Before (07 September) and after (17 September) MODIS false-color Red/Green/Blue (RGB) images

A comparison of “before” (07 September 2013) and “after” (17 September 2013) 250-meter resolution MODIS false-color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above) showed a dramatic change in the appearance of the South Platte River that flows northeastward across northeastern Colorado. Following the record-setting rainfall across much of the Front Range of Colorado (previous blog post), much of this water (darker blue in appearance on the RGB images) was being carried across northeastern Colorado by the South Platte River and its tributaries.

Hydrographs for 3 different locations along the South Platte River in Colorado (Kersey, Balzac, and Julesburg, below) showed the actual and forecast maximum river crests — note that the peak river crest of 18.79 feet at Kersey eclipsed the previous record of 11.7 feet set in 1973.

A comparison of AWIPS images of consecutive overpasses (1 hour and 44 minutes apart) of Suomi NPP VIIRS 1.61 µm near-IR data on 14 September (below) appeared to indicate that the dark signal of the leading edge of the South Platte River flooding could be seen moving rapidly southeastward along that particular portion of the river.

The Suomi NPP VIIRS 1.61 µm image at 19:58 UTC on 17 September (below) showed that the dark signature of flooding along the South Platte River had nearly reached the Colorado/Nebraska border.

Note that the South Platte River flooding is not evident on the corresponding 0.64 µm VIIRS visible channel image (below) — this is due to the fact that there is not much contrast between the sediment-rich water and the adjacent dry soils (much of northeastern Colorado remains in a drought).

Suomi NPP VIIRS 0.64 µm visible channel and 1.61 µm near-IR channel images

===== 18 September Update =====

Suomi NPP VIIRS 1.61 µm near-IR images (17 vs 18 September)

A comparison of AWIPS images of Suomi NPP VIIRS 1.61 µm near-IR images (above) showed the progression of the leading edge of the flooding along the South Platte River in a 24-hour period (from 19:58 UTC on 17 September to 19:42 UTC on 18 September). The dark signal of river flooding had extended northeastward across the Nebraska border.

The hydrograph for Julesburg in far northeastern Colorado (below) showed that the river had already crested above the previous record stage of 10.4 feet.

Julesburg, Colorado hydrograph

View only this post Read Less

GOES-13 10.7 µm IR channel image (with track of Humberto)

A GOES-13 10.7 µm IR channel image with the track of Humberto from the CIMSS Tropical Cyclones site  (above) showed that the storm (which became the first hurricane of the season in the Atlantic Basin on 11 September) had been meandering over the eastern North Atlantic Ocean for several days, eventually weakening to a post-tropical cyclone on 14 September before re-organizing into a tropical storm early in the day on 16 September (NHC advisories archive).

GOES-13 0.63 µm visible channel image (with ASCAT surface scatterometer winds)

A GOES-13 0.63 µm visible channel image with an overlay of ASCAT surface scatterometer winds (above) showed evidence of tropical storm force winds (yellow wind barbs) north of the storm center at 13:17 UTC. Note the presence of a well-defined cloud vortex located southwest of the 18 UTC center fix position.The NHC discussion #26 mentioned that several smaller-scale vortices had been rotating around the mean circulation center of Humberto — and a close-up view using GOES-13 0.63 µm visible channel images (below; click image to play animation) revealed one of these exposed vortices to the southwest of the center of Humberto.

GOES-13 0.63 µm visible channel images (click images to play animation)

See From the Lee Side for a discussion on the role of vertical wind shear on tropical cyclones such as this.

View only this post Read Less

GOES Sounder DPI Lifted Index from 1300 UTC 15 September

The GOES-13 Sounder has been experiencing an increased number of unexpected scan line lengths. This was originally detected on 1 July 2013 and it has become more common since 12 September 2013. The anomaly is manifest in the data by occasional pixel gaps in all channels of the Sounder images. Some images show only a single gap (and therefore just one missing pixel); other images have several gaps. In the image above, the missing values are the black pixels just off the North Carolina coast (for example), or near the Mississippi River in southwestern Mississippi. Note that missing data only is present in the GOES-13 part of the domain. The GOES-15 signal is clean. The root cause of this error is under investigation. From NESDIS: “No telemetry violations have occurred, and all Sounder filter wheel telemetry data, including Filter wheel currents and period monitors, are within expected values.”

The missing pixels are also present in the real-time Sounder data available at CIMSS at this link. An example from September 15th is shown below. Or, click here for a composite (GOES-13/GOES-15) single-band image.

GOES Sounder DPI Lifted Index from 1300 UTC 15 September

A quick analysis at two times (0246 and 1446 UTC) suggests that prior to 28 August, errors per image were limited to 20 or so pixels. Between 28 August and 9 September, fewer than 100 pixels were affected each hour. Since 10 September, the number of pixels affected in each image has increased one some days to more than 200. There are nearly 64000 pixels in each sounder image, so the number of bad points remains a small percentage of the total.

View only this post Read Less

GOES-13 10.7 µm IR channel images (click unage to play animation)

Hurricane Ingrid had been going though a period of slow intensification (ADT plot) off the Gulf Coast of Mexico during the overnight hours on 14 September – 15 September 2013. McIDAS images of GOES-13 10.7 µm IR channel data (above; click image to play animation) showed a few convective bursts within the central dense overcast portion of the storm, with cloud-top IR brightness temperatures as cold as -91º C (yellow color enhancement) at 04:45 UTC. These are unusually cold IR temperatures to be sensed by the coarse 4-km resolution of the IR detectors on the GOES Imager instrument.

An AWIPS image comparison of 375-meter resolution (projected onto a 1-km AWIPS grid) Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel data at 07:30 UTC (below) revealed even colder cloud-top IR brightness temperatures of -95º C (darker violet color enhancement). Another feature of interest was the concentric packet of gravity waves propagating northward away from the intense overshooting tops. These cloud-top gravity waves were best seen on the DNB image, which highlights the “visible image at night” capability of this VIIRS channel data.The intese overshooting tops (produced by the rapid pulsing nature of the convective bursts seen on the GOES imagery above) were likely the source of these gravity waves.

Suomi NPP VIIRS 11.45 µm IR channel and 0.7 µm Day/Night Band images

The bright streak seen on the VIIRS DNB image was due to cloud illumination by intense lightning activity at the time the instrument was scanning that particular location. An overlay of cloud-to-ground (CG) lightning strikes for the 15-minute period ending around the time of the VIIRS overpass (below) did show a small cluster of positive polarity (red) CG strikes centered near the region of highest overshooting tops — but the bright area detected by the DNB sensor was likely due to in-cloud or cloud-to-cloud lightning.

Suomi NPP VIIRS 0.7 µm Day/Night Band image (and cloud-to-ground lightning strikes)

Also evident on the VIIRS DNB image were southward-propagating mesospheric airglow waves, likely generated by the intense overshooting tops of the strong cluster of convection located to the southeast of Ingrid. Note that these mesospheric airglow waves had no signature in the corresponding 11.45 µm IR image (in contrast to the aforementioned cloud-top gravity waves north of the center of Ingrid, which could be seen on the IR image due to their effect on the topography of the convective cloud top).

View only this post Read Less