Sunday , June 13 2021

AGU: Aircraft flying out of snow and rain from the clouds



A radar station in Kerava, Finland, recorded a trail of increased rainfall – the yellow stripe to the left – on the way to the Helsinki-Vantaa Airport (EFHK) in March 2009. With courtesy of AGU / Journal of Geophysical Research: Atmospheres

"Puncher & clouds like this seen in Houston, Minnesota in November 2014, occur when airplanes fly through a thin cloud of supercooled water vapor, causing freezing of water drops. The small crystals fall, leaving a hole to mark the passage of the plane. When such super-cooled clouds cover a lower layer of rainfall, falling crystals can cause heavy rain or snowfall, according to a new study in the Journal of Geophysical Research: Atmospheres. Photo by Jamie Vix

AGU News:

WASHINGTON, D.C. – Airplanes flying over rain or snow can increase rainfall by up to 10 times, according to a new study.

The rain and snow waves are not caused by aircraft emissions but are a special consequence of aircraft wings passing through clouds of overcooled water droplets in clouds over a layer of active rain or snow.

Under the right conditions, this effect may increase rains and snow storms over airports where many planes cross the cloud by approach and downhill.

"Interesting in this feature is that it was caused by airplanes but not caused by pollution," said Dimitriy Moisseev, a researcher at Helsinki University and the Finnish Meteorological Institute and lead author of a new study in the Geophysical Survey Journal of AGU: Atmospheres. "Even if there are absolutely clean planes that have no burning, no fuel or anything else, it will still happen."

Although the precipitation strips are artificially created, the physical process that begins with the crossing of plains can occur naturally, making them useful laboratories for studying the formation of rainfall, according to Mosesev. Observing them can help meteorologists "make" natural rainy and snowy conditions from 2 to 6 hours in the future, which is essential for airport operations.

"When you, like me, look at the radar data every day, something interesting is always happening," Mosesev said. "It is surprising that there are always new things that we can not yet explain."

Moisseev discovered curious streams of increased rainfall from a radio station antenna at the University of Helsinki Kumpula. Unnatural spots of heavy rainfall appeared on a background of lighter rain or snow and seemed to lean toward the nearby Helsinki-Vantaa Airport.

Their shapes seemed intriguing as the reversed form of the clouds, known as falling edges, holes or clouds of canals, phenomena that can occur when planes fly through clouds of water droplets that are colder than 0 degrees Celsius degrees Fahrenheit), but not freezing.

The new study shows that such a phenomenon can improve or cause rain or snowfall from layers of clouds underlying these super-cooling cloud layers.

Supercool

Both small water droplets and ice crystals form clouds. Pure water can remain liquid up to -40 degrees Celsius (-40 degrees Fahrenheit) without dust particles or other suitable surfaces to be present in the crystallization of seeds on ice. Thus, water droplets that condense in clouds can be much colder than the typical freezing point of 0 degrees Celsius (32 degrees Fahrenheit). These super-cooling clouds of liquids are common in low to medium cloud layers.

Changes in air pressure from passing aircraft can trigger these super cooled water drops to freeze in ice crystals. The air expands suddenly beyond the wings and tip of the propeller, resulting in a dramatic decrease in pressure and temperature on the ground. In the cloud of water droplets, which is already cooled between -15 to -20 degrees Celsius (5 to -4 degrees Fahrenheit), the passing airplane can lower the temperature below -40 degrees Celsius (-40 degrees Fahrenheit) and caused the formation of ice crystals.

The new ice crystals help to freeze more droplets of the supercooled water, triggering a crystalline chain reaction in an expanding circle around the airplane's path. When crystals fall, they create holes or stripes of fresh air in the cloud, sometimes opening a blue sky window if the cloud is thin. In most cases, ice crystals evaporate before reaching the ground.

Meteorologists knew that aircraft could freeze water droplets in ice crystals and previous work suggests that the process may increase the rain and snow in the underlying clouds but the effect has not been captured in detail.

Andrew Heimsfield, senior scientist at the Boulder, Colorado, NAM, and a researcher not linked to the new study, noted the potential for accidentally saturating cloudy clouds over airports in a previous article on formation and spreading of holes in aircraft-induced clouds. He observed similar radar rainfalls generated in radar data collected near the former Stapleton airport in Denver in 1992.

"We know that airplanes can cause precipitation. The authors of this study have many instances, with wonderful data from terrestrial instruments – radar, lizard – good particle and concentration information, and radio zone data to show the likely temperature to form, "said Haymsfield. "They managed to document the phenomenon."

In order to find out whether the drilled rainstorms could be caused by airplanes, Mosesev and his colleagues reviewed 11-year data from the Helsinki University meteorological data and found 17 days with recurrent cases of characteristic linear strips between December 2008 and January 2018

They investigated flight flights near the airport to see if the drillers could be caused by an airplane. The flight paths backed up by 2011 confirmed airplanes crossing 2-10 kilometers (1-6 miles) of the intense rollers in most of the observed cases.

"Increased rainfall mainly follows the airplane route above the cloud," Mosesev said. "It could be hundreds of miles, but the cross-section could be 100 meters. So this is a very narrow, long feature.

Sowing snow

Additional ice crystals increase the speed at which crystals collide to form larger snowflakes, increasing snowfall, say the authors.

This could happen if the airplane flies directly over the predominant cloud, but the authors suspect that something more complicated happens because their data locates the initial rainfall and snow improvement far above the layer that is already accelerating.

The new study concludes that ice-generated crystals most likely fall from the overcooled upper cloud layer in a lower layer that is raining or snowing, giving more rain or snow than the lower cloud layer.

Satellite data supports this scenario by showing the top layer of clouds consisting of droplets with too cold or a mixture of ice and water prepared at the right temperature to turn into ice crystals under the influence of planes. This upper layer of super-cool swims at the typical altitude of planes flying at Helsinki-Vantaa Airport.

Rain and snow, artificially reinforced by air traffic, have useful clues to natural rainfall and factors influencing the effectiveness of formation, according to Mosesev. Strases are random experiments that allow researchers to monitor the effect on the airplane's path just outside of it, and ask questions about the types of microphysical processes that are going on.


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