256
S. Ferrarese et al.
3 Turbulent Sensible and Latent Heat Fluxes
The fast response ultrasonic and kripton instruments allow the évaluation of the
turbulent sensible and latent heat fluxes via the direct method, i.e. the time integra! of the cross products of fluctuations of absolute température (TO (or spécifie humidity q') with vertical wind component (w7). As far as the sensible heat flux
is concerned, from the relationship between absolute température T and sonie
température Ts [6,7]:
T=Ts/(l+aq),
(1)
where a is a suitable dimensionless constant and q is the spécifie humidity (in
kilograms per kilogram), the sonie température time sériés can be transformed in
to the absolute température time sériés, from which the absolute température
fluctuation T'can be obtained. As a matter of fact, in the présent analysis, the
cross products ivî' hâve been considered satisfactorily approximated by the
terms wT' [8], owing to the low levels of spécifie humidity during the experiment: q = O( 10 3 - 10’4) kg kg1. In fact, a simple calculation [6] shows that w'Tj
and wT'are related by the approximate relationship:
w'T's = w'T' + 0.515 T w'q'.
(2)
By assuming a maximum value of 350 W m’2 for the latent heat flux (p Lp w'q'),
the différence between these two terms cornes out as being in the order of 10%.
Then, the latent (F) and sensible (H) heat fluxes hâve been expressed by the following relationships:
F=pLpw'q';___
(3a)
H = pcp w'T's pcp wT's
(3b)
where r is the air density and Lp is the latent heat. The averaging time for the
détermination of the quantities defined through Eqs. (3a) and (3b) has been chosen as 30 min. In Fig. 5a and b the time trends of measured vertical turbulent fluxes of sensible heat at 5 and 10 m are represented, while Fig. 6 shows the time trend
of the measured turbulent flux of latent heat at 10 m. Both fluxes refer to observations carried out on the inner mast.
4 Heat Energy Balance
In order to evaluate the enthalpy balance in lower layers of katabatic winds for the
purpose of assessing the heat energy transfer at the air-ice surface boundary
under non-stationary conditions, an atmospheric slab, 5 km long, oriented along
the line linking the two masts and vertically limited by the horizontal surfaces at
10 and 5 m, has been selected. This allowed us to define a ‘control volume’ not connected to the ice surface, so that estimate of the flux in the ice was not necessary.
In a previous paper, Heinemann [9] carried out a thorough investigation of the
S. Ferrarese et al.
3 Turbulent Sensible and Latent Heat Fluxes
The fast response ultrasonic and kripton instruments allow the évaluation of the
turbulent sensible and latent heat fluxes via the direct method, i.e. the time integra! of the cross products of fluctuations of absolute température (TO (or spécifie humidity q') with vertical wind component (w7). As far as the sensible heat flux
is concerned, from the relationship between absolute température T and sonie
température Ts [6,7]:
T=Ts/(l+aq),
(1)
where a is a suitable dimensionless constant and q is the spécifie humidity (in
kilograms per kilogram), the sonie température time sériés can be transformed in
to the absolute température time sériés, from which the absolute température
fluctuation T'can be obtained. As a matter of fact, in the présent analysis, the
cross products ivî' hâve been considered satisfactorily approximated by the
terms wT' [8], owing to the low levels of spécifie humidity during the experiment: q = O( 10 3 - 10’4) kg kg1. In fact, a simple calculation [6] shows that w'Tj
and wT'are related by the approximate relationship:
w'T's = w'T' + 0.515 T w'q'.
(2)
By assuming a maximum value of 350 W m’2 for the latent heat flux (p Lp w'q'),
the différence between these two terms cornes out as being in the order of 10%.
Then, the latent (F) and sensible (H) heat fluxes hâve been expressed by the following relationships:
F=pLpw'q';___
(3a)
H = pcp w'T's pcp wT's
(3b)
where r is the air density and Lp is the latent heat. The averaging time for the
détermination of the quantities defined through Eqs. (3a) and (3b) has been chosen as 30 min. In Fig. 5a and b the time trends of measured vertical turbulent fluxes of sensible heat at 5 and 10 m are represented, while Fig. 6 shows the time trend
of the measured turbulent flux of latent heat at 10 m. Both fluxes refer to observations carried out on the inner mast.
4 Heat Energy Balance
In order to evaluate the enthalpy balance in lower layers of katabatic winds for the
purpose of assessing the heat energy transfer at the air-ice surface boundary
under non-stationary conditions, an atmospheric slab, 5 km long, oriented along
the line linking the two masts and vertically limited by the horizontal surfaces at
10 and 5 m, has been selected. This allowed us to define a ‘control volume’ not connected to the ice surface, so that estimate of the flux in the ice was not necessary.
In a previous paper, Heinemann [9] carried out a thorough investigation of the