CIMSS Satellite Blog

On the morning of 17 July 2008, we received the following email from Ron Miller at the National Weather Service forecast office in Spokane, Washington:

Can you tell me what you think the “clouds” are over the PacNW this morning. You can only see them on the first few visible images of the morning. Looking out out window we can’t detect them. Given the flow pattern over the past 24-48 hours, I have a hard time believing that it’s wildfire smoke from CA. One possibility is Volcanic Ash from Okmok, since after it’s eruption, the ash cloud drifted southeast and essentially hung out in the Gulf of AK trough. Any ideas?

Excellent question, Ron — thanks for bringing this case to our attention! As it turns out, the Okmok volcano erupted in the Aleutian Islands on 12  July (see: Google Earth AVHRR false color image | VISIT Meteorological Interpretation Blog | GOES-11 visible Java animation), and GOES-11 (GOES-West) visible imagery (above)  did indeed reveal a portion of the volcanic plume  (actually, three separate thin plumes) drifting eastward over the Pacific Northwest region early in the day on 17 July. The thin volcanic plumes seen on the GOES visible imagery  — which were likely composed primarily of ice crystals and sulfur dioxide (SO2) — were  high-altitude features (verified to exist at an altitude around 11-12 km by CALIPSO), so they showed up on the visible imagery before  the lower-altitude smoke (from to wildfires that had been burning in northern California) which  became illuminated by the rising sun a bit later in time. Forward scattering was more favorable at the times of the earlier visible images, enhancing the appearance of the volcanic plume features  — then, as the sun angle increased into the mid-morning hours, the thin volcanic plumes became less apparent on the visible imagery (but the thick low-level smoke drifting northeastward across Oregon continued to remain  obvious).

The corresponding GOES-11 10.7-12.0 µm “split window IR difference” product did not show a clear signal of volcanic ash content in the plume. However, an Aqua MODIS IR difference image (below) shows the signature of an SO2-rich plume (darker blue colors) stretching east-northeastward across the Pacific Ocean and  reaching western Washington and Oregon around 11:00 UTC (5am local time). The daily evolution of the SO2 plume can be seen in an animation of Ozone Measuring Instrument (OMI) images. Compare the 3-plume structure seen on the MODIS IR difference image with the similar plume structure seen on an AIRS brightness temperature difference image.

NOAA Air Resources Laboratory HYSPLIT model backward trajectories (below) indicated  that high-altitude air parcels arriving over western Washington and far northwestern Oregon at 11:00 UTC on 17 July had likely been transported southward, then east-northeastward across the Pacific Ocean during the previous 72-hour period.

A comparison of AWIPS images of the MODIS visible, IR window, water vapor, and cirrus detection channels from 18:47 UTC (11:47 am local time) on 17 July (below) show that there was no obvious volcanic plume signature on either the visible or IR images at that time. However, a brighter “plume signal” did show up in the cirrus detection channel image (which seems to correspond roughly to the northernmost of two moist plumes on the water vapor image).

The MODIS cirrus detection channel is a daytime-only “near-IR” channel (centered at 1.6 µm) which is sensitive to particles that are efficient scatterers of light (such as ice crystals, volcanic ash particles, airborne dust or sand, etc). Two consecutive  MODIS cirrus detection images from the Terra (18:47 UTC) and Aqua (20:26 UTC) satellites (below) both show evidence of a subtle volcanic plume  signal (brighter white streaks), which appeared to be moving farther inland over the Pacific Northwest.

By the end of the day on 17 July, a favorable forward scattering angle (with the sun getting lower in the western sky) allowed the volcanic plume to again be seen on GOES visible channel imagery — but this time using the GOES-12 (GOES-East) satellite (QuickTime animation).  The plume had moved eastward along the US/Canada border during the day, drifting across parts of northern North Dakota and even far northwestern Minnesota by sunset!

** 18 July Update: The volcanic plume was again evident on early morning  GOES-11  visible imagery (Animated GIF), stretching from the Pacific Northwest states all the way to southern  Manitoba and southern Ontario in Canada. During the afternoon and early evening hours, there were numerous pilot reports from aircraft encountering the plumes over northern Oregon, southwestern Washington, and other parts of the Pacific Northwest region, prompting the issuance of a Volcanic Ash SIGMET advisory.

A 17:49 UTC AWIPS image of the MODIS “cirrus detection” channel suggested that the Volcanic Ash SIGMET advisory could perhaps have been extended a bit farther northeastward across Montana — note the slightly brighter “streaky” volcanic plume signature  heading northeastward across Idaho into Montana, beyond the eastern boundary of the SIGMET (outlined in yellow).

A GOES-12 IR image from the CIMSS Tropical Cyclones site (above) showed Tropical Storm Bertha continuing to linger over the western Atlantic Ocean on 16 July 2008 — this storm first became “Tropical Depression #2″ over the eastern Atlantic, thirteen days earlier (on 03 July).

GOES-12 visible images with an overlay of SeaWinds data from the QuikSCAT satellite (above) revealed that there were still some bursts of convection near the center of the tropical cyclone, and maximum wind speeds remained near 60 knots. Tropical Storm Bertha was in the process of weakening as it began to encounter increasing environmental wind shear. AWIPS images of the GOES-12 water vapor channel (below) showed that the northeastward motion of Bertha was slowing, with a gradual turn toward the southeast as the tropical cyclone began to interact with a mid-latitude low located off to the  east (this middle to upper level low was drifting southwestward across the North Atlantic Ocean).

It is interesting to note the large pool of cooler Sea Surface Temperatures (cyan to blue colors) in the wake of slow-moving Bertha (below; sourced from Remote Sensing Systems).

A mesoscale convective system (MCS) moved southward across southern Wisconsin during the morning of 11 July 2008, producing a well-defined bowing segment along the southern flank. As the MCS continued to decay into the late morning hours, the signature of a weak Mesoscale Convective Vortex or MCV (see Interesting Clouds on Satellite: “Mesoscale Convective Vortex” from the NWS Milwaukee) became apparent on AWIPS images of the GOES-12 visible and IR channels (above, upper 2 image panels; see also a GOES-12 visible animation from Dave Santek, SSEC). While the presentation of the MCV was fairly impressive on the GOES-12 satellite imagery, this feature was much less obvious on the Milwaukee/Sullivan radar base reflectivity and base velocity data (above, lower 2 image panels).

Northerly winds gusted to 37 mph at the Madison airport KMSN (41 mph on top of the SSEC building in downtown Madison) as the bowing segment convection moved through; however, it is interesting to note that Juneau KUNU later reported a southeasterly wind gust of 43 mph at 17 UTC (Noon local time) as the MCV passed over that area; apparently a boundary layer “wake low” was responsible for producing this brief period of gusty southeasterly winds (which also included gusts to 20 mph at West Bend KETB and 17 mph at Watertown KRYV).

A comparison of MODIS true color and false color images from the SSEC MODIS Today site (below) showed finer detail in the cloud structures of both the leading edge of the convective outflow (the aforementioned bowing segment) and the MCV. The MCV signature was comprised primarily of lower to middle level clouds, and became more obvious as the canopy of high-level cirrus clouds (cyan colors on the MODIS false color image) began to dissipate as the MCS decayed.