Cauliflower cloudtops reveal risky storms
Monday, May 09, 2016, 18:31 GMT - NASA researchers have just given severe weather forecasters a powerful new tool in the detection of hazardous storm conditions on the ground, by focusing their attention on the storms' loftiest heights.
NASA has already been gaining new insights into storms by scanning the rain content of clouds from space, but a new line of research is showing how the tops of clouds could reveal important details about what's going on underneath them.
Kristopher Bedka, a physical scientist at NASA's Langley Research Center, is leading a team that has developed a new method for computers to scan thousands of square kilometres of imagery from orbiting weather satellites in under two minutes, to pick out an indicator of extreme weather known as overshooting tops.
The concept of overshooting tops has been in weather forecasting handbooks for years. As a thunderstorm grows, the warm, humid air that contributes to it remains buoyant and continues to rise as long it is warmer, and thus less dense, than the air above it. With an exceptionally strong storm, it can keep building all the way to the top of the troposphere, and only stops there because temperatures "bottom out" at the tropopause - the boundary between the troposphere and the stratosphere - and start to rise again as you continue to go higher. This structure in the stratosphere - warmer air above cooler air - causes it to be very stable, and that stability halts the storm's vertical growth. As a result, the cloud flattens out, and this shapes the entire storm into something that looks like an immense anvil.
This cumulonimbus incus cloud was captured by Expedition 16 crew on the International Space Station. An overshooting top is circled in red. Credit: NASA
Rising air doesn't just stop on a dime, though. Updrafts in the cloud can reach speeds of over 160 km/h in the largest supercell storms. At speeds like that, although they slow down as they get near the top, these updrafts can pierce the bottom of the stratosphere, carrying part of the cloud with them. The winds quickly rebound back into the cloud, but what's left behind by this overshoot is a lumpy portion of the otherwise smooth cloud top, which looks a bit like a there's head of cauliflower floating at the top of the cloud (circled in red, above).
These are overshooting tops and when you spot these at the top of the cloud, you can be sure that some kind of extreme weather is going on below them, under the base of the cloud - heavy rainfall, strong winds, up to softball-sized hail, and worst of all, powerful, damaging tornadoes. Higher up, overshooting tops can cause turbulence that can be a danger to aircraft flying in the vicinity.
Without the right perspective on the cloud, spotting overshooting tops isn't easy, especially when you have several storms with cloud edges that are all bleeding into one another (as opposed to the lone anvil pictured above). Satellites provide the optimal vantage point to spot them, as they show up in visible imagery simply from the shadows they cast, and infrared images are even more ideal, since overshooting tops are cooler than the smooth cloudtops surrounding them.
NASA Cloudsat imagery - Visible (left) and Infrared (right) - of an anvil cloud over the South Pacific Ocean, May 9, 2008. The grey line notes the path the satellite traced as it passed over the storm. Credit: NOAA/NASA
This makes them stand out nicely in the imagery, but there's still been a persistent problem of timing.
How do you keep weather forecasters, and thus the public, updated on the development of these hazardous weather conditions when you're forced to scan through thousands of square kilometres in geostationary satellite images that arrive every fifteen minutes or so, or wait for lower-flying satellites (such as NASA's CloudSat mission) to fly directly over the region of the storm, which can take up to 90 minutes, depending on the satellite's orbit.
"You can see some of these clouds with your eyes in satellite imagery, but given that the images are changing so rapidly, you need a tool that can automatically identify them to make better forecasts," Bedka said in a NASA statement.
"You can imagine monitoring a storm throughout its lifetime, whether it has overshooting tops and how these tops relate to weather on the ground," he said. "But you could maybe do that for like one storm, and what you really want is to identify whether there’s an entire line of severe weather events corresponding to each of those little bumps."
Visible imagery from the GOES 8 weather satellite, during the May 3, 1999 Oklahoma City tornado outbreak. The overshooting tops show up as darker blotches of shadow against the white smooth cloudtops. Nearly every overshooting top in this animation corresponds to a tornado that touched down in the Oklahoma City area on that day. Credit: NOAA
Even with the benefits this new system could offer forecasters and weather spotters in the United States and Canada, the algorithm developed by Bedka and his team will be even more useful offshore, where radar data and spotting isn't as easily available, and in other parts of the world that don't have the benefit of these other methods of locating and tracking severe weather.
"In the U.S. we are fortunate in that we can track hazardous storms using weather radar data," Bedka said in the NASA press release. "But many regions in the world do not have these radars, so satellite imagery and hazardous storm detection products like mine are often the only data that forecasters can use to warn the public."
The new GOES-R weather satellite, which is scheduled to launch into geostationary orbit in October 2016, will not only have updated imaging capability, to give us a better look at storms and other weather systems, but will also include on-board detection algorithms to spot overshooting tops, which were developed by Bedka and his team.
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