Definition
This product is more specifically designed to analyse and forecast strong to extreme precipitation events. It is based on the Misva (aeris-data.fr) single-layer vertical integral approach in the 925-600 hPa layer. It proposes a panel of 4 maps (Figure below) over 2 possible areas – West Africa or Atlantic – and can be accessed under:
Prévision Synoptique Cartes prévues – Misva (aeris-data.fr) Parameters : LowLevel-Diag – 925-600 – WestAfrica or Atlantic
Interpretation
1. West Africa Domain
- The main information concerns the barotropic vortex (top left), which can be used to identify the strong cyclonic vorticies (warm colours) around which the flow lines wrap.
- Since such strong vorticies have a little vertical slope, they are favourable to strong to extreme precipitation events.
- They correspond to strong AEW troughs, and in the most extreme cases to an AEW breaking.
- They are located on the southern flank of the wind core (AEJ) in accordance with the AEW conceptual scheme. In extreme cases of wave breakings, this strong wind core can wraps completely around the vortex.
- The maps at 600 and 850 hPa (below) confirm the previous barotropic analysis:
- verticality of the vortex (same position at both levels)
- strong AEJ core at 600 hPa
- strong monsoon flux at 850 hPa
- When a powerful vortex coincides with a thick MD monsoon flow (areas in blue, top right), we can anticipate a intense rainfall event.
- The northern boundary of the monsoon flow corresponds to a sudden drop in MD (red to yellow) allowing the identification of monsoon outbreaks over the Sahel.
- The areas of strong shear in the 600-950 hPa layer (thick vectors and isolines) are favourable to squall lines and indicate their speed and direction of propagation (see object Convection – Misva (aeris-data.fr)).
2. Atlantic Domain
The Atlantic domain can be used to track extremes coupled with a powerful vortex over Africa, then over the Atlantic where they can give rise to cyclogenesis in the Cape Verde area and transform into a cyclone as they cross the Atlantic. This product therefore complements the tropical cyclone analyses and forecasts provided by the National Hurricane Center (noaa.gov) by helping forecasters in Cape Verde to anticipate cyclogenesis events caused by the passage of strong AEWs developing over Africa and tracked with the help of the MISVA site.
- This product gives a clear visualisation of the vortex associated with TC tropical cyclones (closed circulation of the flow lines), and allows locating its position and quantifying its intensity (coloured vortex),
- if associated with high humidity on a deep layer (PW and MD)
- and to the absence of strong shear which is not favourable to the formation and maintenance of CTs.
Illustration
1. Extreme case of 30-31 August 2019
- The figure above shows a strong wet vortex over southern Sierra Leone which caused heavy rainfall over the region (Guinea, Sierra Leone and Liberia) on 29 August.
- Heavy rain also fell over Senegal (figure opposite), with ground observations up to xx mm in Dakar on 30-31 August.
- Being to the north of the vortex, below the JEA in a drier zone but with strong shear, this event corresponds to the passage of a squall line over Senegal.
- This vortex was forecast up to 6 days in advance, as confirmed by the 144 h forecast for 24 August below, compared with the analysis.
- However, the vortex forecast is slightly overestimated and a few degrees further north along the Atlantic coast, indicating that the predicted propagation was slightly too rapid.
- Over Senegal, the proximity of dry air, the strong AEJ and intense shear to the north of the vortex allowed anticipating conditions favourable to strong squall lines 6 days in advance.
2. Tropical Cyclone Larry (31 August – 11 September 2019)
The barotropic analysis opposite (from 31 August to 6 September) clearly shows the transformation of the vortex coming from the AEW as it leaves Africa into a TC in line with the “best track” trajectory (above) provided by the National Hurricane Center (noaa.gov)
- On 31 August the wet vortex leaves the West African coast
- On 2 September the JEA wrapped around and became a tropical storm.
- From 3 September it became a TC
- On 4 September, the JEA disappeared and wrapped itself completely around the centre of the TC, which had become mature and very powerful. The vortex is vertical, very wet and concentrated.
- On 6 September, it reached its maximum intensity and its trajectory began to curve northwards.
3. Extreme rainfall event of Ougadougou (1st September 2009)
This exceptional event, with a record rainfall of 263 mm observed in Ougadougou, has been widely documented in several articles (Lafore et al. 2017, Beucher et al. 2019) showing all the multi-scale factors that contributed to the formation of this case. The figure below illustrates the power of MISVA’s barotropic analysis to detect and forecast extremes.
- The mean flow lines in the 950-600 hPa layer form a very powerful vortex (red eddy > 2.10-4 s-1) corresponding to the breaking of the AEW on the southern flank of the AEJ.
- This wet vortex, coinciding with a thick monsoon flow (blue MD>4000 m), is highly favourable to the occurrence of a strong or even extreme rainfall event in this case.
- The previous trough has reached the Atlantic and is also very strong, showing that we have a powerful train of easterly waves.
- Between the 2 troughs the ridge is very dry (warm colours – monsoon flow thickness MD< 2500 m).
- Once triggered, these wet vortexes are highly predictable.
Complementary products
The “barotropic” analysis condenses most of the relevant information for forecasting strong and extreme rainfall events. However, it is essential to take into account other products for an in-depth analysis of the situation, such as: meridional transport, PW hovmöllers, etc. We illustrate below the additional information provided by these products.
1. Extreme case of 30-31 August 2019
The meridional transport highlights the vortex (opposite) centred on Sierra Leone. In addition to the ‘barotropic’ analysis, it shows the phasing between mid-latitude Rossby waves (eastward propagation) and equatorial Rossby waves and AEWs (westward propagation), allowing this latter to be amplified.
The hovmöllers opposite show the propagation of AEWs with their PW anomaly, meridional wind and eddy signatures at 850 hPa. The strongest events combine a strong vorticity (blue) with a moist PW anomaly (blue). However, unlike the ‘barotropic’ analysis :
- These hovmöllers do not allow detecting the verticality of the vortex and are therefore less relevant for extreme cases.
- They are also highly sensitive to the zonal band.
2. Tropical Cyclone Larry (31 August – 11 September 2019)
The hovmöller opposite, taken in a slightly more southerly band to catch the TC Larry’s trajectory, shows its progression and intensification :
- The initial signature (in term of vorticity and PW*) of the trough of the wave appeared over West Africa on 29 August at around 5°E.
- It intensifies strongly as it leaves the African continent from 1st September onwards (cyclogenesis).
- It reaches its peak around 4 September.
- Its trajectory curved northwards and it left the Hovmöller band on 5 September.
- Although small in size, the vortex is accompanied by a large meridional wind doublet, reflecting the wrapping around the TC over a disc more than 2000 km in diameter. This illustrates the interest of the “Meridional Transport” product opposite, which shows the extent of the disturbance to the large-scale circulation caused by the TC on 4 September.
