A southward to southeastward surge of arctic air across the Great Lakes in the wake of a strong cold frontal passage (18 UTC surface analysis) on 27 February 2014 produced widespread lake-effect snow bands and also contributed to a renewed growth of ice. GOES-13 Visible (0.63 µm) images (above) showed (1) a variety of lake-effect cloud bands streaming across Lake Superior, Lake Michigan, Lake Huron, and western Lake Erie, (2) the motion of lake ice, due to strong northerly, northwesterly, and westerly winds across the region, and (3) the rapid formation of new lake ice in the previously ice-free nearshore waters in the far northwestern portion of Lake Superior.

A comparison of Suomi NPP VIIRS Visible (0.64 µm) and False Color “snow/ice vs cloud discrimination” Red/Green/Blue (RGB) images at 19:02 UTC (below) demonstrated how the RGB product could be used to highlight the various cloud features — snow and ice appeared as darker shades of red, while supercooled water droplet clouds were depicted as varying shades of white (glaciated cloud features exhibited a pink to lighter red appearance).

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A number of previous blog posts have demonstrated the ability of the water vapor channel to sense surface features when the atmospheric column is cold and/or dry; in this example, the signal of a thin ribbon of high-altitude Sierra Nevada snow cover can be seen on an AWIPS image of 1-km resolution MODIS 6.7 µm water vapor channel data at 10:05 UTC on 25 February 2014 (above). At that time the middle to upper troposphere over much of southern California was relatively dry, as indicated by the shades of lighter blue to yellow on the water vapor image. The Blended Total Precipitable Water product indicated that TPW values were generally in the 8-10 mm range over the central Sierra Nevada region, which was actually about 130-150% of normal — however, higher resolution TPW values over the Sierra Nevada were as low as 0.7 mm and 1.3 mm according to the GOES-15 sounder and MODIS, respectively.

A similar high-altitude snow signature was seen on 4-km resolution GOES-15 6.5 µm water vapor channel images (below; click image to play animation).

The thin ribbon of high-altitude snow cover showed up as darker blue features on both the MODIS and GOES-15 water vapor images — not because there was more water vapor in that location, but because the temperature of the air above the snow pack was colder than the adjacent lower-elevation bare ground areas. This example helps to underscore the fact that the water vapor channel is essentially an InfraRed (IR) channel, which is sensing the temperature of a layer of moisture (or in this case, the temperature of a colder surface feature).

The altitude (and vertical thickness) of the layer being sensed by water vapor imagery depends on the temperature and moisture profile over that particular region, as well as the satellite viewing angle. The GOES Weighting Functions site allows you to select the rawinsonde profile closest to your area of interest, and a radiative transfer model is then used to calculate the weighting functions for the various GOES imager channels (as well as the 3 GOES sounder water vapor channels). In this case, the rawinsonde profile for Vandenberg Air Force Base (KVBG) in California (below) was the closest sounding site to the pocket of dry air over the Sierra Nevada mountains — and due to the deep layer of dry air aloft, the peak altitude of the GOES-15 6.5 µm water vapor channel weighting function was shifted downward to just below 500 hPa.

In a comparison of 3 regional rawinsonde sites (below), note how the altitude of the GOES-15 6.5 µm water vapor channel weighting function peak (as well as the vertical thickness of the weighting function plot) increases over Elko, Nevada (KLKN) and Tucson, Arizona (KTUS) where more middle to upper tropospheric moisture was present.

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Twitter follower @PedroCFernandez alerted us to an interesting case of mesovortices (original Spanish language blog post | Google translate version) which developed within the circulation of a strong mid-latitude cyclone over the northeastern Atlantic Ocean (moving toward the British Isles) on 24-25 February 2014. McIDAS images of EUMETSAT Meteosat-10 daytime 0.75 µm high resolution visible channel and night-time 10.8 µm IR channel data (above; click to play animation) revealed the series of mesovortices that spun up along the “bent-back” occluded frontal boundary (surface analyses); this storm was producing hurricane force winds during the 12-18 UTC period on 24 February, according to the NWS Ocean Prediction Center. Mesovortex cloud-top IR brightness temperatures were around -40º C (green color enhancement), indicating significant vertical development of the cloud structure associated with these features.

A comparison of the 18:00 UTC Meteosat-10 visible and IR images (above) showed two well-developed mesovortices at that time: #1, located southwest of the storm center, and #2, located west of the storm center;  a new mesovortex #3 was just in the process of forming to the northwest of the storm center (below).

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Hat-tip to Matt Sitkowski and Carl Parker of The Weather Channel for the heads-up on some interesting wave features that could be seen in the vicinity of Guadalupe Island on McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) on 24 February 2014. Apparently a gravity wave had propagated northwestward through the region during the morning hours, perturbing the depth of the marine boundary layer (MBL) such that undulations in the MBL stratocumulus clouds were quite evident. In addition, an unusual “dry pulse” propagated outward from Guadalupe Island (located in the center of the images). These wave features eventually became hidden as layers of middle and high clouds overspread the area from the southwest.

AWIPS images of GOES-15 0.63 µm visible channel data with overlays of Real-Time Mesoscale Analysis (RTMA) surface winds (below) showed that the surface flow was very light or even calm across much of the Guadalupe Island region during the time that the “dry pulse” was most evident on visible imagery.

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