250-m resolution MODIS true color image

Moderate to extreme drought conditions across much of North Dakota during the Spring and Summer of 2008 had caused the water levels at Long Lake (a small  alkaline lake located east-southeast  of Bismarck) to become very low — as a result, bright white “salt flats” began to grow in size, becoming large enough to be seen on 1-km resolution GOES-12 visible imagery as well as on 250-meter resolution MODIS true color imagery from the SSEC MODIS Today site (above) on 18 August 2008. This feature could also be seen on MODIS true color imagery from the previous day (below, viewed using Google Earth).

MODIS true color image (viewed using Google Earth)

AWIPS images of the MODIS visible channel, Land Surface Temperature (LST) product, and Normalized Difference Vegetation Index (NDVI) product from 17 August (below) revealed that LST values were significantly cooler over the high albedo areas of the white “salt flats” (in the upper 70s to low 80s F, compared to 100-110 F over the surrounding areas), and NDVI values were very low over the dry lake bed (less than 0.1, compared to 0.4 to 0.6 in the surrounding areas).

AWIPS images of MODIS visible, LST, and NDVI (Animated GIF)

** 20 AUGUST UPDATE **

Strong southerly winds on 20 August (gusting to 40 mph at Mobridge SD, 44 mph at Jamestown ND, and 36 mph at Bismarck ND) were creating a plume of “blowing dust” off the dry lake bed of Long Lake. A comparison of GOES-12 and GOES-13 Rapid Scan Operations (RSO) visible imagery at 5-10 minute intervals (below) shows the extent of the plume. This image comparison also serves to highlight the improved navigation on the newer GOES-13 satellite — note the excellent image-to-image navigation on the GOES-13 images on the right, compared to the notable wobble on the GOES-12 images on the left.

GOES-12 + GOES-13 RSO visible images (Animated GIF)

MODIS true color imagery (below, viewed using Google Earth) showed that the plume extended northward  to the Turtle Mountain plateau (the darker green feature) along the North Dakota/Canada border! A forecaster from Rapid City SD spoke to his nephew via telephone, who was driving an agricultural combine harvester south of Martin and west of Harvey in central North Dakota — this location appeared to be right in the middle of the plume — and the nephew reported 2-3 miles visibility, but no apparent smell or odor. 

MODIS true color image (viewed using Google Earth)

Hat-tip to Lee Czepyha, Meagan Holm, and Matthew Bunkers of the National Weather Service forecast office at Rapid City, SD for first noting this interesting feature and bringing it to our attention (and getting ground truth reports)!

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MTSAT-1R + GOES-11 visible images (Animated GIF)

The Kasatochi Volcano (located in the Aleutian Islands of Alaska) experienced a series of eruptions on 07-08 August 2008. A comparison of visible channel images from the MTSAT-1R and GOES-11 satellites (above) shows the initial sequence of volcanic plumes from 2 very different satellite viewing angles — note that the third eruption plume (beginning around 05 UTC on 08 August) appeared much darker that the previous 2 plumes, suggesting a higher volcanic ash content.

GOES-11 “split window” IR difference images (below) displayed a volcanic ash signal (yellow to cyan color enhancement) — again, the volcanic ash signal appeared to be more well-defined after the third eruption (beginning around 05 UTC on 08 August).

GOES-11 split window IR difference images (Animated GIF)

About 4 days after the initial eruption, AWIPS images of the GOES-11 + GOES-12 Sounder 7.4 µm channel (below) revealed a signature of a portion of the volcanic plume (lighter blue color enhancement) that was drifting eastward across the northwestern and north-central US on 11-12 August. The GOES Sounder 7.4 µm channel was designed to be used primarily as a lower-tropospheric “water vapor” channel, but it happens to  also be sensitive to sulfur dioxide (SO2).  However, this volcanic plume was also evident on GOES-11 visible channel images (QuickTime animation), which suggests that the “SO2 plume” is an aerosol feature (possibly a plume consisting of supercooled water coated sulfate particles).

AWIPS images of GOES-11 + GOES-12 Sounder 7.4 µm channel (Animated GIF)

NOAA Air Resources Laboratory HYSPLIT model trajectories (below) suggested that the features seen arriving over eastern Wyoming around 00 UTC on 12 August could very well have been transported from the region of the Kasatochi eruption over the Aleutians. There were also a number of pilot reports of volcanic ash over the region during that time period (including an interesting report of “SULFUR SNOW” over northeastern Montana).

HYSPLIT model trajectories

The GOES-13 satellite had just recently been taken out of on-orbit storage for evaluation and testing in early August 2008. A sequence of GOES-13 Sounder IR difference  [7.4 µm (Band 10) minus 13.4 µm  (Band 5] images (below; courtesy of Tony Schreiner, CIMSS) showed a signal of the “volcanic SO2 plume” (darker black enhancement) as it moved eastward from Montana and Wyoming on 11 August to Minnesota and Iowa on 13 August. As the cloud shield cleared over southern Wisconsin on 13 August, Arctic High Spectral Resolution Lidar located at the University of Wisconsin – Madison indicated a layer of aerosol backscatter centered around 12 km, which could very well have been part of the Kasatochi volcanic plume.

GOES-13 Sounder IR difference product (Animated GIF)

In addition, another portion of the “volcanic SO2 plume” could be seen moving southward across Ontario on 12 August, then moving southeastward across New England on 13 August — these particular volcanic plume features were forecast with remarkable accuracy by an Environment Canada Lagrangian transport model. The large hazy feature seen in the northeastern part of the MODIS true color image from the SSEC MODIS Today site (below) was the leading edge of the Ontario “volcanic SO2 / aerosol plume” as it began to move southward over the Great Lakes region on 12 August. According to the NASA Earth Observatory News, this was one of the largest volcanic sulfur dioxide clouds scientists have observed since Chile’s Hudson volcano erupted in August 1991. In addition, this was the second Alaskan volcanic plume in as many months to be observed over the Lower 48 states — the Okmok volcanic plume was seen in mid-July 2008.

MODIS true color image

** 15 AUGUST UPDATE ** Additional lidar data obtained from the University of Wisconsin – Madison on 14-15 August (below) continued to show thin layers of aerosol backscatter with small depolarization ratios (cyan colors) in the upper troposphere that were possibly due to Kasatochi volcanic plumes.

Arctic High Spectral Resolution Lidar data

It is interesting to note the thin “tail” of aerosol backscatter (cyan colors) that extended downward from the main aerosol layer (located between altitudes of 11-12 km) to as low as the 8-9 km altitude range during the 04-08 UTC time period on 15 August. AWIPS images of the GOES Sounder + GOES Imager water vapor channels (below) indicated that strong subsidence was occurring over Wisconsin during that time — warmer water vapor brightness temperatures values (indicative of drier air) were depicted by the blue to yellow to orange colors (depending on which particular water vapor channel was being viewed).

So the Question of the Day is: could the lidar data be showing evidence that some of the volcanic aerosol plume aloft was being transported downward several km by the strong subsidence that was occurring within the middle to upper troposphere over Wisconsin on 15 August? The GOES Sounder total column ozone product showed a lobe of elevated ozone values, concurrent with a lowering of the dynamic tropopause (taken to be the pressure of the PV1.5 surface) to around the 300 hPa pressure level (around 9 km) over the Madison WI area, in agreement with the lidar filament seen extending down to the 8-9 km level — so perhaps a stratospheric intrusion may have helped to transport a portion of the volcanic aerosol plume downward. HYSPLIT back trajectories (EDAS | GDAS) indicated that the transport arriving over Madison WI on 15 August at the 8, 10, and 12 km altitudes all passed over Ontario and Hudson Bay during the preceeding 24 hours, where the thick aerosol feature was seen on GOES and MODIS imagery 2 days earlier.

AWIPS images of GOES Sounder+Imager water vapor channel data

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Mesoscale Convective Vortices (MCVs) will occasionally emerge from under the eroding cirrus canopy of a Mesoscale Convective System (MCS). Typically, an MCS will dissipate shortly after sunrise, but in atmospheres that include plentiful moisture and little vertical wind shear, the MCV that very frequently develops in an MCS can persist and serve to force subsequent convective development. (Previous MCVs have been documented in the CIMSS Satellite Blog here, here and here).

Strong convection that developed over western Texas early in the morning on Monday 11 August grew into a MCS that quickly eroded near sunrise. However, a swirl of mid-level clouds in the IR image loop above, and in the visible loop here, clearly show the persistence of an MCV. Note in the visible loop how subsequent convection develops very near the propagating MCV, first just to the northwest of the vortex (with the convection spreading south) and then to the north and east.

An MCV is driven mostly by the release of latent heat in convective clouds. Such heating will alter the stability and thus force the development of a vortex. The persistence of the vortex is a balance between the effects of ongoing convection releasing latent heat (maintaining the vortex) and the effects of strong vertical wind shear that serves to weaken the vortex. On this day, a rich moisture source is evident, as noted in the plot here of surface (yellow, in degrees Fahrenheit) and 850-mb dewpoints (white, in degrees Celsius). In addition, GFS model output of 850-500mb shear at 1200 UTC and 1800 UTC show minimal values in Texas where the MCV persisted.

Because forcing associated with the MCV can aid subsequent convective development, noting their presence — an easy task in satellite animation — is vital.

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03 August 2008 marked the 22nd  consecutive day of daily high temperatures of 90º F or higher at Denver, Colorado (the old record was 18 consecutive days, set back in 1874 and 1901). AWIPS images of the MODIS Land Surface Temperature (LST) product (above) revealed daytime LST values as high as 140º F in southeastern Colorado and 134º F in southwestern Nebraska — while the surface “skin temperatures” were quite warm, the actual air temperatures (measured within shaded instrument shelters located about 5  feet above the surface) were only the in upper 90s to low 100s F.

It is interesting to note that the LST values were significantly lower across much of southcentral and southeastern Nebraska (in the upper 80s to low 90s F, green to yellow colors), even though the air temperatures were similar to those seen in eastern Colorado at that time (in the upper 90s to low 100s F).  These lower LST values were due to a much higher density of vegetation in eastern Nebraska, as shown by a comparison with the MODIS Normalized Difference Vegetation Index (NDVI) product (below) — NDVI values were greater than 0.7 in Nebraska, compared to 0.1 to 0.3 across much of eastern Colorado. In addition, very dry conditions had prevailed across eastern Colorado — Denver had only received 3.28 inches of precipitation so far in the year (the normal is 10.25 inches for the 01 January  – 31 July period) — in contrast with Hastings in eastern Nebraska, which had received 6.77 inches of precipitation so far in the year (the normal precipitation is 3.81 inches for the 01 January – 31 July period). The combination of drier soils and sparse vegetation  helped contribute to such high MODIS LST values.

MODIS true color imagery from the SSEC MODIS Today site (below) confirmed the presence of a much higher density of vegetation (denoted by darker green colors) across eastern Nebraska, compared to that found across eastern Colorado.

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