Geoscience Reference
In-Depth Information
1.0
r
q
v
q
r
q
v
w
0.5
r
q w
0
Figure 15.7
Typical daytime
vertical profiles of correlation
coefficients through the ABL.
−
0.5
0
Correlation coefficient
0.5
with the fact that the variations in
w
and
q
v
are correlated. The resulting flow of
heat is called the
turbulent flux
of sensible heat. We return to this point below.
In the atmospheric boundary layer (ABL), there are substantial correlations
between the fluctuations in atmospheric variables such as virtual potential
temperature,
q
v
, and specific humidity,
q
, and also between these two variables and
the vertical wind speed,
w
. The strength of these correlations changes with height
and time of day and are associated with height dependent differences in the ABL
itself and with the transport of turbulent fluxes through it. Figure 15.7 shows
typical variations with height (normalized such that the height of the ABL is unity)
in the linear correlations coefficients
r
q
v
,
q
in daytime conditions.
During the day and near the ground, there is usually a significant positive correlation
between fluctuations in vertical wind speed, and those in virtual potential temperature
and specific humidity. These are associated with the upward flow of sensible and latent
heat that is transported by turbulent fluctuations away from the surface. There is also
correlation between fluctuations in virtual potential temperature and specific humidity
near the ground reflecting the fact that the air in parcels moving upward tend to be
both warmer and moister than average, while those moving downward tend to be both
cooler and drier than average. At the top of the ABL the situation is entirely different.
The sign of
r
v
,
w
,
r
q
,
w
and
r
q
qv
,
q
, are negative implying a downward flow of heat energy, in the
opposite direction to that of moisture which is still upward.
qv
,
w
, and
r
Kinematic flux
The frame of reference we adopt when describing atmospheric flows in the ABL
has the Z axis perpendicular to and positive away from the ground, the X axis
parallel to the ground along the direction of the mean wind, and the Y axis parallel
to the ground and perpendicular to the direction of the mean wind.
