Home Contact Us Site Map  
 
       
    next up previous contents
Next: 6.12 Coupling interface for Up: 6. Physical Parameterization and Previous: 6.10.1 Key subroutines, parameters   Contents


6.11 Land package

This package provides a simple land model based on Rong Zhang [e-mail:roz@gfdl.noaa.gov] 2 layers model (see documentation below).

It is primarily implemented for AIM (_v23) atmospheric physics but could be adapted to work with a different atmospheric physics. Two subroutines (aim_aim2land.F aim_land2aim.F in pkg/aim_v23) are used as interface with AIM physics.

Number of layers is a parameter (land_nLev in LAND_SIZE.h) and can be changed.

Note on Land Model
date: June 1999
author: Rong Zhang

This is a simple 2-layer land model. The top layer depth $ z1=0.1m$, the second layer depth $ z2=4m$.

Let $ T_{g1},T_{g2}$ be the temperature of each layer, $ W_{1,}W_{2}$ be the soil moisture of each layer. The field capacity $ f_{1,}$ $ f_{2}$ are the maximum water amount in each layer, so $ W_{i}$ is the ratio of available water to field capacity. $ f_{i}=\gamma z_{i},\gamma =0.24$ is the field capapcity per meter soil$ ,$ so $ f_{1}=0.024m,$ $ f_{2}=0.96m.$

The land temperature is determined by total surface downward heat flux $ F,$

$\displaystyle z_{1}C_{1}\frac{dT_{g1}}{dt}=F-\lambda \frac{T_{g1}-T_{g2}}{(z_{1}+z_{2})/2}$ (6.24)

$\displaystyle z_{2}C_{2}\frac{dT_{g2}}{dt}=\lambda \frac{T_{g1}-T_{g2}}{(z_{1}+z_{2})/2}$ (6.25)

here $ C_{1},C_{2}$ are the heat capacity of each layer , $ \lambda $ is the thermal conductivity, $ \lambda =0.42Wm^{-1}K^{-1}.$


$\displaystyle C_{1}=C_{w}W_{1}\gamma +C_{s}$ (6.26)

$\displaystyle C_{2}=C_{w}W_{2}\gamma +C_{s}$ (6.27)

$ C_{w},C_{s}$ are the heat capacity of water and dry soil respectively. $ % C_{w}=4.2\times 10^{6}Jm^{-3}K^{-1},C_{s}=1.13\times 10^{6}Jm^{-3}K^{-1}.$


The soil moisture is determined by precipitation $ P(m/s)$,surface evaporation $ E(m/s)$ and runoff $ R(m/s).$

$\displaystyle \frac{dW_{1}}{dt}=\frac{P-E-R}{f_{1}}+\frac{W_{2}-W_{1}}{\tau }$ (6.28)

$ \tau =2$ $ days$ is the time constant for diffusion of moisture between layers.

$\displaystyle \frac{dW_{2}}{dt}=\frac{f_{1}}{f_{2}}\frac{W_{1}-W_{2}}{\tau }$ (6.29)

In the code, $ R=0$ gives better result, $ W_{1},W_{2}$ are set to be within [0, 1]. If $ W_{1}$ is greater than 1, then let $ \delta W_{1}=W_{1}-1,W_{1}=1$ and $ W_{2}=W_{2}+p\delta W_{1}\frac{f_{1}}{f_{2}}$, i.e. the runoff of top layer is put into second layer. $ p=0.5$ is the fraction of top layer runoff that is put into second layer.

The time step is 1 hour, it takes several years to reach equalibrium offline.


References

Hansen J. et al. Efficient three-dimensional global models for climate studies: models I and II. Monthly Weather Review, vol.111, no.4, pp. 609-62, 1983

next up previous contents
Next: 6.12 Coupling interface for Up: 6. Physical Parameterization and Previous: 6.10.1 Key subroutines, parameters   Contents
mitgcm-support@dev.mitgcm.org
Copyright © 2002 Massachusetts Institute of Technology