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Land surfaces

Within each grid box there are several different types of ground surface, each covering a proportion of the area (Fig2.1.18 and Fig2.1.19).  The ground types are described by a set of "tiles" for:  

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However, the areal extent of each land surface tile type can vary in a rapid, interactive way during the model run, as rain falls then evaporates, or snow accumulates then melts, etc.  The characteristics of the soil may also change (e.g. infiltration or runoff of rain, temperature structure of soil etc).  The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.

Considerations

• The average ground type within a grid box is not necessarily representative of an individual location.  Land surface characteristics can be very variable within a grid box.  Users and forecasters should take into account the peculiarities of a location when interpreting model output.
• Currently there is no representation of an urban or city surface where extensive concrete and buildings are likely to have very different characteristics from tiles shown above, and possibly also provide a source of heat (the heat island effect) and even moisture (from air-conditioning units).
• The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.

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Fig2.1.19: An example of the variability of land surface within an approximate grid box illustrating the difficulty in assigning a representative HTESSEL "tiles" for the whole grid square area.  The red lines show the extent of a very approximate 9km x 9km schematic HRES and ENS grid square.  The flag locates the grid point.  There is some variation in ground surface type but it is predominantly covered by evergreen needle-leaf trees.  The proportional contribution to the heat, moisture and momentum fluxes are rather simpler to assess.  In winter snow the appropriate tile would be forest snow.  Runoff would be rapid over Rocky Mountain sides, much slower over low-lying river valleys   For this grid box, high vegetation 'tile' covers about 75%, low vegetation 'tile' covers about 5%, lake 'tile' covers about 5%, and the remaining area about 15% is considered a bare ground 'tile'.  This consists of 5% rock area but 10% urban area which is also considered as bare ground.  An  ENS meteogram is interpolated from the four grid points surrounding a given station within the box.  See Section on Selection of gridpoints for Meteograms for details.

Additional sources of information

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

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