Introduction

This module provides the surface forcing for GOTM. For all dynamic equations, surface boundary conditions need to be specified. For the momentum equations described in section 3.2.6 and section 3.2.7, these are the surface momentum fluxes $\tau_x^s$ and $\tau_y^s$ in Nm$^{-2}$. For the temperature equation described in section 3.2.11, it is the total surface heat flux,

$$Q_{tot}=Q_E+Q_H+Q_B$$ (228)

in Wm$^{-2}$ that has to be determined¹. The total surface heat flux $Q_{tot}$ is calculated as the sum of the latent heat flux $Q_E$, the sensible heat flux $Q_H$, and the long wave back radiation $Q_B$. In contrast to the total surface heat flux $Q_{tot}$, the net short wave radiation at the surface, $I_0$, is not used as a boundary condition but as a source of heat, as calculated by means of equation (29), see Paulson and Simpson (1977). For the salinity equation described in section 3.2.12, the fresh water fluxes at the surface are given by the difference of the evaporation and the precipitation, $p_e$, given in ms$^{-1}$, see also the surface boundary condition for salinity, (32). The way how boundary conditions for the transport equations of turbulent quantities are derived, is discussed in section 4.

There are basically two ways of calculating the surface heat and momentum fluxes implemented into GOTM. They are either prescribed (as constant values or to be read in from files) or calculated on the basis of standard meteorological data which have to be read in from files. The necessary parameters are the wind velocity vector at 10 m height in ms$^{-1}$, the sea surface temperature (SST in Celsius), air temperature in Celsius), air humidity (either as relative humidity in %, as wet bulb temperature or as dew point temperature in Celsius) and air pressure (in hectopascal), each at 2 m height above the sea surface, and the wind velocity vector at 10 m height in ms$^{-1}$. Instead of the observed SST, also the SST from the model may be used. For the calculation of these fluxes, the bulk formulae of Kondo (1975) or () are used.