Chapter 5 - The Oceanic Heat Budget

 Chapter 5 Contents

5.3 Direct Calculation of Fluxes

Before we can describe the geographical distribution of fluxes into and out of the ocean, we need to know how they are measured or calculated.

Gust-Probe Measurements of Turbulent Fluxes
There is only one accurate method for calculating fluxes of sensible and latent heat and momentum at the sea surface: from direct measurement of turbulent quantities in the atmospheric boundary layer made by gust probes on low-flying aircraft or offshore platforms. Very few such measurements have been made. They are expensive, and they cannot be used to calculate heat fluxes averaged over many days or large areas. The gust-probe measurements are used only to calibrate other methods of calculating fluxes.

1. Measurements must be made in the surface layer of the atmospheric boundary layer (See §4.3), usually within 30 m of the sea surface, because fluxes are independent of height in this layer.
2. Measurements must be made by fast-response instruments (gust probes) able to make several observations per second on a tower, or every meter from a plane.
3. Measurements include the horizontal and vertical components of the wind, the humidity, and the air temperature.

Fluxes are calculated from the correlation of vertical wind and horizontal wind, humidity, or temperature: Each type of flux is calculated from different measured variables, u', w', t', and q' :

 T = < ρ u 'w '> = ρ < u' w' > = ρ u2* (5.9a) QS = Cp < ρ w' t' > = ρ Cp < w' t' > (5.9b) QL = LE < w' q' > (5.9c)

where the brackets denotes time or space averages, and the notation is given in table 5.1. Note that specific humidity mentioned in the table is the mass of water vapor per unit mass of air.

Table 5.1 Notation Describing Fluxes
 Symbol Variable Value and Units Cp Specific heat capacity of air 1030 J·kg-1 · K-1 CD Drag coefficient (0.44 + 0.063 U10) X 10-3 CL Latent heat transfer coefficient 1.35 X 10-3 CS Sensible heat transfer coefficient 0.9 X 10-3 LE Latent heat of evaporation 2.5 X 106 J/kg q Specific humidity of air kg(water vapor)/kg (air) qa Specific humidity of air 10m above the sea kg(water vapor)/kg (air) qs Specific humidity of air at the sea surface kg(water vapor)/kg (air) QS Sensible heat flux W/m2 QL Latent heat flux W/m2 ta Temperature of the air 10m above the sea K or °C ts Sea-surface temperature K or °C t' Temperature fluctuation °C u' Horizontal component of fluctuation of wind m/s u* Friction velocity m/s U10 Wind speed at 10m above the sea m/s w' Vertical component of wind fluctuation m/s ρ Density of air 1.5 kg/m3 T Vector wind stress Pa
CS and CL from Smith (1988).