Medium level instability above underlying dry zones 

Evaporation of hydrometeors during descent from medium levels through an underlying dry zone depends upon:

  • how vigorous the convection is within the medium levels.  The values of Convective Available Potential Energy (CAPE) give an indication.
  • large drops falling may penetrate to the surface while small drops may well evaporate during descent.
  • moisture convergence can produce more precipitation even at low CAPE.  This can be because:
    • the dry zone becomes more moist as falling rain drops evaporate.
    • the lowest layers become more moist where rain reaches the surface.
    • more moist air at low level is advected into the area.

Use of CAPE in Medium Level Instability

It is important that moist medium level instability is modelled sufficiently as even relatively small CAPE can produce precipitation.  

If the medium level instability has high CAPE values, any heavy precipitation that is developed in reality can have drop sizes sufficiently large that they penetrate through dry air to reach the ground.  But IFS tends to evaporate heavy precipitation from medium levels too much during descent (in part due to limitations of assumed drop size distribution), and consequently insufficient rain is forecast to reach the ground.  

Even if medium level instability has low CAPE values, there can be insufficient evaporation of precipitation falling from the medium level cloud through underlying dry zones.  Also moisture convergence in the model can produce more precipitation even at low CAPE.   Because of this, forecast precipitation fields can show areas of precipitation where none is actually observed.

Because of this, IFS forecast precipitation fields can show areas of precipitation where none is actually observed.

Users should check forecast vertical profiles against local observations and profiles. This can help assess how likely it is that falling hydrometeors will evaporate during descent.  But the natural uncertainty in the real atmosphere should also be taken into account.

Convective Available Potential Energy (CAPE) is very sensitive to the humidity in the boundary layer.   A slight change in dewpoint, particularly within the boundary layer will leads to a significant change in CAPE.  Thus any medium level showers that do penetrate to the surface can locally increase boundary layer moisture - observed surface dew points can become several ºC higher than forecast T2m dew points (up to ~12ºC difference has been observed).  This leads to a local, possibly major, reduction in CIN and an increase in CAPE.   Further instability may then be released inducing further showery activity.   Forecast charts of surface precipitation are not likely to capture all such details.


  

Fig9.6.3-1: Example of a case in April 2025 when IFS forecast excessive convective precipitation falling from medium levels and reaching the ground.  FigA: The observed structure of the atmosphere at Bordeaux at 00UTC 04 Apr 2025 (black) and the analysed structure (red) which appears realistic.  FigB: Forecast vertical profile at a location near Dax in SW France at 12UTC 04 Apr 2025 which shows a realistic dry zone beneath weakly unstable AC based at 750hPa (low CAPE).  FigC: IFS Ensemble Control (ex-HRES) convective precipitation rate at 00UTC 05 Apr 2025 with local heavy rain cells (max forecast 10.5mm/hr in red areas) but no rain was actually observed.  FigD: IFS Ensemble Control (ex-HRES) total 24hr precipitation generally too heavy and widespread.  The shaded area in FigC and FigD shows the area where 1mm was observed in 24hr to 00UTC 05 Apr 2025, locally 4-10mm in the internal area over Spain.

Comparing AIFS and IFS precipitation

In IFS, heavy precipitation developed by medium level instability in reality can have drop sizes sufficiently large that they penetrate through dry air to reach the ground.  But IFS tends to evaporate heavy precipitation from medium levels too much during descent, in part due to limitations of assumed drop size distribution.  Consequently insufficient rain is forecast to reach the ground.  Even with lighter precipitation developed by medium level instability, there can be insufficient evaporation of precipitation falling from the medium level cloud through underlying dry zones.  Also moisture convergence in the model can produce more precipitation. 

Because of this, IFS forecast precipitation fields associated with medium level instability can show areas of precipitation where none is actually observed. See above.

Artificial Intelligence (AIFS) models are not concerned with detailed physics of the medium level cloud nor with the evaporation of falling raindrops.   Forecast surface precipitation is predicted from the structure of the forecast model atmosphere at each forecast time.  It is based upon what has been observed before during the machine learning phase.


Fig9.6.3-2: Example of a case in April 2025. IFS forecast excessive convective precipitation falling from medium levels and reaching the ground.  FigA: IFS Ensemble Control (ex-HRES) total 24hr precipitation generally too heavy and widespread.  FigB: AIFS Single total 24hr precipitation generally approximates closer to reality in this case.  The shaded area in FigA and FigB shows the area where 1mm was observed in 24hr to 00UTC 05 Apr 2025, locally 4-10mm in the internal area over Spain.


Use of forecast lightning charts

Lightning associated with medium level instability is often indicated on forecast charts although no precipitation is forecast at the surface.  Whilst lightning activity tends to be over-predicted (sometimes considerably) it can be a reasonable indicator of the potential for active medium level instability.  Forecast lightning activity often covers a greater area than does forecast surface precipitation. 

Where medium level instability is forecast above a dry lower atmosphere, users should use forecast lightning charts and forecast vertical profiles to extend and improve precipitation forecasts.   Where medium level instability is forecast (even with only moderate CAPE), some additional showers should be forecast within the areas of forecast lightning.  Owing to resolution issues, forecast intensity of lightning strikes gives only a rough idea of regions where there is more active medium level instability but it does not reliably indicate that showers will penetrate to the surface, nor their intensity if they do so.  However, probability of precipitation should be increased.


Fig9.6.3-3: Forecast IFS data for central and northwest Australia 17 Jan 2019.  Local time is about 10hrs ahead of European time zones. The circled triangle locates Alice Springs.

  • Fig9.6.3-3(a): Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019,  T+21 VT 09UTC 17 Jan 2019.
  • Fig9.6.3-3(b): Lightning density in 6hr (flashes 100km-2hr-1) DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019.
  • Fig9.6.3-3(c): ENS probability of total precipitation >1mm: DT 12UTC 16 Jan 2019, T+12 VT 00UTC 17 Jan 19 to T+36 00UTC 18 Jan 2019.
  • Fig9.6.3-3(d): Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019, and Observed lightning flashes VT 09UTC 17 Jan 2019.
  • Fig9.6.3-3(e): Forecast vertical profile at Alice Springs DT 12UTC 16 Jan 19, T+18 VT 06UTC 17 Jan 2019.


In this example, medium level thunderstorms developed and extended well into central parts of Australia (9.6.3-2(b), with observed lightning) but no underlying surface rainfall is forecast (Figs9.6.1-18(a) & 9.6.1-18(d), nor any probability of rain (9.6.3-3(c)).  Forecast lightning flashes (9.6.3-3(b)) is overly extensive in northwest Australia but although there is some indication in central parts it is under-indicated (compare with 9.6.3-3(d)). 

The model boundary layer was generally dry in central Australia but observations showed much higher dew points where showers have occurred.  Near Alice Springs the model T2m dewpoint was 4.2C lower than the observed dew point, and at a location to the northwest the error was 11.8ºC.  Both discrepancies were probably due to storms that the model didn't represent.

The forecast vertical profile for Alice Springs (9.6.3-3(e)) shows possible (surface-based) medium level instability with just moderate CAPE (e.g. cyan line construction).  Note that some ensemble members have higher low-level dew points which means a lower CIN to initiate medium level convection with greater CAPE (e.g. red dashed line construction).  Further, if the boundary layer is moistened after any medium level showers penetrate to the surface then there is a higher likelihood of more energetic convection being released afterwards with much greater CAPE (e.g. black dashed line construction).

In Central Australia, no precipitation is indicated; any precipitation in the model is being evaporated before reaching the ground.  However, the lightning activity chart suggests that, though the deep moist convection isn't very well-organised, scattered thunderstorms appear likely.  This was bourne out by observations.  Note also that the model greatly over-predicted lightning activity over northwest Australia.



(FUG Associated with Cy49r1) 




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