8.5.2. Land package Introduction

This package provides a simple land model based on Rong Zhang [e-mail Rong.Zhang@noaa.gov] two 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 Equations and Key Parameters

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

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 \(f_{1}=0.024\) m, \(f_{2}=0.96\) m.

The land temperature is determined by total surface downward heat flux \(F\),

\[\begin{split}\begin{aligned} z_1 C_1 \frac{dT_{g1}}{dt} & = F - \lambda \frac{T_{g1}-T_{g2}}{(z_1 + z_2)/2}, \nonumber\\ z_2 C_2 \frac{dT_{g2}}{dt} & = \lambda \frac{T_{g1}-T_{g2}}{(z_1 + z_2)/2}, \nonumber \end{aligned}\end{split}\]

here \(C_{1},C_{2}\) are the heat capacity of each layer, \(\lambda\) is the thermal conductivity, \(\lambda =0.42\) W m–1 K–1.

\[\begin{split}\begin{aligned} C_{1} & = C_{w}W_{1}\gamma +C_{s}, \nonumber\\ C_{2} & = C_{w}W_{2}\gamma +C_{s}, \nonumber \end{aligned}\end{split}\]

\(C_{w},C_{s}\) are the heat capacity of water and dry soil respectively. \(C_{w}=4.2\times 10^{6}\) J m–3 K–1, \(C_{s}=1.13\times 10^{6}\) J m–3 K–1.

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

\[\frac{dW_{1}}{dt} = \frac{P-E-R}{f_{1}}+\frac{W_{2}-W_{1}}{\tau},\]

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

\[\frac{dW_{2}}{dt}=\frac{f_{1}}{f_{2}}\frac{W_{1}-W_{2}}{\tau }\]

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. Land diagnostics

<-Name->|Levs|<-parsing code->|<--  Units   -->|<- Tile (max=80c)
GrdSurfT|  1 |SM      Lg      |degC            |Surface Temperature over land
GrdTemp |  2 |SM      MG      |degC            |Ground Temperature at each level
GrdEnth |  2 |SM      MG      |J/m3            |Ground Enthalpy at each level
GrdWater|  2 |SM P    MG      |0-1             |Ground Water (vs Field Capacity) Fraction at each level
LdSnowH |  1 |SM P    Lg      |m               |Snow Thickness over land
LdSnwAge|  1 |SM P    Lg      |s               |Snow Age over land
RUNOFF  |  1 |SM      L1      |m/s             |Run-Off per surface unit
EnRunOff|  1 |SM      L1      |W/m^2           |Energy flux associated with run-Off
landHFlx|  1 |SM      Lg      |W/m^2           |net surface downward Heat flux over land
landPmE |  1 |SM      Lg      |kg/m^2/s        |Precipitation minus Evaporation over land
ldEnFxPr|  1 |SM      Lg      |W/m^2           |Energy flux (over land) associated with Precip (snow,rain) 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 Experiments and tutorials that use land

  • Global atmosphere experiment in aim.5l_cs verification directory.