On the Heat Energy Fluxes in the Non-stationary Surface Boundary Layer
263
the fetch. In conclusion, according to Eq. (7), the magnitudes and signs of the
divergence of turbulent heat flux and those of the horizontal advection term force
the divergence of latent heat flux to take on a magnitude and a sign such as gives
rise to only limited changes of the local température (LOCAL).
During the second period, characterized by the katabatic wind, the term of turbulent sensible heat flux becomes the dominant one and is almost one order of
magnitude larger than the other terms. The DIVH term is large négative (up to
about -36.5 W m’3), showing a tendency to warm the layer. As the advection term
is not able to offset it, so as to produce the limited local change of température
observed during this period, Eq. (7) implies again large values of the divergence
of the latent heat flux, with positive sign. Unfortunately, due to a breakdown of
power supply, during the transitional period in which the katabatic wind took the
place of weak winds and the average air température lowered from about 0°C to
about -6°C degrees, the relevant data were lost, so that it has not been possible to
analyze the relative contribution of each enthalpy budget component to the cooling of air. Our analysis had then to be limited to the 12-h épisode of well-established katabatic wind, from 6 p.m. on day 36 to 6 a.m. on day 37. Figures 7-9 show
that, during this épisode, AHT was small positive (smaller than during the weak
wind period), increasing from about 0 W m'3 to about 4 W m'3 in the first 7 and
then decreasing again to about 1 W m'3 at the end of the period, LOCAL was small
positive (about 0.2-0.4 W m'3) in the first 4h and small négative (decreasing from
about 0 W m-3 to about -0.3 W m'3) in the last 8 h, while DRAD was almost zéro
ail the time.
5 Conclusion
This chapter has described the experimental layout deployed during the tenth
Antarctica Italian expédition to assess the heat energy budget within the non-stationary boundary layer of offshore winds blowing on the Hells Gâte Ice Shelf,
Terra Nova Bay, in the Southern summer of 1994-1995. The temporal data sériés
and the computed turbulent fluxes hâve been presented and an enthalpy budget
including a possible rôle of the latent heat flux has been introduced. The results of
this study are limited, at the présent, by the availability of the measurement of the
latent heat flux at only one level and by the remarkable différence of the sensible
heat fluxes at 10 and 5 m.
The Gill sonie anemometers used in this experiment were calibrated by the
manufacturer and calibrated again before and after the experiment by us, but,
unfortunately, only in low wind conditions (<5 m s1).
Looking at the time sériés of the sensible heat fluxes provided by the two
instruments, and at the time sériés of wind speed and latent heat flux, one can
observe that their average différence is small when the wind speed is from 15 to
25 m s'1 (from day 37 to 37.30), moderate (—35 W m‘2) when the wind speed is
from 10 to 20 m s'1 (from day 35.40 to 35.50) and high (-100 W m ~) when the
wind speed is from 20 to 30 m s’1 (from day 36.50 to 36.90).
This behaviour shows that the différence is not strictly a monotonie fonction
of wind speed: in addition, when the différence is small, also the latent heat ux
263
the fetch. In conclusion, according to Eq. (7), the magnitudes and signs of the
divergence of turbulent heat flux and those of the horizontal advection term force
the divergence of latent heat flux to take on a magnitude and a sign such as gives
rise to only limited changes of the local température (LOCAL).
During the second period, characterized by the katabatic wind, the term of turbulent sensible heat flux becomes the dominant one and is almost one order of
magnitude larger than the other terms. The DIVH term is large négative (up to
about -36.5 W m’3), showing a tendency to warm the layer. As the advection term
is not able to offset it, so as to produce the limited local change of température
observed during this period, Eq. (7) implies again large values of the divergence
of the latent heat flux, with positive sign. Unfortunately, due to a breakdown of
power supply, during the transitional period in which the katabatic wind took the
place of weak winds and the average air température lowered from about 0°C to
about -6°C degrees, the relevant data were lost, so that it has not been possible to
analyze the relative contribution of each enthalpy budget component to the cooling of air. Our analysis had then to be limited to the 12-h épisode of well-established katabatic wind, from 6 p.m. on day 36 to 6 a.m. on day 37. Figures 7-9 show
that, during this épisode, AHT was small positive (smaller than during the weak
wind period), increasing from about 0 W m'3 to about 4 W m'3 in the first 7 and
then decreasing again to about 1 W m'3 at the end of the period, LOCAL was small
positive (about 0.2-0.4 W m'3) in the first 4h and small négative (decreasing from
about 0 W m-3 to about -0.3 W m'3) in the last 8 h, while DRAD was almost zéro
ail the time.
5 Conclusion
This chapter has described the experimental layout deployed during the tenth
Antarctica Italian expédition to assess the heat energy budget within the non-stationary boundary layer of offshore winds blowing on the Hells Gâte Ice Shelf,
Terra Nova Bay, in the Southern summer of 1994-1995. The temporal data sériés
and the computed turbulent fluxes hâve been presented and an enthalpy budget
including a possible rôle of the latent heat flux has been introduced. The results of
this study are limited, at the présent, by the availability of the measurement of the
latent heat flux at only one level and by the remarkable différence of the sensible
heat fluxes at 10 and 5 m.
The Gill sonie anemometers used in this experiment were calibrated by the
manufacturer and calibrated again before and after the experiment by us, but,
unfortunately, only in low wind conditions (<5 m s1).
Looking at the time sériés of the sensible heat fluxes provided by the two
instruments, and at the time sériés of wind speed and latent heat flux, one can
observe that their average différence is small when the wind speed is from 15 to
25 m s'1 (from day 37 to 37.30), moderate (—35 W m‘2) when the wind speed is
from 10 to 20 m s'1 (from day 35.40 to 35.50) and high (-100 W m ~) when the
wind speed is from 20 to 30 m s’1 (from day 36.50 to 36.90).
This behaviour shows that the différence is not strictly a monotonie fonction
of wind speed: in addition, when the différence is small, also the latent heat ux