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Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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Errors in the representation of evaporation impact forecasts of near-surface humidity.  Leaf area index is a measure of vegetation coverage and determines the degree of evapotranspiration.  Higher values mean more evapotranspiration, and thus greater fluxes of moisture into the atmosphere.

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Evaporation over bare soil is also problematic.  Soil temperature and soil moisture is modelled in IFS but there are not a great deal of directly measured observations available.  However, the impact of heat and moisture fluxes can be a significant contributor to 2m and surface temperature errors, and hence have an impact on humidity.

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The effect of lakes is parameterised using FLake and a lake cover mask.  The sub-grid detail may not be completely captured and the energy fluxes may well be incorrectly estimated, particularly where frozen lakes are plentiful and/or forecast snow cover is uncertain.   These aspects can:

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• too little snow-cover and/or too much cloud is analysed then there is a risk forecast temperatures may be too high and humidity too low.
• too extensive snow-cover and/or too little cloud is analysed then there is a risk forecast temperatures may be too low and humidity too high.
• in light winds, humidity over snow and water surfaces is likely to be rather higher than shown in background or forecast fields, particularly where flooding or with melting snowfields.
• the boundary layer structure is not successfully analysed then there is a risk forecast temperatures may correspondingly be in error.
• winds are too strong or too weak then forecast temperatures may have larger errors (particularly at high latitudes in winter where the role of insolation in offsetting radiative cooling is minimal.

(FUG Associated associated with Cy50r1)