RELAMPAGO

Field Campaign Map

Extreme Thunderstorms of Argentina

Central Argentina is home to some of the most extreme thunderstorms in the world in terms of hail frequency, storm height, growth and lightning. These storms develop in the unique environment created along the eastern side of the Andes mountains fed by atmospheric moisture from the Amazon. To understand what makes these storms so severe, over 160 scientists from around the world conducted an atmospheric science field project called RELAMPAGO between 1 November and 16 December 2018, the largest of its type ever outside the US. Mobile and fixed weather radars, weather balloon launchers and car-fixed weather stations were strategically deployed to intercept hailstorms across the Mendoza and Córdoba provinces. The following article follows the experiences of the hail team (Joshua Soderholm (Monash), Nicholas McCarthy (UQ), Matt Kumjian (PSU) and Paula Maldonado(UBA) ).

The hail team was tasked with (1) the deployment of hail pads (polystyrene pads covered in foil) into array to capture hail stone impacts and (2) operating an aerial drone immediately after hail-fall to survey hailstone size and distribution for the Australian HailPixel project.

25 November 2018

After an 8 hour drive from Córdoba to the Mendoza Province, the mobile armada was deployed the following morning to observe hailstorms across the desert plains of the eastern Andes. This consisted of no less than 24 staff operating 3 weather radar trucks, 3 weather balloon vans, 3 mobile mesonets and our humble hail team car. Deployment was coordinated from the operations center, 700 km to the east, in Villa Carlos Paz. Communication was a challenge at the best of times, with patchy cell phone and even satellite coverage.

The strategy focused on utilizing the limited north-south road network to intercept discrete supercells as the moved east off the Andes through the mobile weather radar domain east of San Rafael. This required the deployment of a 15 hail pads into two arrays with 1-2 km spacing along routes 153 and 77.

Deployment Map for 25 November 2018

Red: hail pads. Blue: weather radars. White: mobile mesonets. Pink: Weather balloons. B/W targets: weather station pods

Weather Station Pod

Doppler on Wheels (DOW) mobile weather radar

Hail Pad

Hail Pixel Drone

Downburst timelapse of approaching storm

After a 250 km drive to deploy the hail pad arrays, the hail team positioned ourselves on route 153 shortly after 1900 UTC as a thunderstorm started to moved towards the asset domain. Mobile phone coverage was very limited, but luckily we could visit one of the three mobile radars and watch the imagery feeds in real-time! The target storm quickly developed a large rain-free updraft base, wall clouds and downbursts characteristic of low-precipitation supercells as it approached the hail team. Hail was on cards!

With limited time to spare, additional hail pads were deployed along the southern end of route 153 to increase sample density across the anticipated storm track. After finishing deployments, we moved just north of the updraft track to avoid any hail.

Despite efforts to stay outside the hail, 2-3 cm fell on the hail team vehicle, providing the opportunity to collect a detailed survey hailstone size and aerial survey of hail coverage. Out of the 19 hail pads deployed, only one pad sampled hail, highlighting the fine-scale nature of hail cores within thunderstorms.

The single pad which observed hail

Aerial view of hailstorm

26 November 2018

After an early start to retrieve the hail pads from the previous day (another 250 km driving east of San Rafael), the hail team raced to the west of San Rafael to join the experiment as thunderstorms were already brewing. This new domain was positioned over the vast alluvial plains that stretch up into the Andes - the last place I'd expect to find a hailstorm. Assets were deployed along route 144, anticipating thunderstorms will track along the road.

On the plains there was no mobile phone coverage - our only communication with the operations centre was via satellite messaging, which was flaky at best. Luckily, the absence of hills and vegetation provided fantastic panoramic views for surveying the track and structure of the oncoming storm. With only an east-west and north-south road to choose from, we set off into to intercept the storm.

Deployment Map for 26 November 2018

Red: hail pads. Blue: weather radars. White: mobile mesonets. Pink: Weather balloons. B/W targets: weather station pods

View of approaching hailstorm

As the target storm passed over the mobile radars, reports of 1 cm hail started to filtering through the satellite messaging. Unimpressed with the visual structure, we apprehensively deployed hail pads further to the east along route 144 hoping for further intensification. Shortly after, a shelf cloud rapidly developed along the leading edge of the storm, indicating a clear shift to a stronger system. Hail streaks in the falling precipitation were also apparent along the left flank of the storm - our hopes were lifted!

As the ground-scraping shelf cloud approached, the hail team set off east under it, hoping to catch further intensification at the anticipating crossing of route 144. There's nothing like driving with a rapidly developing thunderstorm on your tail. The intercept location ended up being outside one of the numerous vineyards of the Mendoza province, fortunately protected by Kevlar hail nets (hail is a very serious problem for agriculture). Hail pads were deployed and we positioned ourselves just outside the expected hail core.

Sample of HailPixel survey imagery

Hearing the first 'ding' on the roof meant efforts to avoid the hail weren't entirely successful. The first 'thump' rendered those efforts a complete failure. Regardless, it provided a unique opportunity to watch 4+ cm falling, bouncing and shattering on impact. Immediately after the hail stopped we whirled into action, measuring the hail size using calipers and flying the HailPixel survey drone. The HailPixel imagery provides a unique chance to compare the small sample size of hail pads to a much large sample.

We suspected that the hail pads dropped earlier on route 144 had sampled hail, but we had no idea to what degree.

Measuring hail size

20181126_radarloop.mp4

Mendoza radar (reflectivity) animation. Colour lines show paths of cloud seeding aircraft.

Aerial view of the hail swath

VID_20181126_175831786.mp4

DOW radars departing after the hailstorm

Accumulating large hail

Hail pad sampling 40+mm stones

Hail pad "destroyed" by accumulating large hail

The first hail pad we collected had clearly sampled large hail stones, likely 40+ mm. Success! We continued back long route 144, photographing impacts and collecting hail pads. The last hail pad we collected was truly a shock - it had sampled large hail at an incredible density. The foil was stripped away from the pad and the foam surface was ruptured by the repeated impacts. It's likely several dozen hailstones impacted the small footprint of the hail pad. Moving further down the road we found deep accumulations of large hail that had melted relatively little 45 minutes after falling.

Aerial drone imagery revealed the true extent of the accumulated large hail. It also revealed the remarkable changes in thunderstorm intensity. Over the space of 20 km, the storm had transitioned from producing small 1 cm hail, to deep accumulations of 2+ cm hail and finally sparse 4+ cm hail further east.

Giant Hail in Villa Carlos Paz

Back at the operations centre in Villa Carlos Paz, a local resident had informed us she had saved a giant hailstone collected in January 2018 and kindly let us inspect it. This was the first time we had seen a 11+ cm hailstone! Using a 3D scanner, we were able to capture the hailstone shape for further studies. Little is known about the trajectories giant hail take through a thunderstorm which allow it to grow to such an incredible size. Furthermore, giant hail is often very sparse, which was confirmed for the Villa Carlos Paz event. The sparse nature of giant hail means it is poorly detected by weather radars and an ongoing research focus.

3D scanning the 11+ cm VCP hailstone