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Geosciences LibreTexts

18.3: ABL Structure and Evolution

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    9646
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    Figure 18.8 Components of the boundary layer during fair weather in summer over land. Tan indicates nonlocally statically unstable air, light green (as in the RL) is neutral stability, and darker blues indicate stronger static stability.

    The fair-weather ABL consists of the components sketched in Fig. 18.8. During daytime there is a statically unstable mixed layer (ML). At night, a statically stable boundary layer (SBL) forms under a statically neutral residual layer (RL). The RL contains the pollutants and moisture from the previous day’s mixed layer, but is not very turbulent.

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    Figure 18.9 Typical vertical profiles of temperature (T), potential temperature (θ), mixing ratio (r, see the Water Vapor chapter), and wind speed (M) in the ABL. The dashed line labeled G is the geostrophic wind speed (a theoretical wind in the absence of surface drag, see the Atmos. Forces & Winds chapter). MBL is average wind speed in the ABL. Shading corresponds to shading in Fig. 18.8: tan is statically unstable, green is neutral, blue is statically stable, and darker blues are more stable.

    The bottom 20 to 200 m of the ABL is called the surface layer (SL, Fig. 18.9). Here frictional drag, heat conduction, and evaporation from the surface cause substantial variations of wind speed, temperature, and humidity with height. However, turbulent fluxes are relatively uniform with height; hence, the surface layer is known as the constant flux layer.

    Separating the free atmosphere (FA) from the mixed layer is a strongly stable entrainment zone (EZ) of intermittent turbulence. Mixed-layer depth zi is the distance between the ground and the middle of the EZ. At night, turbulence in the EZ ceases, leaving a non-turbulent layer called the capping inversion (CI) that is still strongly statically stable.

    Typical vertical profiles of temperature, potential temperature, humidity (mixing ratio), and wind speed are sketched in Fig. 18.9. The “day” portion (Fig. 18.9a) corresponds to the 3 PM time indicated in Fig. 18.8, while “night” (Fig. 18.9b) is for 3 AM.

    Next, we will look at ABL temperature, winds, and turbulence in more detail.

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