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
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{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}
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{aligned}
C_{1} & = C_{w}W_{1}\gamma +C_{s}, \nonumber\\
C_{2} & = C_{w}W_{2}\gamma +C_{s}, \nonumber
\end{aligned}
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.
------------------------------------------------------------------------ <-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)
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
- Global atmosphere experiment in aim.5l_cs verification directory.