source: palm/trunk/DOC/misc/prandtl_layer.tex @ 375

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%\begin{titlepage}
%\author{Gerald Steinfeld}
%\title{Prandtl layer parameterisation in PALM}
%\end{titlepage}
\begin{document}
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\chapter{Prandtl layer parameterisation in PALM}

The friction velocity $u_{\ast}$ is a velocity scale that is defined by the relation 
\begin{equation}
u_{\ast}=\left ( \left | \tau / \overline{\rho} \right | \right )^{\frac{1}{2}}, 
\end{equation}
where $\tau$ is the Reynolds stress and $\rho$ is the air density.
Using the surface kinematic momentum fluxes in the x and y directions 
$\left ( -\overline{u'w'}, -\overline{v'w'} \right )$ to represent 
surface stress, the friction velocity can be written as   
\begin{equation}
u_{\ast}=\left ( (-\overline{u'w'})^2 + (-\overline{v'w'})^2 \right )^{\frac{1}{4}}.
\end{equation}
Based on the definition of the friction velocity $u_{\ast}$ the vertical turbulent 
momentum flux $\overline{u'w'}$ can be determined from 
\begin{equation}
{u^2_{\ast}} \cos \left ( \alpha_0 \right ) = - \overline{u'w'},
\end{equation}
while the vertical turbulent momentum flux $\overline{v'w'}$ can be determined 
from  
\begin{equation}
{u^2_{\ast}} \sin \left ( \alpha_0 \right ) = - \overline{v'w'}.
\end{equation}
The angle $\alpha_0$, that is assumed to be constant in the Prandtl layer, is the 
angle between the x-direction and the direction of the mean horizontal wind and can 
be evaluated by   
\begin{equation} \label{win}
\alpha_0 = \arctan \left ( \frac{\overline{v}(z_p)}{\overline{u}(z_p)} \right ).
\end{equation}
According to the similarity theory of Monin and Obukhov the following relationship for 
the profile of the mean horizontal wind is valid
\begin{equation} \label{hmpr}
\begin{split}
\frac{\partial \left | \overline{\vec{v}} \right |}{\partial z} &= \frac{u_{\ast}}{\kappa z} \phi_m \left ( \frac{z}{L} \right ) \\
                                                                &= u_{\ast} \frac{1}{\kappa z} \phi_m \left ( \frac{z}{L} \right ).
\end{split}  
\end{equation}
In equation \ref{hmpr} $L$ denotes the Monin Obukhov length and $\kappa$ denotes the von Karman constant, while $\phi_m$ denotes the profile 
or Dyer-Businger functions for momentum:
\begin{equation}
\phi_m = 
\begin{cases}
1+5 \text{Rif} & \text{if $\text{Rif} > 0$}, \\
1 & \text{if $\text{Rif} = 0$}, \\
\left ( 1 - 16 \text{Rif} \right )^{-\frac{1}{4}} & \text{if $\text{Rif} < 0$}. 
\end{cases}
\end{equation} 
Rif denotes the dimensionless Richardson flux number.

By integrating equation \ref{hmpr} over $z$ from $z_0$ to a height $z$ the following relationship for the friction velocity can be deduced:    
\begin{equation} \label{usb}
u_{\ast} =
\begin{cases}
\frac{\left | \overline{\vec{v}} \right | \kappa}{\left ( \ln \left ( \frac{z}{z_0} \right ) + 5 \text{Rif} \left ( \frac{z-z_0}{z} \right ) \right )} & \text{if $\text{Rif} \ge 0$}, \\
%\frac{\left | \overline{\vec{v}} \right | \kappa}{\left ( ln \left ( \frac{1+B}{1-B} \frac{1-A}{1+A} \right ) + 2 \left ( arctan(B) - arctan(A) \right ) \right )} & \text{if $Rif<0$}
\frac{\left | \overline{\vec{v}} \right | \kappa}{\ln \left ( \frac{z}{z_0} \right ) - \ln \left ( \frac{ \left ( 1+A \right )^2  \left ( 1+A^2 \right ) }{ \left ( 1+B \right )^2  \left ( 1+B^2 \right ) }\right ) + 2 \left ( \arctan(A) - \arctan(B) \right )} & \text{if $\text{Rif}<0$}
\end{cases} 
\end{equation}
with
\begin{equation}
%A=\left ( 1 - 16 Rif \right )^{-\frac{1}{4}}
A=\left ( 1 - 16 \text{Rif} \right )^{\frac{1}{4}} 
\end{equation}
and
\begin{equation}
%B=\left ( 1 - 16 Rif \frac{z_0}{z} \right )^{-\frac{1}{4}}.
B=\left ( 1 - 16 \text{Rif} \frac{z_0}{z} \right )^{\frac{1}{4}}.
\end{equation}
In fact, equation \ref{usb} is used in PALM to determine the friction velocity $u_{\ast}$.

The following paragraph is a short digression dealing with the integration of equation \ref{hmpr} that finally leads to equation \ref{usb}. The integration 
is only shown for the profile function for unstable stratification. According to PAULSON (1970) the general result of an integration of $\phi=\frac{\kappa z'}{u_{\ast}} 
\frac{\partial \overline{u}}{\partial z'}$ over $z'$ from $z_0$ to $z$ is
\begin{equation}
\overline{u}(z) = \frac{u_{\ast}}{\kappa} \left [ \ln \left ( \frac{z}{z_0} \right ) - \Psi \right ] 
\end{equation}
with
\begin{equation}
\Psi = \int_{\frac{z_0}{L}}^{\frac{z}{L}}d \left ( \frac{z'}{L} \right ) \frac{1-\phi \left ( \frac{z'}{L} \right )}{\frac{z'}{L}}.
\end{equation}  
Applying the profile function for unstable stratification, $\phi=\left ( 1 - 16 \frac{z}{L} \right )^{-\frac{1}{4}}$, $\Psi$ can be determined as follows:
\begin{equation}
\begin{split}
\Psi &= \int_{\frac{z_0}{L}}^{\frac{z}{L}}d \left ( \frac{z'}{L} \right ) \frac{1- \left ( 1 - 16 \frac{z'}{L} \right )^{-\frac{1}{4}} }{\frac{z'}{L}} \text{ | substitution: } x=\frac{1}{\phi \left ( \frac{z'}{L} \right ) } \\
     &= \int_{\left ( 1-16\frac{z_0}{L} \right )^{\frac{1}{4}}}^{\left ( 1-16\frac{z}{L} \right )^{\frac{1}{4}}} dx \left ( 4 \frac{x^2 - x^3}{1 - x^4} \right ) \\
     &= \int_{\left ( 1-16\frac{z_0}{L} \right )^{\frac{1}{4}}}^{\left ( 1-16\frac{z}{L} \right )^{\frac{1}{4}}} dx \left ( 2 \left ( \frac{1}{1+x} + \frac{x}{1+x^2} - \frac{1}{1+x^2} \right ) \right ) \\
     &= \left [ 2 \ln \left ( \frac{1+x}{2} \right ) + \ln \left ( \frac{1+x^2}{2} \right ) -2 \arctan(x) \right ]_{\left ( 1-16\frac{z_0}{L} \right )^{\frac{1}{4}}=B}^{\left ( 1-16\frac{z}{L} \right )^{\frac{1}{4}}=A} \\
     &= \ln \left ( \frac{\left ( 1 + A \right )^2}{\left ( 1 + B \right )^2} \frac{1+A^2}{1+B^2} \right ) - 2 \left ( \arctan(A) - \arctan(B) \right )
\end{split}
\end{equation} 

Making use of equation \ref{win} and \ref{hmpr}, it is also possible to deduce a relationship for the profile of the mean u-component of the wind velocity
\begin{equation} \label{upr}
\begin{split}
\frac{\partial \overline{u} }{\partial z} &= \frac{\partial \left | \overline{\vec{v}} \right |}{\partial z} \cos \left ( \alpha_0 \right ) \\
                                          &= \frac{u_{\ast}}{\kappa z} \phi_m \left ( \frac{z}{L} \right ) \cos \left ( \alpha_0 \right ) \\
                                          &= \frac{1}{u_{\ast}} {u^2_{\ast}} \cos \left ( \alpha_0 \right ) \frac{1}{\kappa z} \phi_m \left ( \frac{z}{L} \right ) \\
                                          &= \frac{-\overline{u'w'}}{u_{\ast}} \frac{1}{\kappa z} \phi_m \left ( \frac{z}{L} \right )
\end{split} 
\end{equation}
and accordingly a relationship for the profile of the mean v-component of the wind velocity
\begin{equation} \label{vpr}
\begin{split}
\frac{\partial \overline{v} }{\partial z} &= \frac{\partial \left | \overline{\vec{v}} \right |}{\partial z} \sin \left ( \alpha_0 \right ) \\
                                          &= \frac{u_{\ast}}{\kappa z} \phi_m \left ( \frac{z}{L} \right ) \sin \left ( \alpha_0 \right ) \\
                                          &= \frac{1}{u_{\ast}} {u^2_{\ast}} \sin \left ( \alpha_0 \right ) \frac{1}{\kappa z} \phi_m \left ( \frac{z}{L} \right ) \\
                                          &= \frac{-\overline{v'w'}}{u_{\ast}} \frac{1}{\kappa z} \phi_m \left ( \frac{z}{L} \right ).
\end{split} 
\end{equation}
As the right-hand sides of equation \ref{upr} and \ref{vpr} differ only by prefactors that are (assumed to be) constant within the Prandtl layer from the 
right-hand side of equation \ref{hmpr}, we can directly make use of the integration that led to equation \ref{usb} in order to obtain   
\begin{equation} \label{uswsb}
C=\frac{-\overline{u'w'}}{u_{\ast}} =
\begin{cases}
\frac{\overline{u} \kappa}{\left ( \ln \left ( \frac{z}{z_0} \right ) + 5 \text{Rif} \left ( \frac{z-z_0}{z} \right ) \right )} & \text{if $\text{Rif} \ge 0$}, \\
%\frac{\overline{u} \kappa}{\left ( ln \left ( \frac{1+B}{1-B} \frac{1-A}{1+A} \right ) + 2 \left ( arctan(B) - arctan(A) \right ) \right )} & \text{if $Rif<0$}.
\frac{ \overline{u} \kappa}{\ln \left ( \frac{z}{z_0} \right ) - \ln \left ( \frac{ \left ( 1+A \right )^2  \left ( 1+A^2 \right ) }{ \left ( 1+B \right )^2  \left ( 1+B^2 \right ) }\right ) + 2 \left ( \arctan(A) - \arctan(B) \right )} & \text{if $\text{Rif}<0$}.

\end{cases} 
\end{equation}
and
\begin{equation} \label{vswsb}
D=\frac{-\overline{v'w'}}{u_{\ast}} =
\begin{cases}
\frac{\overline{v} \kappa}{\left ( \ln \left ( \frac{z}{z_0} \right ) + 5 \text{Rif} \left ( \frac{z-z_0}{z} \right ) \right )} & \text{if $\text{Rif} \ge 0$}, \\
%\frac{\overline{v} \kappa}{\left ( ln \left ( \frac{1+B}{1-B} \frac{1-A}{1+A} \right ) + 2 \left ( arctan(B) - arctan(A) \right ) \right )} & \text{if $Rif<0$}.
\frac{\overline{v} \kappa}{\ln \left ( \frac{z}{z_0} \right ) - \ln \left ( \frac{ \left ( 1+A \right )^2  \left ( 1+A^2 \right ) }{ \left ( 1+B \right )^2  \left ( 1+B^2 \right ) }\right ) + 2 \left ( \arctan(A) - \arctan(B) \right )} & \text{if $\text{Rif}<0$}.

\end{cases} 
\end{equation}
Both equations, \ref{uswsb} and \ref{vswsb}, are used in PALM. In order to get an information on the turbulent vertical momentum fluxes within the Prandtl layer, 
equation \ref{uswsb} and equation \ref{vswsb} need only to be multiplied by $-1$ and the friction velocity $u_{\ast}$, so that finally the followings two steps 
have to be executed in PALM: 
\begin{equation}
\overline{u'w'} = -C u_{\ast}
\end{equation}
and
\begin{equation}
\overline{v'w'} = -D u_{\ast}.
\end{equation}

In case that no near-surface heat flux has been prescribed by the user of PALM, the near-surface heat flux $\overline{w'\Theta'}_0$ is evaluated from
\begin{equation} \label{hfe}
\overline{w'\Theta'} = - u_{\ast} \theta_{\ast}.
\end{equation}
Here, $\theta_{\ast}$ is the so-called characteristic temperature for the Prandtl layer. In case of no preset near-surface heat flux the characteristic 
temperature $\theta_{\ast}$ is determined in PALM from the integrated version of the profile function for the potential temperature. According to the 
similarity theory of Monin and Obukhov the following relationship for the vertical gradient of the potential temperature is valid:
\begin{equation} \label{hmpt}
\frac{\partial \overline{\Theta}}{\partial z} = \frac{\theta_{\ast}}{\kappa z} \phi_h.
\end{equation}  
In equation \ref{hmpt} $\phi_h$ denotes the profile or Dyer-Businger functions for temperature:
\begin{equation}
\phi_h = 
\begin{cases}
1+5 \text{Rif} & \text{if $\text{Rif} > 0$}, \\
1 & \text{if $\text{Rif} = 0$}, \\
\left ( 1 - 16 \text{Rif} \right )^{-\frac{1}{2}} & \text{if $\text{Rif} < 0$}. 
\end{cases}
\end{equation} 
By integrating equation \ref{hmpt} over $z$ from $z_0$ to a height $z$ the following relationship for the characteristic temperature in the Prandtl 
layer can be deduced:    
\begin{equation} \label{tsb}
\theta_{\ast} =
\begin{cases}
\frac{\kappa \left ( \overline{\Theta}(z) - \overline{\Theta}(z_0) \right )}{\ln \left ( \frac{z}{z_0} \right ) + 5 \text{Rif} \left ( \frac{z-z_0}{z} \right )} & \text{if $\text{Rif} \ge 0$}, \\
\frac{\kappa \left ( \overline{\Theta}(z) - \overline{\Theta}(z_0) \right )}{\ln \left ( \frac{z}{z_0} - 2 \ln \left ( \frac{1+A}{1+B} \right )\right )} & \text{if $\text{Rif}<0$}
\end{cases} 
\end{equation}
with
\begin{equation}
%A=\left ( 1 - 16 \text{Rif} \right )^{-\frac{1}{4}}
A=\sqrt{1 - 16 \text{Rif}} 
\end{equation}
and
\begin{equation}
%B=\left ( 1 - 16 \text{Rif} \frac{z_0}{z} \right )^{-\frac{1}{4}}.
B=\sqrt{1 - 16 \text{Rif} \frac{z_0}{z}}.
\end{equation}
Note, that the temperature at the height of the roughness length that is required for the evaluation of $\theta_{\ast}$ in equation \ref{tsb} is saved 
in PALM at the first vertical grid level of the temperature with index k=0. The height that is assigned to this vertical grid level is -0.5$\Delta z$, 
where $\Delta z$ is the grid length in the vertical direction.   
In case of a near-surface heat flux $\overline{w'\Theta'}_0$ that has been prescribed by the user, the evaluation of $\theta_{\ast}$ is not needed for 
the integration of the model. Instead $\theta_{\ast}$ is only determined for the evaluation of statistics of the turbulent flow. In that case it is 
simply derived by a transformation of equation \ref{hfe} into  
\begin{equation}
\theta_{\ast} = - \frac{\overline{w'\Theta'}_0}{u_{\ast}}.
\end{equation}    
In case that PALM is run in its moist version, the evaluation of the characteristic humidity for the Prandtl layer is evaluated in a way correspondent to 
that of the determination of the characteristic temperature.


  
  
 
        

 

 


 







 
  

 

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