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Ice storms are very bad for trees! NOAA image from here.

(I have no idea how blogger put the "U" on this post, nor any idea how to get rid of it....Grrrr....)

The CNN  headline today is "Forecast: Historic, crippling, catastrophic ice: Atlanta prepares for the worst." It is well known that freezing rain storms occur frequently in the southeastern part of the U.S. They are beautiful, but dangerous and costly.

And, they are not all that rare. Montreal, Quebec, typically receives freezing rain more than a dozen times a year. In 1998 the great North American ice storm of January 5-9 was one of the most damaging and costly ice storms in North American history, causing massive power outages on the east coast. Eastern Canada bore the brunt of the storm. Millions were without power for days to weeks to even months. 35 people died, a significant number from carbon monoxide poisoning from generators they used to try to keep themselves warm. The effort to reconstruct the power grid led to the biggest deployment of Canadian military personnel since the Korean War.
What makes an ice storm? The attached graphic from Gay and Davis summarizes the types of precipitation nicely and, when I read their paper, I learned a new word: "hydrometeor." It is "any water or ice particles that have formed in the atmosphere or at the Earth's surface as a result of condensation or sublimation." Examples are clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow and blowing spray.

Vertical temperature profiles in the atmosphere and
the kind of storms that they produce. From Gay and Davis,
1993 here.

The graphs shown here summarize the general conditions under which snow, sleet, freezing rain, and rain land on the ground within the context of the atmospheric temperature distribution.** Consider the situation when a warm front moves in.  If warm front isn't too strong, the atmosphere remains cold (below freezing) throughout, and precipitation falls in the form of snow. But, as a warm front moves in, an inversion layer develops with cold air near the surface under the warm air aloft. If snow starts falling aloft and encounters this warm air, the snowflakes melt. A mixture of frozen and unfrozen "hydrometeors" develops in the warm layer (left side of the Figure shown here). As these hydrometeors fall into the near-surface cold layer, they get supercooled. Any icy snowflakes that didn't melt as they traveled through the warm layer become efficient sites for refreezing, and a mixture of snow, ice, and some liquid falls to the ground, i.e., sleet. As the warm layer develops (gets warmer and thicker), all of the snowflakes melt as they travel through it. Without nearby ice particles to serve as nuclei, these become supercooled as they fall through the cold layer near the ground, i.e., they are supercooled liquid. When they land on cold ground, they freeze, producing freezing rain. If the liquid droplets formed in the warm layer reach ground that is above freezing temperature, the precipitation is cold rain.

As the warm front develops, it is common to see a sequence of precipitation progress from snow to sleet to freezing rain to rain.  The reverse situation occurs with cold front events in the southern Plain states.

A few factlets from Wiki: The thickest recorded ice accumulation from a single ice storm in the U.S. is 8 inches (northern Idaho, January 1961). In February 1994 a severe ice storm caused over $1 billion damage in the southeast.

**This discussion is from David Gay and Robert Davis, "Freezing rain and sleet climatology of the southeastern USA," Climate Research, vol. 3, 209-220, 1993. Notably, they comment that at the time this paper was written, relatively little was known about freezing rain and sleet climatology.

Meteorite impact craters and their rays

Martian impact crater formed between July 2010 and
May 2012. NASA image ESP-034285_1835. 

NASA just released this beautiful image of a fresh Martian impact crater. The image came from HiRISE on NASA's Mars Reconnaissance Orbiter taken on November 19, 2013. The age range was pinpointed through the orbiter's "Context Camera" that revealed a change in appearance at that site between July 2010 and May 2012. The crater is about 30 m in diameter, and the ejecta extends out to 15 km. The blue color in this image is attributed by the HiRISE team to removal of reddish dust in the area. Alternatively, I'm wondering if it sue to the veneer of fresh excavated ejecta covering the reddish dust.

In discussing this with a colleague, I pointed out that many of the studies of impact ejecta processes date back to the 1960's and 1970's, and were in the context of where to send an astronaut to explore on the Moon.  If you wanted to sample material from deep in the crust, it would be too hazardous for an astronaut to climb down the walls of an impact crater (believe me, having scrambled around the walls of Meteor Crater in Arizona many times, you do not want to be wearing a space suit while climbing down into an impact crater!). One thought was that you could sample the ejecta by going to the rays of a crater. For example, from this source:

"Lunar crater rays are those obvious bright streaks of material that we can see extending radially away from many impact craters. Historically, they were once regarded as salt deposits from evaporated water (early 1900s) and volcanic ash or dust streaks (late 1940s). Beginning in the 1960s, with the pioneering work of Eugene Shoemaker, rays were recognized as fragmental material ejected from primary and secondary craters during impact events. Their formation was an important mechanism for moving rocks around the lunar surface and rays were considered when planning the Apollo landing sites. A ray from Copernicus crater crosses the Apollo 12 site in Oceanus Procellarum. Rays of North Ray and South Ray craters cross near the Apollo 16 site in the Descartes Highlands and a ray from Tycho crater can be traced across the Apollo 17 site in the Taurus-Littrow Valley on the eastern edge of Mare Serenitatis. There is still much debate over how much ejecta comes from the primary impact site or by secondary craters that mix local bedrock into ray material."

In a 1971 article, Verne Obereck concluded that the bright rays "only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters:"

Observations of high resolution photographs of part of one of the prominent rays of the lunar crater Copernicus show that there is a concentration of small bright rayed and haloed craters within the ray. These craters contribute to the overall ray brightness; they have been measured and their surface distribution has been mapped. Sixty-two percent of the bright craters can be identified from study of high resolution photographs as concentric impact craters. These craters contain in their ejecta blankets, rocks from the lunar substrate that are brighter than the adjacent mare surface. It is concluded that the brightness of the large ray from the crater Copernicus is due to the composite effect of many small concentric impact craters with rocky ejecta blankets. If this is the dominant mechanism for the production of other rays from Copernicus and other large lunar craters, then rays may not contain significant amounts of ejecta from the central crater or from large secondary craters. They may in fact only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters.

Recently, Valery Shuvalov proposed a ray production mechanism based on a large supercomputer simulation. In this simulation, the hypothesis was that rays result from interaction between the shock wave associated with a developing crater and nonuniformities in the target surface. The results of a simulation of the formation of a crater by a 5-km diameter asteroid on the Moon at an impact velocity of 15 km/s are shown in the adjacent figure. This impact would have produced a crater approximately the size of Tycho, a famous rayed crater on the Moon. The target and projectile material were both assumed to have the mechanical properties of granite.

When the shock wave from the developing primary crater hits a depression (preexisting small crater) a jet of material is spalled off the wall of the small crater proximal to the primary crater (upper left in the simulation sequence shown).  In contrast to the effect of a depression on ray formation, a ray-suppressing effect is seen if there is a nearby elevation.

Eruption of Mount Sinabung, in North Sumatra, Indonesia

Photo from CNN.Com by Einsar Bakkara/AP
in the cited article in text
(if I read the credit correctly)

Mount Sinabung, an Indonesian volcano dormant since 1600 came to life in 2010 and, on Saturday, spewed forth pyroclastic flows that killed at least 14 people. Tragically, it appears that these people had been evacuated last summer and only the day before this eruption, had been allowed to return to their villages. The Wiki site for Mount_Sinabung appears to be updated in a timely way, so I won't go into details here.
     It is difficult to tell what the source of the erupted material is in detail, but from photos of the volcano (a classic beautifully conical stratovolcano) and the lack of any indication of lateral bulges on the flank, a good assumption is that the flows are originating in a summit crater. A question/assumption, is whether they are being driven by volatiles (presumably H3) from magma or whether or not groundwater is involved. According to the Wiki article, in late December, a lava dome had formed on the summit.
     The eruption gas/ash material from lava domes results in eruptions known as "Pelean" or "Merapi"-type pyroclastic flows. Two processes contribute to the high-velocities observed from such eruptions: gravitational collapse (supplemented by heating and expansion of entrained air), and sudden expansion of pressurized gases from inside the domes. If gravity controls the energy transfer, then areas affected can be predicted on the basis of topography. If gas expansion adds a significant contribution, which is likely in the proximal region around a dome, then velocities beyond those acquired by acceleration in a gravitational field, exist, and these imply that much larger areas are at risk than might be predicted from the gravitational forces alone.
     In 1993, Jonathan Fink and I published a paper "Estimate of pyroclastic flow velocities resulting from explosive decompression of lava domes," Nature, v. 363, pp. 612-615, 1993.  In this paper we examined the two processes above, and concluded that the decompression process produces velocities comparable to those acquired by gravitational accelerations. In snapshots, such as that in the photo in this post, my guess is that the flow is clearly already some distance down the slopes of the volcano where it has assume the classic profile of a dense gravitational flow with air entrainment. More proximal regions have already been hit, and are, apparently, where the casualties have occurred. With the complicated sequence of recurring explosions/eruptions from the summit, it may never be possible to reconstruct the dynamics of the flows in the proximal region.