easyclimate.field.boundary_layer.aerobulk

easyclimate.field.boundary_layer.aerobulk#

Estimate turbulent air-sea fluxes

AeroBulk is a FORTRAN90-based library and suite of tools (including a C++ interface) that feature state of the art parameterizations to estimate turbulent air-sea fluxes by means of the traditional aerodynamic bulk formulae.

These turbulent fluxes, namely, wind stress, evaporation (latent heat flux) and sensible heat flux, are estimated using the sea surface temperature (bulk or skin), and the near-surface atmospheric surface state: wind speed, air temperature and humidity. If the cool-skin/warm-layer schemes need to be called to estimate the skin temperature, surface downwelling shortwave and longwave radiative fluxes are required.

See also

Bulk formula and their parameterizations#

AeroBulk relies on the following traditional set of bulk formula to estimate turbulent fluxes at the air-sea interface:

\[\vec{\tau} = \rho\ C_D \ \vec{U}_z \ U_B\]
\[E = \rho \ C_E \ \big[ q_s - q_z \big] \ U_B\]
\[Q_L = -L_v \ E\]
\[Q_H = \rho \ C_H \ C_P \ \big[ \theta_z - T_s \big] \ U_B\]

where \(\rho\) is the density of air. The \(z\) subscript relates to the reference height above the air-sea interface (generally z=10m). :math:` ec{U}_z` is the wind speed vector at the reference height. \(U_B\) is the bulk scalar wind speed at the reference height (very close to \(|\vec{U}_z|\) in most cases). \(\theta_z\) and \(q_z\) are the potential temperature and specific humidity of air at the reference height, respectively. \(T_s\) and \(q_s\) are the absolute (= potential) temperature and specific humidity of air immediately at the air-sea interface (z=0), respectively; if the cool-skin/warm-layer scheme is used, these two are deduced from the skin temperature, otherwise they are deduced from the bulk SST (default).

Any decent level of accuracy from this set of formula can only be achieved through a consistent estimate of the 3 bulk transfer coefficients: \(C_D\), \(C_E\), and \(C_H\) (drag, evaporation, and sensible heat coefficients). In AeroBulk, these bulk coefficients can be estimated thanks to a collection of bulk parameterizations a.k.a bulk algorithms, which relate the value of these coefficients to the near-surface atmospheric stability, the wind speed, and (ideally) the roughness of the sea surface.

The following figure provides a schematic overview on the way turbulent fluxes are computed in AeroBulk:

Currently, in AeroBulk, 5 bulk parameterizations are available to compute \(C_D\), \(C_E\), and \(C_H\) used in the bulk formula:

In the COARE and ECMWF algorithms, a cool-skin/warm layer scheme is included and can be activated if the input sea-surface temperature to be used is the bulk SST (usually measured a few tenths of meters below the surface). Activation of these cool-skin/warm layer schemes requires the surface downwelling shortwave and longwave radiative flux components to be provided. Other parameterizations, such as NCAR, are meant to be used with the bulk SST, and do not feature a cool-skin/warm layer scheme.

Beside bulk algorithms, AeroBulk also provides a collection of functions (module mod_phymbl.f90) to accurately estimate relevant atmospheric parameters such as: density of air, different expressions of the humidity of air, viscosity of air, specific humidity at saturation, Obukhov length, bulk Richardson number, wind gustiness, etc…

The focus in AeroBulk is readability, efficiency, and portability towards modern ocean & atmosphere GCMs (Fortran 90, set of modules and a library).

Fig 1/ Comparison of the stability correction profiles :math:`Psi(zeta)` as used in 4 different bulk algorithms.

Fig 2/ Comparison of the neutral drag (thick lines) and evaporation coefficients (thinner lines) as a function of the neutral wind speed at 10m.

Functions

calc_turbulent_fluxes_skin_correction(...[, ...])

Aerobulk with skin correction.

calc_turbulent_fluxes_without_skin_correction(...)

Aerobulk without skin correction.
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