By HansPeter Roesli (Switzerland) and Luca Nisi (MeteoSwiss)
Low-level convergence over the Po Valley is a recurrent weather situation, where on the alpine south-side a possible moderate north-foehn flow breaks down and is replaced by a low-level backflow of cooler wetter air moving up the Po Valley from the Dinar Alps and the Adriatic Sea. This eastern backflow (EBF), actually an extension of the Adriatic Bora winds, creeps under the airmass present in the central/western Po Valley (Piedmont and Italian Switzerland). The airmass above the EBF is then simultaneously lifted and blocked in the arc formed by the Western Alps and (to a lesser part) by the northern Apennines. The resulting weather change over the area may be quite dramatic: from clear skies to low cloud cover and drizzle at places in the cooler months or, in the warmer months, convective rains/thunderstorms not directly forced by the diurnal cycle.
On 27-28 May 2022 low pressure covered Scandinavia and the Baltic countries. On its western side polar air flowed from the Norwegian Sea southward and further on turned to the south-east. The associated cold front arrived over Central Europe in the night of 27-28 May. While the cold front continued to move south over eastern Europe, its western tail weakened under anticyclonic influence and got blocked along the Alpine north-side.
The surface charts by the Deutscher Wetterdienst (Figure 1) depict the situation on 28 May at 06:00 UTC and 18:00 UTC, with the frontal tail inside the red rectangle. The animation with side-by-side SEVIRI HRV and false-coloured IR10.8 images between 04:00 UTC and 20:45 UTC shows the evolution of the cloudiness over the west-central Alps and North Italy (Figure 2).
Concentrating on the area framed by the red rectangles (essentially the western parts of the Alps and Po Valley), a north-south pressure gradient across the Alps dominated at 06:00 UTC, while at 18:00 UTC it was rivalled by a similar gradient along the Dinar Alps. As a consequence the initial north-foehn tendency got quenched and the Bora-type gradient started to dominate, driving an EBF into the Po Valley.
Based on a paper by Gao and Kaufman* ratios of MODIS bands 17 (0.905μm) and 2 (0.8585μm) are a simple way to depict low-level moisture. Both bands are solar channels in the near-infrared region. While band 17 sits in a weak water-vapour absorption region, band 2 is in a water-vapour window. Thus, the images of the ratio band17/band2 may be used to monitor lower-level moisture content in a qualitative way in cloud-free places.
Applying this simple algorithm to the two MODIS overpasses in daylight (Terra and Aqua) and colouring the ratio values with an appropriate colour map from moist-blue over green shades to dry-yellow, the progress of the EBF in the 105-minute interval between the overpasses stands out quite well (Figure 3).
Although considerable cloudiness hampers the identification of the subtle transition from moister (in blue shades) to less moist (tending to green shades) is not easy to see, the overlay with winds and equipotential temperature at 975hPa from ERA5 confirms the posited progress of the EBF across the Po Valley. The ERA5 analysis at 10:00 UTC (left side) even shows the dry foehn flow that initially penetrated the western parts and really is difficult to make out in the underlying ratio image. Under clear sky and stronger moisture contrast ratio imagery gives a much clearer picture, as shown in the B/W image of a strong north-foehn case in the same area on 14 April 2020 (Figure 4).
The ERA5 analyses put the upper limit of the EBF at about 825hPa (not shown). The radio soundings at Cameri (west) and Udine (east) on 12:00 UTC (28 May) and 00:00 UTC (29 May) (Figure 5) confirm the penetration of the EBF below ~800hPa.
As already shown by the IR10.8 images in the SEVIRI animation above, important convection set in over the Piedmont Alps at after 17:00 UTC (inside red ellipse in Figure 6).
Details may be gleaned from the animated sequence of weather radar pictures covering the whole day (Figure 7); in the late evening back-building convective rain cells popped up in this area that, due to their stationarity, led locally to considerable rain amounts.
Finally they drifted eastward driven by the west winds blowing above the then weakening EBF.
Before the new century forecasting (the timing of) these weather changes was a difficult task and needed much experience. The lack of good and timely observations and deficient modelling restrained a reliable tracking of the EBF across the Po Valley. Nowadays modelling has made considerable progress and allows to pinpoint the EBF in a reliable way, as illustrated by the ‘most probable’ COSMO-1 ensemble forecasts of 10m winds with lead times of 6 (15UTC) and 9 hours (18UTC). The EBF is found inside the red ellipses (Figure 8).
In the near future FCI on MTG-I will offer bands very similar to the ones used here from MODIS. It is posited that regular imaging by FCI, i.e. animated image sequences at 10-minute intervals or less, will improve the identification of subtle signals such as moisture gradients even under cloudy conditions (based on expertise gained with e.g. Dust RGB sequences from SEVIRI). Thus, ratio imagery promises to empower nowcasting skills for this and similar types of forecasting challenges, as it was already anticipated a long time ago by the MTG Expert Team on Nowcasting that originally proposed the inclusion of the NIR0.9 band in FCI.
* Gao, B.-C., and Y.J. Kaufman, Water vapor retrievals using Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared channels, J. Geophys. Res., 108(D13), 4389, doi:10.1029/2002JD003023,2003