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Energy Fluxes - Surface

Surface exchanges of energy have a strong impact on the large-scale evolution of the atmosphere.  However, since these exchanges are active at scales smaller than the resolved scales of the model grid, a parameterisation technique has to be used.  This involves both statistical methods and simplified mathematical-physical models (e.g. the air closest to the earth’s surface exchanges heat with the surface through turbulent diffusion or convection, which adjusts stability in the lowest layers).  Sub-grid scale physical processes have to be treated in simplified ways which impacts on forecast accuracy.  Future higher and more appropriate resolution will allow air-surface interaction to be described more explicitly.

The IFS atmospheric models need information about the underlying boundary conditions to model their effect upon fluxes of momentum, radiation and moisture.  Some conditions remain fairly constant with time in the IFS, for example the extent of forests or sizes of lakes (lake extent that varies seasonally and from year to year is not accounted for yet).  Some conditions vary with the seasons in the IFS, for example solar energy reaching the earth or the extent of vegetation (inter-annual variations in vegetation are not accounted for yet).  Some other conditions vary considerably through the forecast period due to results from the forecasts themselves.  For example, the snow cover may alter due to snowfall produced during the forecast or due to melting with forecasted warmer temperatures.  It is important that surface/atmosphere energy interactions are represented as well as possible in order that energy, momentum and moisture sources and sinks are well modelled.  For this reason it is necessary to have a fairly detailed representation of the characteristics of the land or sea surface within each grid box and which can be updated by climatological changes or model output throughout the forecast period.  The Land-Sea Mask defines the extent of each surface and remains constant within IFS.  The proportion of land and water is calculated by using satellite derived 300m resolution dataset, so should be quite precise when  aggregated to HRES relution of 9km and the albedo is a climatological 10-day average.  However the user should be alert to potential local temporary vatiations due to tides that can cover and uncover extensive areas of sand or mud (e.g. around the Frisian Islands) or where the extent of a lake varies substantially during the year (e.g. Lake Kati Thanda–Lake Eyre, Australia).

Energy fluxes or exchanges of heat, moisture and momentum with the atmosphere vary with location and time.   Sea, lakes and soil each have their own characteristics regarding energy exchange with the atmosphere and many of these can change through the period of the forecast due to e.g. deposition of snow or rain by the model, by the drying action of model winds, by radiative warming or cooling, by formation of ice.  The energy fluxes over land are modelled by HTESSEL, over lakes and coastal waters by FLake, and over oceans by similar processes within NEMO.  To assess these fluxes it is necessary to have values for the lake- and sea-surface temperature, extent of ice and snow, soil temperature and moisture, and the albedo of the surface.  These data are initially analysed but then vary according to the model evolution.  

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