Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
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2 Theory<br />
2.1 Introduction to the <strong>at</strong>mospheric boundary <strong>layer</strong><br />
One of the most important fe<strong>at</strong>ures of the ABL is the continuous turbulence in the whole<br />
region, which is mostly produced by vertical wind shear and buoyancy. Turbulence mixes<br />
quantities like humidity, temper<strong>at</strong>ure and aerosols in the ABL, transmitting them from the<br />
surface through the ABL to the rest of the <strong>at</strong>mosphere.<br />
Figure 1: Vertical cross section of the ideal ABL structure over land during summer in cloud<br />
free conditions. NBL is the nocturnal boundary <strong>layer</strong>. Figure from Garr<strong>at</strong>t (1992).<br />
The daily evolution of the ABL over land on a clear day in a high pressure region may be<br />
seen in Figure 1. The structure of the ABL turbulence is strongly influenced by the surface<br />
he<strong>at</strong>ing and cooling. The surface <strong>layer</strong> is seen in the figure as the lowest part of the ABL,<br />
its <strong>height</strong> is approxim<strong>at</strong>ely 10% of the ABL <strong>height</strong>. In the surface <strong>layer</strong> the mean fluxes of<br />
momentum, he<strong>at</strong> and moisture are near constant. The vertical wind gradient is large and the<br />
wind speed in the surface <strong>layer</strong> may be expressed <strong>with</strong> a logarithmic wind profile under neutral<br />
<strong>at</strong>mospheric conditions 1 (Larsen, 2009; Stull, 1988).<br />
In the morning shortly after sunrise, the convective ABL starts to form (indic<strong>at</strong>ed by marker<br />
A in Figure 1). The sun he<strong>at</strong>s the surface leading to thermal instability and convection in<br />
the surface <strong>layer</strong>. Above the convective surface <strong>layer</strong> the mixed <strong>layer</strong> (ML) is formed. The<br />
ML is domin<strong>at</strong>ed by convective motion and buoyant turbulence gener<strong>at</strong>ion including large<br />
turbulent eddies. Mean vertical profiles of potential temper<strong>at</strong>ure, humidity and horizontal<br />
wind are nearly constant <strong>with</strong>in the ML as seen in Figure 2(a). Potential temper<strong>at</strong>ure is<br />
defined as the temper<strong>at</strong>ure an air parcel would have if it were brought adiab<strong>at</strong>ically to a<br />
reference pressure (usually 1000 mb). Aerosols th<strong>at</strong> often origin<strong>at</strong>e from the surface get<br />
mixed through the convective ABL as well. At the top of the ML is a strong capping inversion<br />
called the interfacial <strong>layer</strong> (Wyngaard, 2010; Garr<strong>at</strong>t, 1992) or the entrainment zone (Gryning<br />
1 i.e. when the he<strong>at</strong> flux is negligible<br />
7