|
Understanding of the atmospheric boundary-layer regimes and proper parameterization
of the surface fluxes are of obvious relevance for climate modeling, weather forecasting,
and other important applications in the Arctic region.
This study focuses on behavior of the near-surface
atmospheric turbulence in the limit of very strong stable
stratification. Obtained results are based on data
collected at five levels on a 20-m tower over the
Arctic pack ice during the Surface Heat Budget of the
Arctic Ocean experiment (SHEBA).
As Richardson number approaches its critical
value, turbulence decays and vertical fluxes vanish.
The reduction in the surface friction is responsible for
main features of the atmospheric boundary layer in the
limit of very strong stability. First, this regime is associated
with the strong influence of the Earth's rotation.
Frictional effects become negligible and the influence
of the Coriolis effect comes into play. Observed wind
speeds show features of the Ekman spiral even near
the surface (surface Ekman layer). Second, the stress
falls off faster with increasing stability than the heat
flux, and the stress (or friction velocity) ceases to be a
relevant scaling parameter in the limit of very strong
stability (frictionless). This leads to new scaling relationships
for non-dimensional vertical gradients ('stability
functions') for velocity and potential temperature.
Stability function for wind speed follows to '1/3'
dependence in the very stable regime (in log-log coordinates),
whereas the temperature stability function
initially increases with increasing z/L, reaches a maximum
at z/L = 10 and decreases with a slope close to -
1/3 (L is the Obukhov length).
Similar arguments applied to other variables
such as the variances or the standard deviations for
velocity components and the air temperature.
Proposed new scaling relationships for the stability
functions the standard deviations are in good agreement
with the SHEBA profile data (see Figure below
the stability functions).
Thus, our SHEBA data demonstrate that both
stability functions for wind and temperature profiles
do not follow the z-less behavior (a linear function of
(1954) for the very strong stability, z/L >> 1.
Accordingly the concept of z-less stratification is not
valid in general.
However, even in the supercritical stable
regime, where the Richardson number exceeds its critical
value 0.2 and continuous turbulence is collapsed,
some sporadic and intermittent turbulence persists and
there is no evidence of a transition to a laminar
Ekman layer on average.
|
CIRES Research Theme
Climate System Variability
Project Personnel
A. A. Grachev (CIRES), C. W.Fairall (NOAA/ETL), P. O. G. Persson (CIRES),
E. L Andreas (CRREL, Hanover NH), P. S. Guest
(NPS, Monterey CA)
Funding Source(s)
NSF, NOAA, US Department
of Army
Publications
Grachev, A. A., C. W. Fairall, P. O.
G. Persson, E. L Andreas, and P. S. Guest, 2003:
Stable boundary-layer scaling regimes: the
SHEBA data. Boundary-Layer Meteorol. (submitted).
|
