Italian heatwave ends with severe thunderstorms

By Jochen Kerkmann, HansPeter Roesli (EUMETSAT) and Sheldon Kusselson (NOAA NESDIS)

According to Estofex , a cold front, which had started to invade the western European mainland on Saturday 25 August 2012, wrapped around the Alps in the night to Sunday 26 August (see animation of Meteosat-9 Airmass RGB blended with the IR10.8 channel, 25 Aug 15:00 UTC–26 Aug 10:00 UTC) and then moved on through Poland, Slovakia, Hungary, northern Italy, Slovenia and western Balkan. It brought an end to the heatwave, which had plagued Italy and other Mediterranean countries for many weeks (with brief interruptions, see Tmax values from Firenze, source: Wetteronline).

As the low-level air ahead of the front was warm and very moist, with MLCAPE values of up to 1500–2200J/kg (in the lee of the Alps), Estofex had issued a level 2 warning (source: Estofex) for parts of northern Italy, Slovenia and Croatia. This level of warning was probably justified as the database of the European Severe Weather Database (ESWD) for this day (26 August 2012, source: ESWD) shows several reports of large hail and severe winds in the Po Valley and along the Adriatic coast of Italy, but also an F1 tornado close to Ostia/Fiumicino and a tornado close to Porec (Croatia).

The Meteosat-8 rapid scan loop below (HRV channel) shows the movement of this cold front (from north-west to south-east). On its way south, the front triggers several large thunderstorms with overshooting tops and convective outflow boundaries. In the afternoon, when the cold front reaches the Gargano Peninsula, a second wave of thunderstorms forms in northeastern Italy and clears the Po Valley from the remaining moisture that had not been removed with the passage of the cold front. This second Cb system is probably related to the passage of the upper level trough (see Airmass RGB image from 18:00 UTC with height of PV 1.0, source: EUMeTrain).

The moist air ahead of the cold front can be nicely seen in the Dust RGB product shown below. For this special case, the ranges of the RGB input beams have been modified to get a better contrast between dry and moist air. The range for the red beam (IR12.0–IR10.8) is -3 to 0K (instead of -4 to +2K), the range for the green beam (IR10.8–IR8.7) 0 to +4K (instead of 0 to +15K) and the range for the blue beam (IR10.8) is 223 to 300K (instead of 261 to 289K). These narrower ranges for the red and the green beams optimise the contrast between the different airmasses, but of course at the cost of losing the contrast for other features like dust clouds.

The moist air ahead of the cold front can be seen as an atmospheric (moisture) river, i.e. a moisture plume in the atmosphere that, under certain conditions, can lead to heavy rainfall and flashfloods (see e.g. case study of the catastrophic flash flood on Madeira Island, source: EUMeTrain, and the moisture plume of this event as seen in the NOAA blended TPW product, source: S. Kusselson).

According to Wikipedia, an atmospheric river is: "a narrow corridor or filament of concentrated moisture in the atmosphere.

Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans. The term was originally coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s, to reflect the narrowness of the moisture plumes involved. Atmospheric rivers are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon River.".

The animation of the RGB product (25 Aug 18:00–27 Aug 00:00 UTC) gives details of the atmospheric moisture at 15-minute time intervals. It shows how the moist air is pushed eastward, and how the dry air enters the Mediterranean area, starting in the Gulf of Lions and then quickly progressing towards Corsica and Sardinia and from there to the Italian mainland. Interestingly, some pockets of moist air remain downstream of the higher mountain ranges after the front has passed, either because the mountain ranges modify the regional airflow and pressure (including lee vortices, like to the lee of Corsica, see situation at 14:00 UTC) or because the post-frontal dry air does not descend down to the valley like in the Po Valley (see also the situation at 9:00 UTC).

Amendment (Sheldon Kusselson, 11 September 2012)

The NOAA blended TPW product (see NOAA's Blended TPW webpage) for this case is shown in this presentation. Slide 1 shows the blended TPW product for around 12/13 UTC to allow for some time for morning passes to get in the blended TPW product*. Slide 2 shows the same product in percent of normal. The TPW plume shows values between 125 and 190 percent of normal. It shows the TPW plume/atmospheric river well. Slide 4 shows the RAOB TPW for 12 UTC, closest upper air time to the RGB image shown below, and slide 4 is the hand drawn TPW analysis from these RAOB observations, with help from the blended TPW product.

Slides 5, 6 and 7 (which show the 850hPa, 700hPa and 500hPa analysis, respectively, all for 12 UTC) have been included to see if there was any good moisture transport. In summary, they show little moisture transport at 700 and 500hPa, nearly no moisture transport at 850hPa, thus not a great case for flooding as the moisture transport is poor.

Also the ASCAT surface winds (slide 8) show weak winds in the area of the moisture plume. Moisture transport is best when winds and height lines are directly parallel to the highest moisture or plume or 'atmospheric river'. However, the very sharp TPW gradient between the high moisture over northern Italy and nearby areas and low TPW centered over France and western Alps makes it a good case for severe thunderstorms, hail and maybe even a tornado.

*Note: Since NOAA does not yet have a product sector set up over Europe, the global sectors have been used to connect western Europe with eastern Europe. This makes the image a bit grainy, but it shows the main atmospheric river/moisture plumes well. Just for info, all TPW over land over most of the globe outside the US land area is from the MIRS method of deriving TPW and that is from Metop-A, NOAA-19 and NOAA-18 and maybe also F-16 of SSMIS.