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Structure of the lower atmosphere and boundary layer

The IFS atmospheric model has many levels in the lower atmosphere to capture the all important boundary layer but precision is difficult.  But modelling the Modelling the structure of the lowest, near surface, layers is very important for analysis ( and forecasting), especially for cold temperatures at 2m.  The detailed  For this reason there are many model levels in the boundary layer.   Capturing the structure of the temperature in these lowest layers and moisture can be difficult and often imprecise.  The structure is particularly difficult to define where there are low level inversions or over complex orographic areasorography.  Wind  Modelling wind shear and mixing processes at lower levels can also have has an impact.

This affects Forecasting the development and persistence of cloud , and hence also is very important.  Cloud coverage affects both the albedo and radiative balance between surface and boundary layer air.  

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

• super-cooled liquid water is important for radiation considerations.
• super-cooled liquid water 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.

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

Defining anticyclonic inversions is often a problem, particularly in winter with colder or sub-zero temperatures in the lower boundary layer.  Low level cloud cover tends to be shallower in the forecast and break too easily.  Greater incoming radiation can raise the near surface temperatures too much and/or too quickly.  Sub-zero temperatures can be very low to rise.  

Users should assess how well the model has analysed the structure of the inversion and take into account the extent and persistence of the cloud area under the inversion during the past few daysThe user should assess carefully 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.

 
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Fig9.1-2: Examples of the difficulty of describing the boundary layer temperature and moisture structure.  Dew-point is used here as a measure of moisture.  Values as analysed or forecast Forecast values shown in bluered; observed values shown in orangeblack.  DT:00UTC 19 Jan 2025; T+12 VT12UTC 19 Jan 2025.  There was a large anticyclone over Europe at this time with extensive low cloud but with some breaks mainly around the edges.  In this case the Ensemble control (Ex-HRES) handled the boundary layer evolution rather poorly.

• At

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• Budapest the boundary layer was observed to be saturated with thick low cloud but the forecast had thinner cloud with a higher base.  Forecast brighter conditions allowed the near surface temperature to rise a little.  The error in 2m temperature was about 2°C (Temperatures: Observed -3°C, forecast -1°C).
• At Wien the boundary layer was observed to be mostly saturated with low cloud but the forecast had broken the cloud.  Forecast morning insolation allowed the near surface temperature to rise a rather more.  The error in 2m temperature was about 4°C (Temperatures: Observed -3°C, forecast 1°C).
• At Sofia was observed to have fog in the morning and to remain with low cloud all day despite the thin layer of low cloud shown on the vertical profile.  The forecast had readily dispersed the cloud. and unhindered morning insolation allowed the near surface temperature to rise a great deal.  The error in 2m temperature was about 12°C (Temperatures: Observed -4°C, forecast 8°C).

Consequences from an error in the forecast location of a front 

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