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The IFS atmospheric model has many levels in the lower atmosphere to capture the all important boundary layer but difficulties in modelling the detail of radiation exchanges at the surface and lack of uniform and widespread observations makes precision difficult.  This in turn affects  the development and persistence of cloud, which in turn affects the albedo and radiative balance between surface and boundary layer air.  

Cloud containing super-cooled liquid water (SLW) is frequently observed by aircraft and by remote sensing.  However, the processes associated with super-cooled liquid water within the cloud and their consequent effects are difficult to model precisely because:

  • SLW is radiatively important and can increase cloud lifetime (liquid drops can remain suspended while ice crystals grow and fall out),
  • there is a fine balance between turbulent production of water droplets, nucleation of ice, deposition growth and fallout,
  • there are uncertainties in turbulent mixing, ice microphysics, vertical resolution.

The structure of the boundary layer is crucial, particularly where there is a well-marked inversion, with or without a sharp change in humidity with height (hydrolapse).  Users should note:

  • stratocumulus tends to be under-predicted over land in anti-cyclones or may dissipate too quickly,

  • incorrect definition of the boundary layer in the physics schemes can mean incorrect identification, formation or dispersal of low cloud,
  • observed, analysed and forecast temperatures can be very different to one another in hilly or mountainous regions.

 The problems in handling low cloud can have a significant impact on the temperature and moisture structure of the boundary layer, and importantly also 2m temperatures.  A revised warm-phase microphysics and revised boundary layer clouds and shallow convection were introduced in 2018.

The user should critically assess the model representation of temperature and moisture structure in the lower atmosphere.

Fig9.1.1: A comparison of observed (orange) and model analysed/forecast (green) temperature and dewpoint structures.  Errors are due to assimilation issues coupled with the difficulties handling the cloud physics.  In this case the surface cool and moist layer was analysed to be slightly deeper than in reality.   This retarded fog clearance and therefore delayed heating and overturning of the boundary layer through the morning.  So by 12UTC the forecast inversion was too low compared with reality and it had also not captured the stratocumulus from the convective overturning within the boundary layer.  Consequently the true radiation balance around midday was not captured.

 

Fig9.1.2: Examples of the difficulty of describing accurately the temperature and dewpoint (as a measure of moisture) structure of the all-important boundary layer (blue as analysed or forecast, orange as observed).  At Stuttgart, Budapest and Nis the lowest ~500m is poorly represented.  Then the availability of solar radiation, because of the presence or absence of fog or low stratus cloud, is depleted or enhanced in the atmospheric model.  Differences in boundary layer moisture have a strong influence on the likelihood of development of stratocumulus as surface temperatures rise during the morning.  At Bucaresti the temperature structure of the lowest layers is modelled quite well, but the distribution of the moisture near the subsidence inversion is not so well represented. 


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