Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) 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.

Assimilation of screen temperature and derivation of forecast temperatures

Assimilation of 2m temperatures 

Assimilation of observed 2m (screen) temperatures began with Cy49r1 introduced in autumn 2024.  Observed temperatures, nominally at 2m height above the ground, are adjusted from station height to model height using a lapse rate of 5.5ºC/km.  However, to avoid introducing anomalous temperature information:

• Only data from stations between 400m below and 200m above the model height are used.
• Assimilation of data is limited to the first 6hr of the 4D-Var window for observations.
• Large departures of observed 2m temperatures from the background have reduced weight.
• 2m temperatures that differ from the background field by >7.5ºC are not used. (This might have an impact where a strong surface inversion exists over snow).

Occasionally lower tropospheric temperature data has been given low weight during the analysis process.  Usually this relates to problems with assimilating the boundary layer structure in situations with a strong inversion, coupled with the fact that the background is a long way from the truth.  The analysis procedures tend to give lower weight to observations that show major departures from the first guess.  In particularly lower weight is given where there is little support from adjacent observations.  Such data can even be rejected completely.

Derivation of forecast 2m temperatures

Forecast temperatures, including for meteograms, are derived from interpolation between model forecasts of 10m and skin (surface) temperatures:

• in neutral or unstable conditions are adjusted to the station height by using a 5.5ºC/km lapse rate applied across the difference between the station height and the model height taken from the ensemble orography.
• under stable conditions the interpolation process (introduced in Cy49r1) attempts to better match the smoother theoretical stable profile.  

Departures from the atmosphere's lapse rate will result in errors in the meteogram 2m temperature.   The algorithm attempts to avoid always using a fixed value of stability in the lowest atmosphere.  In the past this has occasionally caused abrupt jumps in forecast 2m temperature from one forecast to the next.  Nevertheless, such errors can occur where there is an strong inversion at low altitude above the local surface and especially over snow.

However, generally there is an improvement in forecast 2m temperatures.  But at times the analysed temperature structure of the boundary layer may only move a small way towards correcting errors in the background (Fig9.2.1-7).  From a mathematical standpoint it is also (unfortunately!) more difficult to correctly assimilate data near the surface than data higher up.

Fig9.2.1-7: Examples of the difficulty of assimilating temperature and humidity data in the lowest layers.

• At Kryvyi the first guess (blue) is too warm, and also too dry (in relative humidity terms).  The analysed structure (red) after assimilation of the observed data (black) is slightly less warm.  It has captured saturation within the inversion base but remains generally drier (in relative humidity terms) and warmer in the boundary layer.  The forecast boundary layer temperature is too warm (by ~5°C) and the cloud cover is not represented.
• At Lulea the analysed temperature structure remains similar to the first guess (blue).  This is despite observation of much colder near-surface temperature and warmer inversion top.  The inversion top and the moisture (in relative humidity terms) are not well identified.  The surface temperature is too warm (by ~5°C) but had more cooling been forecast near the surface then the very lowest layers could have been correctly captured.

Differences between observed and first guess values such as these may lead to very low weight being given to the observation, or to it even being rejected. In many cases the analysed temperatures remain similar to first guess values despite the observations. Users beware!

Additional Sources of Information

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

• See the impact of Cycle 47R3 on representation of soil moisture and cloud cover effects.

(FUG Associated with Cy49r1)