GeoLog

clouds

Imaggeo on Mondays: Isolated storm

Imaggeo on Mondays: Isolated storm

Clouds and storms are formed when warm, moist air rises. This causes the air to expand and cool: forming clouds as the moisture condenses onto particles suspended in the air (called cloud condensation nuclei). Normally, air rises from surface heating, or when warm and cold air pockets collide, or if air is pushed upwards when passing over hills or mountains. If this heating, and subsequent rising, is rapid enough then thunderstorms can form.

This imaggeo on Mondays photo shows an isolated thunderstorm roughly 50 km North of Vienna. The difference between an isolated thunderstorm and scattered storms is how much coverage the clouds have over a given area. If less than 10-20 % is covered then these storms are described as isolated. Scattered storms occur when coverage is at least 30-50 %. These storms can lead to downpours lasting a few minutes that then leads to sunny spells, only to have another rain storm occur again shortly afterwards.1

Globally, there are roughly 16 million thunderstorms each year, and at any given moment, there are ~2,000 thunderstorms in progress.2 The visible dark grey anvil shape and the fact that the lighter clouds above appear to be being ‘pulled into’ the storm suggests that this is a ‘severe’ thunderstorm. This means that the storm is self-supporting3 and can cause more extreme impacts than a normal thunderstorm. Rainfall is more intense and can cause flash flooding. In some cases, hail over 2.5 cm large can fall and tornados can even be formed.4 For more information about severe thunderstorms please check out the further reading list below.

By Sarah Connors, EGU Science Policy Officer

Further reading / sources

[1] – Aerostorms Scattered vs. Thunderstorms – http://www.aerostorms.com/scattered-vs-isolated-thunderstorms-what-is-the-difference/

[2] – Thunderstorm Basics – http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/

[3] – Royal Meteorological Society Thunderclouds presentation – http://www.met.reading.ac.uk/~sgs02rpa/CONTED/WEATHER09_thunder.pdf

[4] – Frequently Asked Questions About Thunderstorms – http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/faq/

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Geosciences Column: Recent and future changes in the Greenland Ice Sheet

Geosciences Column: Recent and future changes in the Greenland Ice Sheet

Over the past few decades, the Arctic region has warmed more than any other on Earth. The Greenland Ice Sheet is losing mass faster than ever before, and is expected to keep melting with consequences for global sea-level rise and ocean circulation. At a media briefing, during the EGU’s General Assembly in April (stream it here), researchers presented new results on the factors that influence the Greenland Ice Sheet’s rapid and profound changes – from glacial lakes to clouds and snow darkening.

The vast expanse of the Greenland Ice Sheet covers an area of 1.71 million km2 (approximately a tenth of the size of Russia), and holds a staggering volume of ice: 2.85 million km3. The ice sheet is only rivalled in size by one other: the Antarctic Ice Sheet. Scientist have calculated that the Greenland Ice Sheet stores enough freshwater to raise sea level by 7.4m, should all the ice melt, so understanding what causes the ice to melt now and in the future is critical!

The importance of clouds

When you think of clouds, you probably think of them as purveyors of rain and bad weather. But that is not all; clouds form an intrinsic part of the climate system which is more complex than simply how they affect day to day weather. In Greenland, (as elsewhere across the globe), clouds are a source of precipitation, bringing all-important snow which accumulates on the ice sheet and makes it grow in size.

Southern Tip of Greenland.  Satellite Image by  NASA. Source: Wikimedia Commons

Southern Tip of Greenland. Satellite Image by NASA. Source: Wikimedia Commons

Clouds also affect temperatures: on a clear day you’ll feel the warmth of the sun on your back, but as night falls temperatures start dropping quickly as heat is lost to the atmosphere. However, if in the late afternoon the clouds started rolling in, the night would be warmer, as clouds stop heat being lost to the atmosphere. If they stick around long enough though, they promote cooling, as they reflect sunlight away from the Earth’s surface.

“On a global scale, clouds (on average) tend to cool the Earth’s surface, but there are many regional differences”, explained Kristof Van Tricht,(a PhD student at the University of Leuven in Belgium), during the press conference.

It turns out that, in Greenland, the warming effect of clouds dominates, and warming of the surface encourages melting of the ice sheet. However, the remoteness of the area means that direct observations of just how much the clouds warm the surface and to what extent this impacts on the ice sheet has been limited. Until now.

Using satellite observations, Van Tricht and his team have been able to study the warming effect of clouds in more detail than ever before. Their models show that, in the presence of clouds, the Greenland Ice Sheet can be up to 1.2°C warmer, which can cause substantial melting. Compared to models ran without cloud cover, the ice sheet could melt up to 38% more. This equates to 12% more runoff from the ice sheet into the oceans, solely due to the presence of clouds.

Predictions of what the findings mean for the ice sheet in the future are tricky though. The scientists’ model is based on real-time observations and so it isn’t possible to look into the future. For that, improved cloud model simulations are needed.

Beautiful lakes

Lakes form, seasonally, on the surface of the Greenland Ice Sheet as a result of run-off water pooling in depressions in the ice. Although beautiful to look at, because they are darker than the surrounding ice, they attract more heat. The lakes also drain sporadically, and when they do, some of the water they hold drains through the ice making its way to the base of the ice sheet. Once there, the water lubricates the base of the ice sheet and promotes it to flow more easily and quickly towards the ocean. Combined, these two effects affect the dynamics of the ice sheet.

 Drained Supraglacial Lake Bed. This lake has drained through the bottom for several years in a row. The large block was initially formed in summer of 2006, but large cracks run through it from subsequent lake drainages.  Credit: Ian Joughin (distributed via  imaggeo.egu.eu )

Drained Supraglacial Lake Bed. This lake has drained through the bottom for several years in a row. The large block was initially formed in summer of 2006, but large cracks run through it from subsequent lake drainages.
Credit: Ian Joughin (distributed via imaggeo.egu.eu )

At present, the lakes generally form within the ablation zone – the low-altitude regions towards the edges of the ice sheets where ice is lost through melting, evaporation, calving and other processes – where it is already warmest on the ice sheet.

At the press conference, Andrew Sheperd presented research carried out by Amber Leeson, on how the location on the ice at which the supraglacial (meaning they form on the surface of the ice) lakes form might change with a warming climate and what this means for the Greenland Ice Sheet.

As the climate warms, higher altitude regions on the ice sheet will too. Through building a hydrological model, Leeson found that the lakes spread father inland. According to Leeson’s simulation

“by 2050, the lakes have spread about 50 to 100 km further inland, so more of the ice sheet is potentially exposed to this lubrication effect,” added Shepard.

This is equivalent to an estimated 48–53% increase in the area over which they are distributed across the ice sheet as a whole.

Previous studies of how the ice sheet might respond to a warming climate do not consider the effects of the added melt water volume at the base of the ice sheet as a result of more lakes at the surface. Leeson’s findings mean that these models need to be re-run so that scientists can fully understand the potential implications. This is particularly true in terms of the lubrication effect at the base of the ice and whether the ice will more readily slip towards the oceans, potentially heightening the risk of sea level rise.

 By Laura Roberts, EGU Communications Officer

 

Further reading and information

You can stream the full press conference by following this link: http://client.cntv.at/egu2015/PC9.

Details of the speakers at the press conference are available at: http://media.egu.eu/press-conferences-2015/#greenland

This blog post presents only some of the findings which were discussed during the press conference. Other aspects of this press conference where covered in the media, you can find more on those here and by following this link.

Kristof Van Tricht, Gorodetskaya, I.V., L’Ecuyer, T. et al. Clouds enhance Greenland ice sheet mass loss, Geophysical Research Abstracts Vol. 17, EGU2015-12737-1, 2015 (conference abstract).

Amber A. Leeson, Sheperd, A., Briggs, K. et al. Supraglacial lakes on the Greenland ice sheet advance inland under warming climate, Nature Climate Change, 5, 51–55, doi:10.1038/nclimate2463, 2015.

Amber A. Leeson, Sheperd, A., Briggs, K. et al. Supraglacial lakes advance inland on the Greenland ice sheet under warming climate, Geophysical Research Abstracts, Vol. 17, EGU2015-934-1, 2015 (conference abstract).

Imaggeo on Mondays: Fuelling the clouds with fire

Wildfires frequently break out in the Californian summer. The grass is dry, the ground parched and a small spark can start a raging fire, but burning can begin even when water is about. Gabriele Stiller sets the scene for a blaze beside Mono Lake, exploring the events that got it going and what it may have started in the sky… 

While on shores of Mono Lake in the summer of 2012, I spotted something strange in the distance: a great blaze on the other side of the lake. We were on a trip through the southwestern states (a long tour through California, Nevada, Utah and Arizona). All the days before we had been continuously accompanied by thunderstorms that broke out during the afternoon. The photo was taken before the daily thunderstorm, and the large convective system already hinted to the next storm to come – and indeed it did, just a few hours later.

Desert fires close to Mono Lake, California. (Credit: Gabriele Stiller via imageo.egu.eu)

Desert fires close to Mono Lake, California. (Credit: Gabriele Stiller via imageo.egu.eu)

It was not clear if this convective cloud system was generated by uplift of heated air initiated by the fire, a process known as pyro-convection, or if it was simply a coincidence. After all, thunderstorms were a regular occurrence throughout our trip. This could have been the storm of the day, and the related convection could have transported the air and smoke from the fire upwards. Or a combination of both could have been behind it. The cumulus cloud was quite isolated, with clear sky surrounding it, but you can already see a small anvil developing (the area where ice is formed in the cloud) above the cauliflower-like cumulus – a hint towards a developing thunderstorm. Such a development would make the cloud into a cumulonimbus cloud.

So what caused the blaze? On 8 August 2012, the wildfire was started through lightning ignition by a thunderstorm coming from the Sierra Nevada, and it burned for several days on open grassland, far from human infrastructure. Due to these circumstances, firefighting was not particularly difficult for the authorities. However, more than 13000 acres were burned, and more than 500 people fought the fire. One of the priorities was to keep the amount of sage-grouse habitat burned to a minimum.

By Gabriele Stiller, Karlsruhe Institute of Technology, Karlsruhe, Germany

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.