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Fig9.1.4.9-2: The interpolation function (α) shown as a function of stability. 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. α=1 implies that T2m equals the temperature at the lowest model level (TL137 at 10m); α=0 implies that T2m equals the surface (skin) temperature. For practical purposes, the orange line shows the function that is used to interpolate between the temperature at 10m and the skin temperature.
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. Temperatures at stations >200m higher or >400m lower than model contour height are not used. Stations on mountain tops and deep valleys are thus excluded while retaining the majority of observations including in shallow valleys where many stations are located.
Assimilation of other surface variables
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