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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.  This is because there are difficulties  But modelling the structure of the lowest, near surface, layers is very important for analysis (and forecasting), especially for cold temperatures at 2m.  The detailed structure of the temperature in these lowest layers can be particularly difficult to define where there are low level inversions or over complex orographic areas.  Wind shear and mixing processes at lower levels can also have an impact.

detail of radiation exchanges at the surface the lack of uniform and widespread observations.  This affects the development and persistence of cloud, and hence also the albedo and radiative balance between surface and boundary layer air.  

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Errors in the forecast location of a front lead to large errors in the predicted airmass and surface parameters at a given location.  When a front is misplaced, dissimilar air masses either side of the front mean the structure of the atmosphere can be very different from that actually experienced.   The differences between forecast and observations can be over the whole depth of the atmosphere when a front is misplaced.  Large 2-metre temperature errors can occur.

The user should consider carefully the advance of frontal zones and use ensemble output to assess the uncertainty in timing.  Model representation of temperature and moisture structure in the lower atmosphere can be considerably different from actual observed values if a front is faster or slower than predicted.    

An example - passage of cold front in the Balkans late June 2023

The cold front was associated with very thick cloud cover and patchy rain.  2m temperatures were low for this time of year under the frontal cloud, and in the post-frontal cold air temperatures were only between 13°C and 20°C.  The T+72 forecast for Bucharesti Bucaresti showed no cloud, temperature 29°C, and SW'ly low-level winds.  But observations showed thick cloud layers, temperature 19°C, and NE'ly low-level winds.  However, the ensemble members showed uncertainty in the position of the cold front and the spread in temperature was very large in the whole boundary layer.

FigXXXFig9.1-3: Errors in 2m temperature over the Balkans T+72 DT12UTC 25 June, VT 12UTC 28 June 2023.  The cold front advanced further than predicted with extensive cloud depressing temperatures ahead of the front and allowing an inflow of colder air to the rear.  Colours show where forecast temperatures are too warm (red and purple), too cold (blue).

FigXXXFig9.1-4: Vertical profile showing differences structure of the airmass at Bucharestiat Bucaresti.  The T+72 forecast for Bucharesti Bucaresti (red) showed no cloud, temperature 29°C, and Sw'ly low-level winds.  The radio sonde data showed thick cloud layers, temperature 19°C, and NE'ly low level winds.