E2 Surface Turbulent Flows

Topic

Surface turbulent fluxes are the exchanges of sensible heat, latent heat, and momentum occurring between the land surface and the atmosphere, driven by turbulence within the atmospheric boundary layer. Sensible heat is the flow of thermal energy directly into the air, whereas latent heat is the energy flux associated with water evaporation or condensation; together, these two fluxes determine both air temperature and precipitation, as evapotranspiration supplies the water vapor that eventually precipitates, thereby closing the hydrological cycle.

The Penman-Monteith equation serves as the standard physical framework for calculating both reference and actual evapotranspiration; it does so by combining atmospheric demand—driven by wind, temperature, and humidity—with the resistance the surface itself offers to water vapor transport.

Within this equation, aerodynamic resistance depends on wind and surface roughness, while canopy resistance (also known as stomatal resistance) controls the release of water vapor from leaf interiors into the atmosphere; for bare soil, stomatal resistance is replaced by a soil-specific resistance that increases as the soil dries.

Monin-Obukhov similarity theory describes the behavior of wind, temperature, and humidity profiles within the surface boundary layer in relation to the Obukhov length—a parameter quantifying the thermal stability of the entire atmospheric boundary layer. A negative length indicates unstable or convective conditions (where the surface is warmer than the air above), while a positive length indicates stable conditions (where the surface is cooler than the air).

The expected behavior of these turbulent fluxes includes logarithmic wind profiles under neutral conditions—profiles that are modified by specific stability functions when conditions shift to stable or unstable states. Thus, ranging from the sensible and latent heat with which this document begins to the wind profiles with which it concludes—and encompassing the Penman-Monteith equation and the Obukhov length—surface turbulent fluxes ensure that turbulence, invisible to the naked eye, ultimately determines both air temperature and the water cycle itself, which completes its loop with each new precipitation event.

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