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\subsection{Land package} |
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\label{sec:pkg:land} |
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\begin{rawhtml} |
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<!-- CMIREDIR:package_land: --> |
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\end{rawhtml} |
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\subsubsection{Introduction} |
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This package provides a simple land model |
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based on Rong Zhang [e-mail:roz@gfdl.noaa.gov] 2 layers model |
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(see documentation below). |
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|
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It is primarily implemented for AIM (\_v23) atmospheric physics |
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but could be adapted to work with a different atmospheric physics. |
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Two subroutines ({\it aim\_aim2land.F} {\it aim\_land2aim.F} |
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in {\it pkg/aim\_v23}) are used as interface with AIM physics. |
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Number of layers is a parameter ({\it land\_nLev} in {\it LAND\_SIZE.h}) |
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and can be changed. |
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%--------------------------------------------------------------------- |
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% \documentclass[12pt,thmsa]{article} |
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% \begin{document} |
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\begin{center} |
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{\bf Note on Land Model}\\ |
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date: June 1999\\ |
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author: Rong Zhang\\ |
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\end{center} |
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% \baselineskip19pt |
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\subsubsection{Equations and Key Parameters} |
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This is a simple 2-layer land model. The top layer depth $z1=0.1m$, the |
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second layer depth $z2=4m$. |
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|
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Let $T_{g1},T_{g2}$ be the temperature of each layer, $W_{1,}W_{2}$ be the |
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soil moisture of each layer. The field capacity $f_{1,}$ $f_{2}$ are the |
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maximum water amount in each layer, so $W_{i}$ is the ratio of available |
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water to field capacity. $f_{i}=\gamma z_{i},\gamma =0.24$ is the field |
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capapcity per meter soil$,$ so $f_{1}=0.024m,$ $f_{2}=0.96m.$ |
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The land temperature is determined by total surface downward heat flux $F,$ |
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|
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\begin{equation} |
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z_{1}C_{1}\frac{dT_{g1}}{dt}=F-\lambda \frac{T_{g1}-T_{g2}}{(z_{1}+z_{2})/2} |
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\end{equation} |
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|
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\begin{center} |
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\begin{equation} |
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z_{2}C_{2}\frac{dT_{g2}}{dt}=\lambda \frac{T_{g1}-T_{g2}}{(z_{1}+z_{2})/2} |
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\end{equation} |
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\end{center} |
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|
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here $C_{1},C_{2}$ are the heat capacity of each layer , $\lambda $ is the |
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thermal conductivity, $\lambda =0.42Wm^{-1}K^{-1}.$ |
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|
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\begin{center} |
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\bigskip |
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\begin{equation} |
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C_{1}=C_{w}W_{1}\gamma +C_{s} |
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\end{equation} |
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|
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\begin{equation} |
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C_{2}=C_{w}W_{2}\gamma +C_{s} |
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\end{equation} |
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\end{center} |
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|
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$C_{w},C_{s}$ are the heat capacity of water and dry soil respectively. $% |
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C_{w}=4.2\times 10^{6}Jm^{-3}K^{-1},C_{s}=1.13\times 10^{6}Jm^{-3}K^{-1}.$ |
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|
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\bigskip |
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|
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The soil moisture is determined by precipitation $P(m/s)$,surface |
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evaporation $E(m/s)$ and runoff $R(m/s).$ |
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|
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\begin{equation} |
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\frac{dW_{1}}{dt}=\frac{P-E-R}{f_{1}}+\frac{W_{2}-W_{1}}{\tau } |
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\end{equation} |
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|
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$\tau =2$ $days$ is the time constant for diffusion of moisture between |
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layers. |
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|
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\begin{equation} |
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\frac{dW_{2}}{dt}=\frac{f_{1}}{f_{2}}\frac{W_{1}-W_{2}}{\tau } |
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\end{equation} |
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|
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In the code, $R=0$ gives better result, $W_{1},W_{2}$ are set to be within |
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[0, 1]. If $W_{1}$ is greater than 1, then let $\delta W_{1}=W_{1}-1,W_{1}=1$ |
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and $W_{2}=W_{2}+p\delta W_{1}\frac{f_{1}}{f_{2}}$, i.e. the runoff of top |
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layer is put into second layer. $p=0.5$ is the fraction of top layer runoff |
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that is put into second layer. |
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|
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The time step is 1 hour, it takes several years to reach equalibrium offline. |
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|
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\subsubsection{Land diagnostics} |
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\label{sec:pkg:land:diagnostics} |
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|
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{\footnotesize |
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\begin{verbatim} |
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|
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------------------------------------------------------------------------ |
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<-Name->|Levs|<-parsing code->|<-- Units -->|<- Tile (max=80c) |
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------------------------------------------------------------------------ |
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GrdSurfT| 1 |SM Lg |degC |Surface Temperature over land |
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GrdTemp | 2 |SM MG |degC |Ground Temperature at each level |
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GrdEnth | 2 |SM MG |J/m3 |Ground Enthalpy at each level |
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GrdWater| 2 |SM P MG |0-1 |Ground Water (vs Field Capacity) Fraction at each level |
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LdSnowH | 1 |SM P Lg |m |Snow Thickness over land |
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LdSnwAge| 1 |SM P Lg |s |Snow Age over land |
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RUNOFF | 1 |SM L1 |m/s |Run-Off per surface unit |
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EnRunOff| 1 |SM L1 |W/m^2 |Energy flux associated with run-Off |
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landHFlx| 1 |SM Lg |W/m^2 |net surface downward Heat flux over land |
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landPmE | 1 |SM Lg |kg/m^2/s |Precipitation minus Evaporation over land |
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ldEnFxPr| 1 |SM Lg |W/m^2 |Energy flux (over land) associated with Precip (snow,rain) |
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\end{verbatim} |
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} |
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|
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\subsubsection{References} |
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|
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Hansen J. et al. Efficient three-dimensional global models for climate |
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studies: models I and II. \emph{Monthly Weather Review}, vol.111, no.4, pp. |
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609-62, 1983 |
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|
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% \end{document} |
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