Discretisation of soil layers

The previous major modification to the land surface model was introduced in August 1993 with Cycle 48; The land surface in ERA15 correspond to the Cycle 48 model.

Finally, before August 1993 the soil is discretized in 3 layers (Cycle 47) and only the values of temperature and water in the two top layers evolve during the forecast, while the values for the lowest layer are kept constant.

The table below summarises the soil layers distribution and, for each layer, the depths of its top and bottom.

	MARS Name	Cy 47 Top (m)	Cy 47 Bottom (m)	MARS Name	Cy 48 Top (m)	Cy 48 Bottom (m)	MARS Name	TESSEL Top (m)	TESSEL Bottom (m)
1	ST/SSW	0	0.07	STL1/SWL1	0	0.07	STL1/SWVL1	0	0.07
2	DST/DSW	0.07	0.49	STL2/SWL2	0.07	0.21	STL2/SWVL2	0.07	0.28
3	CDST/CDSW	0.49	0.91	STL3/SWL3	0.21	1.00	STL3/SWVL3	0.28	1.00
4	-	-	-	STL4/SWL4	1.00	2.89	STL4/SWVL4	1.00	2.89

Soil water units
Since the introduction of TESSEL, the MARS archived soil water (SWVLi) have volumetric units.

To convert MARS values for layer i, with depth d_i to water content in the layer i, the following operation should be performed:

Water content

Before TESSEL soil water was stored in MARS with units of meters of water scaled to the depth of the top soil layer ((m of water)/0.07).

The following table describes the operations to convert old soil water at layer i, (MARS name SWLi), into other units.

Parameter	Symbol	Formula
Old MARS soil water content :
Volumetric water content :
Water content :

Some characteristic values (in TESSEL and cycle 48)
The maximum value of water in any layer corresponds to saturation (0.47 m3/m3). However, saturation can only occur during very short periods, because the model will lose water due to bottom drainage. Field capacity corresponds to the maximum value that the model can sustain for more than a few hours, in the absence of precipitation. The vegetated fraction of the soil evaporates at the maximum rate for soil wetness larger than field capacity (0.32 m3/m3) and stops evaporating below the permanent melting point (0.17 m3/m3), with a continuous variation between the two extremes.