A comparison of AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel and 0.7 µm Day/Night Band (DNB) data (above) demonstrated the superior ability of the broadband spectral response of the DNB to detect the plumes of airborne aerosols which were being lofted by strong offshore winds in the southern Alaska Panhandle region on 01 March 2014.

These strong offshore winds were the result of the strong pressure gradient between a ridge of high pressure inland over Canada and a trough of low pressure located off the coast (below). The air was quite cold and dry across inland Canada (plot of minimum temperatures), and this air experienced further drying as it was forced through the various mountain passes and then descended toward the coast.

McIDAS images of GOES-15 6.5 µm water vapor channel data (below; click image to play animation) showed a trend of strong middle-tropospheric drying during the day, as seen by the growth in areal coverage of the warmer (yellow-enhanced) region moving southwestward over the Alaska Panhandle region.

It is interesting to note that the surface observations at Klawock, Alaska (below) included “Snow” as the wind speeds increased and the aerosol plume became evident on satellite imagery. However, given the relatively warm surface air temperatures and the very low dew points (along with the fact that the sky conditions were reported as “Clear” during the entire day), it is likely that automated sensors mistook the airborne aerosol particles as snow.

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McIDAS images of 4-km resolution GOES-15 6.5 µm water vapor channel data (above; click image to play animation; also available as an MP4 animation) showed the development of a strong and rapidly-occluding storm off the coast of California during the 27 February – 28 February 2014 period.

An AWIPS image of 17:30 UTC GOES-15 water vapor channel data with overlays of 17:28 UTC Metop ASCAT surface scatterometer winds and the 18:00 UTC tropical surface analysis (below) showed satellite-sensed surface winds as strong as 51 knots in the southwestern quadrant of the storm.

Greater detail in the storm structure could be seen in 1-km resolution MODIS 6.7 µm water vapor channel images at 10:32 UTC and 21:40 UTC (below).

One indication of the strength of the storm was the high amounts of GOES-15 sounder Total Column Ozone associated with the circulation (below; click image to play animation), which reached levels as high as 440-450 Dobson Units (lighter red color enhancement). Such high levels of total column ozone are also often associated with potential vorticity anomalies and a dramatically lowered tropopause — in this case, the GFS40 model indicated the the dynamic tropopause (taken to be the pressure of the PV1.5 surface) was as below the 480 hPa pressure level at 18:00 UTC. It is interesting to note that there was a pilot report of moderate turbulence at 36,000 feet, along the sharp western gradient of the total column ozone (and the sharp gradient of the PV1.5 pressure) — the pilot noted that the turbulene lasted for 10 minutes.

A comparison of 375-meter resolution Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images at 22:08 UTC (below) revealed a few convective elements offshore which exhibited cloud-top IR brightness temperatures as cold as -50º C (yellow color enhancement), with a few cloud-to-ground lightning strikes being detected.

===== 01 March Update =====

A comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 10:22 UTC or 2:22 AM local time on 01 March (above) revealed the presence of numerous arc-shaped mesospheric airglow waves in the western semicircle of the storm circulation on the DNB image — note that there was no corresponding wave signature on the IR image.

These vertically-propagating mesospheric airglow waves were likely generated by the 140-knot jet streak that was moving southward around the rear side of the storm  (below).

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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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