Spring convection and associated severe weather returned to the southern Plains on April 22nd. Did predictors of convection do a good job in locating the severe cells?

CIMSS has recently started to produce synthetic satellite imagery from the Weather Research and Forecasting (WRF) model run at the National Severe Storms Laboratory (NSSL). Output from daily runs at 00 UTC is produced for 9 infrared bands that correspond to those of the Advanced Baseline Imager (ABI) that will fly on GOES-R. The hourly loop of the 11.2-micrometer channel, above, for the period between 1800 UTC 22 April and 00 UTC 23 April, shows convection initially forming along the dryline in the Texas Panhandle between 1900 and 2000 UTC before progessing northeastward into Oklahoma and Kansas. Synthetic imagery of the middle of 3 ABI water vapor channels (6.95 microns), show a similar story. Model predictions give clues on where to look for convective development. How did real-time predictors of convective development perform?

The UW Convective Initiation algorithm combines observations of 10.8-micron cooling (from GOES-13) with cloud phase changes. When cooling occurs as cloud phase is changing (suggesting growing cumulus towers that are glaciating), GOES-13 pixels are flagged as showing convective initiation. Depending on the cloud phase — all water, mixed water and ice, or all ice, the initiation is flagged in the screengrabs from N-AWIPS above as pre-CI cloud growth (blue), CI likely (green), or CI occurring (yellow). Once glaciation has occurred, CI detection turns off. A previous blog entry on this method is here.

UWCI does flag individual cells that subsequently develop, ignoring adjacent towering cumulus. Thus, it can draw forecaster attention to the updrafts that, for whatever reason, are the most vigorous. For example, the image at 1701 UTC show convective initiation indicated in one spot along the dryline in west Texas. By 1745 UTC, convection has developed. Shortly after 1800 UTC, UWCI identifies individual cells along a line from the extreme western portion of the Oklahoma panhandle northward into east central Colorado. These cells subsequently spawn severe weather. UWCI also flags nascent convective development for cells that eventually develop into an arc of convection over central Kansas at 2301 UTC. Note also that UWCI flags specific convective towers within a large cumulus field over the southern Panhandle. (Consider the three images at 2131 UTC and 2145 UTC and at 2231 UTC; convection initiation flagged at the earlier two images develops most vigorously as shown in the final image). This can focus forecaster attention to the clouds that are growing most rapidly.

The two images above show where convective initiation was diagnosed to be ongoing at some time on 22 April, as well as a preliminary Storm Report from the Storm Prediction Center. Note the good general overlap of UWCI points over the High Plains and storm reports. That more Storm reports exist than UWCI points reflects the UWCI philosophy of keeping the false alarm rate low, perhaps at the expense of detection.

There are several UWCI hits over the northeast on 22 April as well. There, cold air at upper levels promoted self-destructing sunshine and shower and thunderstorm development. Clear skies early in the day (1431 UTC) gave way to cumuliform development. The strongest updrafts likely yield the strongest cloud-top-cooling signal (as shown in this loop) and evolve into the most vigorous shower or thundershower. Even though severe weather was not reported with these cells, lightning was produced, starting around 1900 UTC as shown here. Cloud-top cooling can give a forecaster a head’s up that a particular cell might become vigorous enough to electrify.

(Note: this post has been corrected to remove images from before 1645 UTC on 22 April that may have included mis-navigated regions of convective initiation).

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A large explosion aboard the offshore oil rig Deepwater Horizon — located about 52 miles southeast of Venice, Louisiana — occurred around 03 UTC on 21 April 2010. McIDAS images of GOES-13 (GOES-East) Shortwave Infrared (3.9 µm) and GOES-13 Visible (0.65 µm) data (above) showed a “hot spot” (darker black pixels) associated with the post-explosion fire during the nighttime hours — then several hours later with the onset of daylight, the smoke plume could be seen drifting southeastward in visible imagery. The transport of sediment flowing out of the Mississippi River Delta region could also be seen on the GOES-13 visible imagery. Note how the initial hot spot (darker black pixels) transitioned to colder values (lighter gray pixels) as pyro-cumulus clouds formed at the top of the rapidly-rising smoke plume.

A 250-meter resolution MODIS True Color Red-Green-Blue (RGB) image from the SSEC MODIS Today site (below) showed the smoke plume around 16:07 UTC.

===== 25 April Update =====

A 3-image sequence of MODIS True Color RGB images (below) showed the smoke plume drifting southeastward from the burning oil rig site on 21 April, followed by a small oil slick on 22 April (thin bright feature meandering eastward from the oil rig site), and finally a much larger oil slick on 25 April (which had grown in size and spread to the north and northeast).

However, it is important to point out that the oil slick feature was easy to detect if it was located within the sun glint portion of the MODIS image swath (where the reflection of solar energy off the thin oil surface makes it appear as a bright feature) — on 25 April, this was the case with the 18:56 UTC overpass of the Aqua satellite. However, about 95 minutes earlier, the oil slick feature was not very apparent on the 17:21 UTC overpass of the Terra satellite, since the sun glint region was located in a different portion of the image swath, such that there was no reflection of solar radiation off the oil slick region reaching the satellite at that time (below).

===== 26 April Update =====

AWIPS images of the MODIS and AVHRR Sea Surface Temperature (SST) products (below) indicated the the SST values within the oil slick feature were often as much as 5º to 10º F cooler (darker green color enhancement) than the surrounding waters in the northern Gulf of Mexico. The very warm SST values (upper 70s to low 80s F, darker red colors) associated with the Gulf of Mexico Loop Current could be seen in the lower right corner of some of the images.

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The eruption of the Eyjafjallajokull volcano on southern Iceland continued on 19 April 2010 (in addition, see the previous CIMSS Satellite Blog entries published on 15 April and 21 March). A McIDAS Red/Green/Blue (RGB) image composite using Aqua MODIS channels 01/04/03 (above) revealed yet another large ash plume streaming southward over the North Atlantic Ocean. According to the London Volcanic Ash Advisory Center (VAAC), the ash from this latest eruption was generally confined to 10,000-15,000 feet and lower. With the volcanic ash plume drifting to the south, air traffic in the immediate vicinity of Reykjavik-Keflavik International Airport (station identifier BIKF) was not affected — and after a 5-day shut-down of air traffic across much of Europe, some airports there were finally beginning to allow limited flights to resume.

The corresponding volcanic ash retrieval products (below, courtesy of Mike Pavolonis, NOAA/NESDIS/STAR/CoRP/ASPB) indicated that the total ash loading was 75.82 kilotons, the maximum ash height was 7.37 km, and the mean ash particle effective radius was 3.51 micrometers. Note that these volcanic ash retrieval products are available in near-realtime at this NOAA/NESDIS/STAR/CIMSS site.

In addition to MODIS instruments on Terra and Aqua, the AVHRR instrument on the NOAA polar orbiters can give information on the state of the eruption. NOAA-19 passed over Iceland at 04:08 UTC 19 April, and NOAA-16 passed over at 09:16 UTC 19 April. What do the two views suggest?

Only the NOAA-16 pass occurred during daylight, and that image, below, centered on the Volcano, shows a plume extending southward from Iceland in the wake of a low pressure system (the cyclonic swirl of clouds in the eastern half of the image) departing to the east.

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Infrared imagery from 0408 UTC and 0916 UTC today suggest how the ash cloud may be changing with time. The 3.74 micron imagery and the 10.8-micron imagery, below, both show an increase in the area covered by the plume. This suggests an ongoing eruption. The 10.8-micron imagery in particular shows a lengthening of the volcanic plume southward from Iceland. The 3.74 micron imagery is affected by radiation reflected from the Sun. The 0408 UTC image occurred before sunrise. Only radiation emitted by the Earth, or clouds, or ash, is detected by the satellite. Note the warm (dark) spot that colocates with the volcano: brightness temperatures there are 20 K warmer than surrounding pixels. The 0916 UTC occurred during daylight, and as such, solar radiation at 3.74 microns can be reflected off the Earth and detected by the satellite. The sum total of radiation (emitted plus reflected) will always be greater than only the emitted radiation, thus the scene will appear warmer (and in the greyscale enhancement, darker): the “extra” radiation detected by the satellite is interpreted to mean a warmer emitting surface. Note the striking appearance of the plume. It is very dark (warm) because the particles in the plume are highly reflective. At 0916 UTC, the volcano still retains its dark spot presence in the 3.74 micron imagery. The brightness temperature remains about 20 K warmer than at surrounding pixels.

===== 20 APRIL UPDATE =====

Eyjafjallajökull continued to erupt on 20 April 2010. An Aqua MODIS Red/Green/Blue (RGB) image (above) showed a thin but well-defined cloud plume (likely a plume of volcanic steam) arcing southeastward, with a hint of a broader volcanic ash plume spreading out southward from the volcano. The corresponding MODIS 3.7 µm shortwave IR image displayed a pronounced “hot spot” (yellow to red color enhancement) associated with the source of the eruption.

An animation of Meteosat-9 volcanic ash retrieval products (below) indicated that the cloud heights decreased rapidly with time (likely a result of relatively large particles), and the ash cloud quickly became undetectable with increasing distance from the source volcano, due to its low optical depth and obstruction by meteorological clouds.

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Eyjafjallajokull, a now-active volcano in southern Iceland that erupted in late March, has recently erupted again, ejecting significant volcanic ash into the atmosphere. Iceland is at high enough latitudes (between 63 and 66.5 degrees north Latitude) that views from geostationary satellites are not as helpful in diagnosing evolving events such as ash clouds as they would be for lower-latitude events. Meteorologists instead rely on polar orbiters to observe the atmosphere surrounding the eruption.

For example, A Terra overpass yesterday allowed MODIS to image the eruption, shown as a true color composite below.

Ash from volcanoes is a significant aviation hazard if it is drawn into jet turbines. For that reason, all flights at London’s Heathrow (and at other airports throughout northern Europe) have been grounded as of mid-afternoon London time on 15 April. The volcanic ash cloud is visible from satellite. The imagery above shows 10.8- and 12.0-micron imagery from a NOAA-18 pass at 0342 UTC on 15 April. The volcanic plume is visible as colder cloud tops arcing eastward from Iceland towards northern Scotland. The color enhancement in the loop shows that the 12.0-micron image has colder brightness temperatures than the 10.8-micron image. For example, the coldest point (red pixels) just off the coast of Iceland have 12.0-micron brightness temperatures of 212.6 K; 10.8-micron temperatures in that region are closer to 214.5 K. This difference in temperature arises because volcanic ash has a lower emissivity at 12.0 microns than at 10.8 microns. Thus, proportionally less radiation compared to a blackbody is being emitted at 12.0 microns than at 10.8 microns. When that emitted radiation is detected by the satellite, the proportionally smaller values at 12.0 microns yield cooler blackbody temperatures.

Indeed, a difference between the two channels can yield a rough approximation of the ash cloud outline, and that is shown above. Colored pixels here have 10.8-micron brightness temperatures at least 2 K warmer than the 12.0-micron brightness temperature. Maximum temperature differences exceed 10 K.

15-16 April Update: The SEVIRI instrument on Meteosat-9, with more spectral resolution than AVHRR, can yield more information about the ash cloud, including total mass, maximum height, and effective radius. These derived products (courtesy of Mike Pavolonis, NOAA/NESDIS/STAR/CoRP/ASPB) are shown for 15 April (above; also available as a QuickTime movie), and for 16 April (below; also available as a QuickTime movie).

18 April Update: below are individual quantitative volcanic ash product images that show characteristics of the volcanic ash features at various times and locations during the 16-18 April period.

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A McIDAS image of a 500-meter resolution Aqua MODIS Red/Green/Blue (RGB) composite using channels 01/04/03 (below) shows a beautiful view of the volcanic ash plume streaming southward on 17 April 2010. Annotated on the image are the tiny village of Skógar, as well as the Mýrdalsjökull Glacier. As an aside, it is interesting to note that a great deal of lightning has been observed associated with the volcanic ash cloud.

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