Nocturnal inversion

This occurs when the nocturnal inversion is dissipated by heat from the morning sun. The lapse layer usually starts at the ground and works its way upward (less rapidly in winter than in summer). Fumigation may also occur in sea-breeze circulations during late morning or early afternoon. The shaded zone of strong concentration is that portion of the plume which has not yet been mixed downward. 

Transport. The turbulent and advective transport of air parcels (horizontally and vertically) is often the most important factor (mixing and dilution). Transport processes show another wide range of causes, but are mainly driven by density gradient (hence, T gradients). Thus, it is clear that characteristic daily and seasonal patterns can be observed (for example, the vertical down-mixing of air in the morning after a nocturnal inversion). 

Current and historical meteorological monitoring-network data confirm that under night-time conditions, cold air from the mountains typically flows downward into the canyons and arroyos and fans onto the flat terrain. These cold-air-drainage winds can be very shallow, with depths occasionally less than 50 or 100 m (164 to 328 ft). In add rtion, nocturnal cooling complicates the area s temperature-inversion profile. 

FIGURE 20.3 One-compartment box models of the air over Indianapolis, Indiana during (a) night during a nocturnal temperature inversion, and (b) day with mixing to a planetary boundary layer. Dimensions are not drawn to scale. 

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