# 8.5.2. Land package¶

## 8.5.2.1. 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

## 8.5.2.2. 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 \(F\),

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}.

\(C_{w},C_{s}\) 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).

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

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.

## 8.5.2.3. 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)
```

## 8.5.2.4. 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

## 8.5.2.5. Experiments and tutorials that use land¶

Global atmosphere experiment in aim.5l_cs verification directory.