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Stability in the lowest layers is taken into account using an interpolation function (α) derived using Monin-Obukhov similarity theory.  The stability measure is taken as the ratio of height above ground (z) to the Monin-Obukhov length (L).  The Monin-Obukhov length (L) is itself a function of, among other parameters, horizontal wind speed and upward ground heat flux.

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Fig9.1.4.9-3: Illustration of temperature observations which are accepted for use in analysis of T2m by 4D-Var.  Temperatures are adjusted by 5 K/km from station height up or down to the model orography height.  Temperatures at stations >200m higher or >400m lower than model orography height are not used.  Stations on mountain tops and in deep valleys are thus excluded while retaining the majority of observations, including those in shallow valleys where many stations are located.

Assimilation of other surface variables

Winds from ships and moored buoys continue to be assimilated.  However, 10 m wind observations over land are not assimilated as it has proved difficult to get a positive impact on forecasts.

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• actual and model station altitude in mountainous areas may differ.
• strong surface inversions, particularly over snow, may not be well modelled.
• extent of low cloud cover may not be captured by the model.

Users should assess the potential for deficiencies in low-level parameters and adjust forecast values as necessary.

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