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2019-11-30T15:48:32+01:00 (12 months ago)
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Started to re-write the "sbcblk" par ot the doc…

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 r11831 \documentclass[../main/NEMO_manual]{subfiles} \usepackage{fontspec} \usepackage{fontawesome} \begin{document} \label{sec:SBC_flx} % Laurent: DO NOT mix up bulk formulae'' (the classic equation) and the bulk % parameterization'' (i.e NCAR, COARE, ECMWF...) \begin{listing} \nlst{namsbc_flx} See \autoref{subsec:SBC_ssr} for its specification. %% ================================================================================================= %% ================================================================================================= \pagebreak \newpage \section[Bulk formulation (\textit{sbcblk.F90})]{Bulk formulation (\protect\mdl{sbcblk})} \label{sec:SBC_blk} \end{listing} In the bulk formulation, the surface boundary condition fields are computed with bulk formulae using atmospheric fields and ocean (and sea-ice) variables averaged over \np{nn_fsbc}{nn\_fsbc} time-step. The atmospheric fields used depend on the bulk formulae used. In forced mode, when a sea-ice model is used, a specific bulk formulation is used. Therefore, different bulk formulae are used for the turbulent fluxes computation over the ocean and over sea-ice surface. For the ocean, four bulk formulations are available thanks to the \href{https://brodeau.github.io/aerobulk/}{Aerobulk} package (\citet{brodeau.barnier.ea_JPO16}): the NCAR (formerly named CORE), COARE 3.0, COARE 3.5 and ECMWF bulk formulae. The choice is made by setting to true one of the following namelist variable: \np{ln_NCAR}{ln\_NCAR}, \np{ln_COARE_3p0}{ln\_COARE\_3p0},  \np{ln_COARE_3p5}{ln\_COARE\_3p5} and  \np{ln_ECMWF}{ln\_ECMWF}. For sea-ice, three possibilities can be selected: a constant transfer coefficient (1.4e-3; default value), \citet{lupkes.gryanik.ea_JGR12} (\np{ln_Cd_L12}{ln\_Cd\_L12}), and \citet{lupkes.gryanik_JGR15} (\np{ln_Cd_L15}{ln\_Cd\_L15}) parameterizations In the bulk formulation, the surface boundary condition fields are computed with bulk formulae using prescribed atmospheric fields and prognostic ocean (and sea-ice) surface variables averaged over \np{nn_fsbc}{nn\_fsbc} time-step. % Turbulent air-sea fluxes are computed using the sea surface properties and % atmospheric SSVs at height $z$ above the sea surface, with the traditional % aerodynamic bulk formulae: %%% Bulk formulae are this: \subsection{Bulk formulae} % In NEMO, when the bulk formulation is selected, surface fluxes are computed by means of the traditional bulk formulae: % \begin{subequations}\label{eq_bulk} \begin{eqnarray} \mathbf{\tau} &=& \rho~ C_D ~ \mathbf{U}_z  ~ U_B \label{eq_b_t} \\ Q_H           &=& \rho~C_H~C_P~\big[ \theta_z - T_s \big] ~ U_B \label{eq_b_qh} \\ E             &=& \rho~C_E    ~\big[    q_s   - q_z \big] ~ U_B \label{eq_b_e}  \\ Q_L           &=& -L_v \, E  \label{eq_b_qe} \\ % Q_{sr}        &=& (1 - a) Q_{sw\downarrow} \\ Q_{ir}        &=& \delta (Q_{lw\downarrow} -\sigma T_s^4) \end{eqnarray} \end{subequations} %lulu % From which, the the non-solar heat flux is $Q_{ns} = Q_L + Q_H + Q_{ir}$ % $\theta_z \simeq T_z+\gamma z$ $q_s \simeq 0.98\,q_{sat}(T_s,p_a )$ where $\mathbf{\tau}$ is the wind stress vector, $Q_H$ the sensible heat flux, $E$ the evaporation, $Q_L$ the latent heat flux, and $Q_{ir}$ the net longwave flux. % $Q_{sw\downarrow}$ and $Q_{lw\downarrow}$ are the surface downwelling shortwave and longwave radiative fluxes, respectively. % Note: a positive sign of $\mathbf{\tau}$, $Q_H$, and $Q_L$ means a gain of the relevant quantity for the ocean, while a positive $E$ implies a freshwater loss for the ocean. % $\rho$ is the density of air. $C_D$, $C_H$ and $C_E$ are the BTCs for momentum, sensible heat, and moisture, respectively.  $C_P$ is the heat capacity of moist air, and $L_v$ is the latent heat of vaporization of water.  $\theta_z$, $T_z$ and $q_z$ are the potential temperature, temperature, and specific humidity of air at height $z$, respectively. $\gamma z$ is a temperature correction term which accounts for the adiabatic lapse rate and approximates the potential temperature at height $z$ \citep{Josey_al_2013}.  $\mathbf{U}_z$ is the wind speed vector at height $z$ (possibly referenced to the surface current $\mathbf{u_0}$, section \ref{s_res1}.\ref{ss_current}). The bulk scalar wind speed, $U_B$, is the scalar wind speed, $|\mathbf{U}_z|$, with the potential inclusion of a gustiness contribution (section \ref{s_res2}.\ref{ss_calm}). $P_0$ is the mean sea-level pressure (SLP). $T_s$ is the sea surface temperature. $q_s$ is the saturation specific humidity of air at temperature $T_s$ and includes a 2\% reduction to account for the presence of salt in seawater \citep{Sverdrup_al_1942,Kraus_Businger_1996}. Depending on the bulk parameterization used, $T_s$ can be the temperature at the air-sea interface (skin temperature, hereafter SSST) or at a few tens of centimeters below the surface (bulk sea surface temperature, hereafter SST). The SSST differs from the SST due to the contributions of two effects of opposite sign, the \emph{cool skin} and \emph{warm layer} (hereafter CSWL). The \emph{cool skin} refers to the cooling of the millimeter-scale uppermost layer of the ocean, in which the net upward flux of heat to the atmosphere is ineffectively sustained by molecular diffusion. As such, a steep vertical gradient of temperature must exist to ensure the heat flux continuity with underlying layers in which the same flux is sustained by turbulence. The \emph{warm layer} refers to the warming of the upper few meters of the ocean under sunny conditions. The CSWL effects are most significant under weak wind conditions due to the absence of substancial surface vertical mixing (caused by \eg breaking waves). The impact of the CSWL on the computed TASFs is discussed in section \ref{s_res1}.\ref{ss_skin}. %%%% Second set of equations (rad): where $a$ and $\delta$ are the albedo and emissivity of the sea surface, respectively. Thus, we use the computed $Q_L$ and $Q_H$ and the 3-hourly surface downwelling shortwave and longwave radiative fluxes ($Q_{sw\downarrow}$ and $Q_{lw\downarrow}$, respectively) from ERA-Interim to correct the daily SST every 3 hours. Due to the implicitness of the problem implied by the dependence of $Q_{nsol}$ on $T_s$, this correction is done iteratively during the computation of the TASFs. \subsection{Bulk parameterizations} Accuracy of the estimate of surface turbulent fluxes by means of bulk formulae strongly relies on that of the bulk transfer coefficients: $C_D$, $C_H$ and $C_E$. They are estimated with what we refer to as a \emph{bulk parameterization} algorithm. ... also to adjust humidity and temperature of air to the wind reference measurement height (generally 10\,m). Over the open ocean, four bulk parameterization algorithms are available: \begin{itemize} \item NCAR, formerly known as CORE, \citep{large.yeager_rpt04} \item COARE 3.0 \citep{fairall.bradley.ea_JC03} \item COARE 3.6 \citep{edson.jampana.ea_JPO13} \item ECMWF (IFS documentation, cy41) \end{itemize} ~ % In a typical bulk algorithm, the BTCs under neutral stability conditions are % defined using \emph{in-situ} flux measurements while their dependence on the % stability is accounted through the \emph{Monin-Obukhov Similarity Theory} and % the \emph{flux-profile} relationships \citep[\eg{}][]{Paulson_1970}. BTCs are % functions of the wind speed and the near-surface stability of the atmospheric % surface layer (hereafter ASL), and hence, depend on $U_B$, $T_s$, $T_z$, $q_s$ % and $q_z$. \subsection{Cool-skin and warm-layer parameterizations} As oposed to the NCAR bulk parameterization, more advanced bulk parameterizations such as COARE3.x and ECMWF are meant to be used with the skin temperature $T_s$ rather than the bulk SST (which, in NEMO is the temperature at the first T-point level). % So that, technically, the cool-skin and warm-layer parameterization must be activated (XXX) to use COARE3.x and ECMWF in a consistant way. \subsection{Air humidity} Air humidity can be provided as three different parameters: specific humidity [kg/kg], relative humidity [\%], or dew-point temperature [K] (LINK to namelist parameters)... ~\\ The atmospheric fields used depend on the bulk formulae used.  In forced mode, when a sea-ice model is used, a specific bulk formulation is used.  Therefore, different bulk formulae are used for the turbulent fluxes computation over the ocean and over sea-ice surface. % thanks to the \href{https://brodeau.github.io/aerobulk/}{Aerobulk} package (\citet{brodeau.barnier.ea_JPO16}): The choice is made by setting to true one of the following namelist variable: \np{ln_NCAR}{ln\_NCAR}, \np{ln_COARE_3p0}{ln\_COARE\_3p0}, \np{ln_COARE_3p6}{ln\_COARE\_3p6} and \np{ln_ECMWF}{ln\_ECMWF}.  For sea-ice, three possibilities can be selected: a constant transfer coefficient (1.4e-3; default value), \citet{lupkes.gryanik.ea_JGR12} (\np{ln_Cd_L12}{ln\_Cd\_L12}), and \citet{lupkes.gryanik_JGR15} (\np{ln_Cd_L15}{ln\_Cd\_L15}) parameterizations Common options are defined through the \nam{sbc_blk}{sbc\_blk} namelist variables. j-component of the 10m air velocity  & vtau           & $m.s^{-1}$         & T     \\ \hline 10m air temperature                  & tair           & \r{}$K$            & T     \\ 10m air temperature                  & tair           & $K$               & T     \\ \hline Specific humidity                    & humi           & \%                 & T     \\ Specific humidity                    & humi           & $-$               & T     \\ Relative humidity                    & ~              & $\%$              & T     \\ Dew-point temperature                & ~              & $K$               & T     \\ \hline Incoming long wave radiation         & qlw            & $W.m^{-2}$         & T     \\ Downwelling longwave radiation       & qlw            & $W.m^{-2}$         & T     \\ \hline Incoming short wave radiation        & qsr            & $W.m^{-2}$         & T     \\ Downwelling shortwave radiation      & qsr            & $W.m^{-2}$         & T     \\ \hline Total precipitation (liquid + solid) & precip         & $Kg.m^{-2}.s^{-1}$ & T     \\ Its range must be between zero and one, and it is recommended to set it to 0 at low-resolution (ORCA2 configuration). As for the flux formulation, information about the input data required by the model is provided in As for the flux parameterization, information about the input data required by the model is provided in the namsbc\_blk namelist (see \autoref{subsec:SBC_fldread}). %% ================================================================================================= \subsection[Ocean-Atmosphere Bulk formulae (\textit{sbcblk\_algo\_coare.F90, sbcblk\_algo\_coare3p5.F90, sbcblk\_algo\_ecmwf.F90, sbcblk\_algo\_ncar.F90})]{Ocean-Atmosphere Bulk formulae (\mdl{sbcblk\_algo\_coare}, \mdl{sbcblk\_algo\_coare3p5}, \mdl{sbcblk\_algo\_ecmwf}, \mdl{sbcblk\_algo\_ncar})} \subsection[Ocean-Atmosphere Bulk formulae (\textit{sbcblk\_algo\_coare.F90, sbcblk\_algo\_coare3p6.F90, sbcblk\_algo\_ecmwf.F90, sbcblk\_algo\_ncar.F90})]{Ocean-Atmosphere Bulk formulae (\mdl{sbcblk\_algo\_coare}, \mdl{sbcblk\_algo\_coare3p6}, \mdl{sbcblk\_algo\_ecmwf}, \mdl{sbcblk\_algo\_ncar})} \label{subsec:SBC_blk_ocean} Four different bulk algorithms are available to compute surface turbulent momentum and heat fluxes over the ocean. COARE 3.0, COARE 3.5 and ECMWF schemes mainly differ by their roughness lenghts computation and consequently COARE 3.0, COARE 3.6 and ECMWF schemes mainly differ by their roughness lenghts computation and consequently their neutral transfer coefficients relationships with neutral wind. \begin{itemize} This is the so-called DRAKKAR Forcing Set (DFS) \citep{brodeau.barnier.ea_OM10}. \item COARE 3.0 (\np[=.true.]{ln_COARE_3p0}{ln\_COARE\_3p0}): See \citet{fairall.bradley.ea_JC03} for more details \item COARE 3.5 (\np[=.true.]{ln_COARE_3p5}{ln\_COARE\_3p5}): See \citet{edson.jampana.ea_JPO13} for more details \item COARE 3.6 (\np[=.true.]{ln_COARE_3p6}{ln\_COARE\_3p6}): See \citet{edson.jampana.ea_JPO13} for more details \item ECMWF (\np[=.true.]{ln_ECMWF}{ln\_ECMWF}): Based on \href{https://www.ecmwf.int/node/9221}{IFS (Cy31)} implementation and documentation. Surface roughness lengths needed for the Obukhov length are computed following \citet{beljaars_QJRMS95}.