1995) defined submeso in the atmosphere within the context of dispersion in an urban setting associated with horizontal scales of 10 to 200 meters in the lowest 20 meters. The model SUBMESO (e.g., http://www.mi.uni-hamburg.de/Submeso.337.0.html) allows examinations of flows on scales smaller than O(km) where nonhydrostatic effects and noncomplicance with incompressible mass continuity may become important (Anquetin et al., 1998). The direct Coriolis effect is considered to be unimportant for submeso motions. Beluŝić and Mahrt (2008) and Mahrt (2009) referred to observations of nonturbulent motions on horizontal scales less than a few kilometers in the stable boundary layer as submeso, which includes motions on horizontal scales as small as 10-20 meters and vertical scales as small as a few meters.  The flow characteristics seem to vary continuously with scale such that these scales boundaries are only rough guidelines.  The physics remains poorly understood.  

Technical description.  Submeso are distinct from turbulence.  Turbulence is fully three dimensional and thus includes vortex stretching and diffusion.  In contrast, submeso motions are more two-demensional and do not effectively diffuse scalars except indirectly through shear generation of turbulence. However, submeso motions can dominate horizontal dispersion. Excluding microfronts, submeso often appear to be quasi-linear processes with weak interaction between modes.  In contrast,  turbulence is highly nonlinear with strong interaction between modes.   Hybrid motions have properties between those of turbulence and submeso motions (e.g., unstable waves or highly nonlinear wave-wave interactions).
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