Geoscience Reference
In-Depth Information
Figure 11.14 Mean radiosonde profiles of potential temperature and specific
humidity measured in the midday, clear-weather CBL in Sterling, Colorado (left)
and Wangara (right). The center, heavier lines are the mean over 33 days (Sterling)
and 9 days (Wangara), and the thinner lines represent
±
1 standard deviation. From
Mahrt ( 1976 ).
Water vapor mixing ratio in the CBL over an evaporating surface provides an
everyday counterexample of an “unmixed” mean scalar profile. A typical water-
vapor flux profile, with the top flux also positive due to the entrainment of drier air
aloft, is sketched in Figure 11.1 . Here the two terms on the rhs of Eq. (11.38) are
of the same sign, which typically gives a detectable mean gradient of water-vapor
mixing ratio in the mixed layer.
Figure 11.14 illustrates this point through mean profiles of specific humidity (a
conserved water-vapor variable) and potential temperature in the mixed layer. The
averaging of 33 radiosonde profiles has removed much of the instantaneous detail,
revealing mean profiles that suggest a significant vertical gradient of mean specific
humidity but an essentially zero gradient of mean potential temperature.
11.4 The interfacial layer
11.4.1 The entrainment flux
The top of the CBL is locally and instantaneously quite thin; averaging over time,
space, or an ensemble produces a much thicker interfacial layer . Its thickness
h
=
h 2
h 1 (Figure 11.1) is a good fraction of z i , according to the convection-
tank experiments of Willis andDeardorff ( 1974 ). It maintains the entrainment fluxes
of momentum, potential temperature, water vapor, and trace constituents at the top
of the mixed layer.
 
Search WWH ::




Custom Search