A late-season winter storm brought heavy snowfall to much of the central US — Rapid City, South Dakota (station identifier RAP) set records that included 20.0 inches on 09 April 2013 (most snowfall on a calendar day) and 28.2 inches on 08 April – 10 April (greatest muti-day snowfall). McIDAS images of 4-km resolution GOES-13 6.5 µm water vapor channel data covering the 08 April – 10 April period (above; click image to play animation; also available as a QuickTime movie) showed the development of several convective elements that helped to enhance snowfall rates as they moved northward across the region on 09 April, as well deformation bands that formed as the circulation of the upper-level low slowly migrated over western South Dakota and western Nebraska on 10 April.

An AWIPS image of 1-km resolution MODIS 0.65 µm visible channel data (below) showed some of the convective elements responsible for producing a period of heavy snow at Rapid City on 09 April. Large thunderstorms were also seen at the time over notheastern Nebraska and southeastern South Dakota.

A surface meteorogram (below) shows the conditions at Rapid City Regional Airport during the 08-10 April period.

===== 12 April Update =====

Widespread cloudiness masked a good view of the areal extent of the resulting snow cover across South Dakota, but farther to the south over Nebraska and far northern Kansas an AWIPS comparison of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel and false-color Red/Green/Blue (RGB) images (below) showed interesting detail in a number of mesoscale bands of snow cover. Snow appears as white on the visible image, and as darker shades of red on the RGB image; supercooled water droplet clouds are lighter shades of white, while ice crystal clouds appear as shades of pink on the false-color image.

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A comparison of AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel and false-color Red/Green/Blue (RGB) images (above) showed the extent of ice in the Bering Sea (snow and ice appear as shades of red in the RGB image), along with curved bands of cloud streets due to cold air advection as arctic high pressure moved toward the area from Siberia on 08 April 2013.

Consecutive VIIRS 0.64 µm visible channel images from 22:39 on 07 April and 00:21 UTC on 08 April (below) showed the amount of sea ice motion in the short time span (less than 2 hours) between the 2 images.

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Continuing on the theme of the previous blog post, AWIPS images of Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) data (above) showed another large mesoscale convective system over the eastern Gulf of Mexico at 06:45 UTC or 4:45 AM local time (06 UTC surface analysis) on 05 April 2013. The Day/Night Band revealed numerous bright streaks denoting cloud tops illuminated by lightning activity — and the coldest cloud-top infrared brightness temperature was -89C (darker violet color enhancement).

Overlays of conventional lightning data (below) showed that there were 1493 negative and 180 positive cloud-to-ground strikes detected within a 15-minute period across that region.

One subtle feature seen on the VIIRS Day/Night Band image was the presence of concentric waves propagating northward through eastward away from the core of the convective complex (below). These features were “mesospheric airglow waves” — essentially “night glow” emitted from a variety of high-altitude (80-105 km) gases (primarily the sodium layer) near the mesopause — that were forced by the intense convective complex in the troposphere. Note that these mesospheric airglow waves do not show up on the corresponding VIIRS Infrared Window image.

The signature of mesospheric airglow waves was also seen on the previous night (below), again forced by intense convection in the Gulf of Mexico.

Thanks to Steve Miller (CIRA) for offering an explanation of these wave features, and providing the reference “Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities”

===== 08 April Update =====

A few days later, yet another example of mesospheric airglow waves was seen in Day/Night Band imagery — but in this case, the waves were forced not by deep tropospheric convection, but by the interaction of strong winds with the rugged topography of the Southwest US. The flow throughout the depth of the troposphere was quite strong as a 120-knot jet upper-tropospheric jet streak was moving southward along the west side of a trough that was deepening over the western US on 08 April 2013. A series of mesospheric airglow waves could be seen traveling southeastward across the Gulf of California and adjacent portions of Baja California and Mexico at 09:10 UTC or 2:10 AM local time (below). Again, note that there was no wave signature apparent on the corresponding VIIRS Infrared Window image.

During the subsequent daytime hours, GOES-15 (GOES-West) and GOES-13 (GOES-East) Water Vapor (6.5 µm) images (below) revealed a complex pattern of mountain waves across the region — surface wind gusts were as high as 88 mph in California, 70 mph in Nevada, and 63 mph in Arizona.

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One of the first strong Mesoscale Convective Systems of the Spring Season traversed the Gulf of Mexico during the day on April 3rd. Suomi/NPP VIIRS imagery of the 0.7 µm Day/Night Band and the 11.45 µm IR channel, from 0722 UTC, above, showed elements that are characteristic of these convective features. For example, the thick convective clouds were able to obscure most of the city lights of southern Louisiana on the Day/Night Band image, and the 11.45 µm IR imagery showed very cold cloud tops — colder than -80 C — over the Gulf of Mexico, along with evidence of cloud-top gravity waves over southern Louisiana (and the adjacent coastal waters) as well as over east Texas.

The active convection was generating considerable lightning activity: there were 1275 negative and 186 positive cloud-to-ground (CG) strikes detected over the region within a 15-minute period. Cloud tops illuminated by lightning were depicted as bright smears of light in the Day/Night Band (DNB) image, indicative of the very fast VIIRS sensor scanning motion. Of particular interest was a pair long black streaks immediately downstream of two large areas of lightning-illuminated cloud tops: one over Louisiana, and another farther south over the Gulf of Mexico (magnified image). These black lines represented a post-saturation “recovery period” after the DNB sensor detected very bright areas associated with intense lightning activity. It is also important to note that there is not always a direct correspondence between DNB image cloud-top lightning signatures and clusters of CG lightning activity — only one positive GC strike was seen close to the bright Louisiana DNB image lightning streak, while numerous negative and positive CG strikes were in the vicinity of the Gulf of Mexico DNB lightning streak.

A toggle between the high-resolution (1 km) Suomi/NPP VIIRS 11.45 µm IR imagery and the nominal 4-km imagery of the 10.7 µm IR from the GOES-13 Imager, above, demonstrates the importance of higher spatial resolution in detecting features that are important to aviation. Only the Suomi/NPP VIIRS image cleanly depicts the transverse bands that herald the potential presence of turbulence in the cirrus canopy.

Despite limitations related to resolution, GOES data can be used to automatically detect overshooting tops. The image above shows the GOES 10.7 µm image from AWIPS. Auto-detected overshooting tops are also shown, and they are spread out along the southern flank of this convective system, a region where convective development was ongoing. (Click here to see a toggle between the 10.7 µm image with and without the auto-detected overshooting tops). That persistent convective growth was also shown by the UW Cloud-top Cooling product, shown below, a product that highlights the most rapidly cooling growing convective towers.

The convective system has persisted through the early afternoon on April 3rd, as shown in the loop of different MODIS channels, above (including the visible, water vapor, cirrus channel, and 10.7 µm IR). This system is unusually far south into the Gulf of Mexico for early April.

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