Leaf Area Index

The impact of vegetation on interception of rainfall and on solar radiation is modelled by a Leaf Area Index (LAI).  It is one of the parameters in the model that describes land surface vegetation.

The leaf area index is derived from climatological data.  This takes into account seasonality (growing, mature, senescent and dormant phases) of four types of forest vegetation (evergreen trees, deciduous trees, mixed forest/woodland, and interrupted forest).  It is also be calculated daily from satellite data.  The leaf area index has a value of 0 over bare ground or where there are no leaves.  

The leaf area index is a non-dimensional number representing the square metres of leaf area per square metre of the earth's surface.  It is as it determines the degree of evapotranspiration and, for example, how much rainwater will be intercepted by the vegetative canopy, rather than falling to the ground.

In IFS the leaf area index varies only with climatology, month by month.  Anomalous weather, widespread fire damage, etc have no effect.

Considerations

Users need to be aware of the potential effect of any day-to-day anomalies.

  • Widespread storm damage or defoliation will reduce evapotranspiration from model predictions.
  • Vegetation growth and decay is climatologically variable but there is currently no interactive biology in the model.  Anomalously early extensive vegetation growth in spring or widespread wilting in heatwaves are not modelled. 
  • Transitory local but significant variations in surface characteristics (e.g. extensive burnt vegetation, seasonal variation in lake extent) are not represented.

Additional sources of information

(Note: In older material there may be references to issues that have subsequently been addressed)


Fig2.1.4.7-1: Example leaf area index (LAI) as shown in the surface parameters charts. The LAI is taken from climatological data.  Anomalous weather has not had an effect.  Higher values mean more evapotranspiration, and therefore greater fluxes of moisture into the atmosphere.  An additional contributor to these moisture fluxes is bare-ground evaporation - this is not represented here.  HTESSEL deals with these aspects in more detail.  See the current leaf area index chart.