Suomi-NPP Imagery: Visible (0.64µm), Day Night Band (0.70µm) and near-IR (0.86µm) images (click to enlarge)

Visible Imagery over the Great Lakes on Friday April 17th showed mostly open waters over the five lakes, with regions that could be ice confined to coastlines of Lakes Superior, Huron, Erie and Michigan. The animation above is of Suomi NPP VIIRS visible (0.64µm and 0.70µm) and near-infrared (0.86µm) data. Can you tell with certainty which of the white features over the lakes are clouds vs. ice?

Suomi-NPP Infrared Imagery (3.74 µm) (click to enlarge)

Infrared data can give clues. The 3.74 µm imagery, above, shows the brightness temperature. Note how the white regions over Lakes Superior, Michigan and Ontario are about the same temperature as the surrounding water. In contrast, white regions over Lakes Erie and Ontario are much darker (warmer) in the 3.74 µm than the surrounding water. This is testimony to the superior scattering abilities around 3.74 µm of water-based clouds compared to lake ice. More solar radiation scattered towards the satellite by the clouds means a warmer temperature is detected.

Suomi-NPP Imagery: Visible (0.64µm) and near-IR (1.61 µm) (click to enlarge)

The 1.61 µm near-infrared channel is useful because ice strongly absorbs solar radiation at that wavelength, appearing dark. The toggle above, of visible (0.64) and near-infrared (1.61) neatly distinguishes between clouds and ice. Ice (dark in the 1.61 µm because it does not reflect; at that wavelength, it absorbs) is apparent over eastern Lake Superior, eastern and northern Lake Huron and some small bays in northern Lake Michigan. There is no ice apparent on Lakes Erie or Ontario: features there exhibit signatures which are white in both visible and at 1.61 µm.

Another method to aid in the discrimination of snow/ice vs supercooled water droplet clouds is the creation of Red/Green/Blue (RGB) products. The example below toggles between the 0.64 µm visible image and an RGB image (which uses the VIIRS 0.64 µm/1.61 µm/1.61 µm data as the R/G/B components) — snow cover and ice appear as darker shades of red on the RGB image (in contrast to supercooled water droplet clouds, which are brighter shades of white). The snow depth on the morning of 17 April was still 13 inches at Munising in the Upper Peninsula of Michigan.

Suomi NPP VIIRS 0.64 µm visible and false-color RGB images (click to enlarge)

On this day there was only 1 pass of the Landsat-8 satellite over any of the ice-covered portions of the Great Lakes; the 15-meter resolution panchromatic visible (0.59 µm) image below shows a very detailed view of the far western portion of the ice that was north of the Keweenaw Peninsula in Lake Superior (zoomed image).

Landsat-8 panchromatic visible (0.59 µm) image (click to enlarge)

Terra and Aqua both carry the MODIS sensor, and MODIS can detect radiation at 1.38 µm, a wavelength at which cirrus is highly reflective. A 1.38 µm image from the 17th, below, shows the horizontal extent of cirrus.

MODIS Imagery: near-IR (1.38 µm) (click to enlarge)

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

GOES-13 (GOES-East) 0.63 µm visible channel images (click image to play animation; also available as an MP4 movie file) showed the hazy signature of a cloud of thick blowing dust moving southward across southern Nevada and parts of southern California, along and behind a strong cold frontal boundary on 14 April 2015.

Areas where the dust cloud was more dense could be identified using the Terra and Aqua MODIS 11-12 µm IR brightness temperature difference (BTD) product (below). The 12 µm IR channel is no longer available on the imager instrument of the current series of GOES satellites — however, the ABI instrument on the upcoming GOES-R satellite will have a 12 µm IR channel, allowing the creation of such BTD products to aid in the identification and tracking of similar dust features.

Terra and Aqua MODIS 11-12 µm IR brightness temperature difference

At 1833 UTC, a pilot reported that the top of the dust cloud was at 11,500 feet near its leading edge (below). Farther to the south, strong winds interacting with the terrain were causing pockets of moderate to severe turbulence.

Terra MODIS 11-12 µm IR brightness temperature difference, with pilot reports

The blowing dust cloud was also evident on true-color Red/Green/Blue (RGB) images from MODIS and VIIRS, as visualized using the SSEC RealEarth web map server (below).

MODIS and VIIRS true-color RGB images

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GOES-13 0.63 µm visible images, with Cloud-Top Cooling Rate, Overshooting Tops Detection, and SPC storm reports (click to play animation)

A deepening area of low pressure (21 UTC surface analysis) was moving northeastward across the Midwest region of the US on 09 April 2015; GOES-13 0.63 µm visible images combined with the Cloud-Top Cooling Rate and Overshooting Tops Detection products (above; click image to play animation) showed a line of severe thunderstorms which quickly developed along the associated cold frontal boundary as it moved eastward across Iowa and Missouri during the afternoon hours. Cloud-Top Cooling Rates with some of the storms in Missouri were in excess of 50º C per 15 minutes (violet color enhancement) during their early stage of development (18:25 UTC image).

A comparison of Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images at 18:51 UTC or 1:51 PM local time (below) showed that the line of thunderstorms was beginning to produce a number of cloud-to-ground lightning strikes.

Suomi NPP VIIRS 11.45 µm IR channel image and 0.64 µm visible channel image with cloud-to-ground lightning strikes

Focusing our attention on eastern Iowa and northern Illinois — where there were widespread reports of large hail, damaging winds, and tornadoes (SPC storm reports) — the organization of large, discrete supercell thunderstorms can be seen on GOES-13 0.63 µm visible channel images (below; click image to play animation), which exhibited numerous overshooting tops.

GOES-13 0.63 µm visible channel images, with SPC storm reports (click to play animation)

The corresponding GOES-13 10.7 µm IR channel images (below; click image to play animation) showed that the coldest cloud-top IR brightness temperatures were -67º C (darker black enhancement).

GOES-13 10.7 µm IR images, with Overshooting Top Detection and SPC storm reports (click to play animation)

The NOAA/CIMSS ProbSevere product (below; click image to play animation) gauges the likelihood of a storm first producing severe weather (of any kind) within the next 60 minutes. It combines information about the environment (Most Unstable CAPE, Environmental Shear) from the Rapid Refresh Model, information about the growing cloud (Vertical Growth Rate as a percentage of the troposphere per minute and Glaciation Rate, also as a percentage per minute), and Maximum Expected Hail Size (MESH) from the MRMS. In this event, the ProbSevere product performed well for the storm that spawned the EF-4 tornado, although due to the cloudiness of the satellite scene the ProbSevere model was unable to diagnose vertical growth rate and glaciation rate (which diminished the potential lead-time). Below is a chronological timeline of events for that storm:

2308 UTC: first ProbSevere > 50%
2310 UTC: first ProbSevere > 70%
2311 UTC: NWS Severe T-Storm Warning
2312 UTC: ProbSevere = 88%
2323 UTC: 1.00″ hail 2 SE Dixson (15 min lead-time for ProbSevere@50, 13 min for ProbSevere@70, 12 min for NWS Svr Warning)
2335 UTC: NWS Tornado Warning (ProbSevere = 94%)
2340 UTC: Tornado report 2 NE Franklin Grove

Radar reflectivity with NOAA/CIMSS ProbSevere model contours and NWS warning polygons (click to play animation)

In spite of widespread cloudiness, the GOES-13 Sounder single-field-of-view Lifted Index (LI), Convective Available Potential Energy (CAPE), and Total Precipitable Water (TPW) derived product images (below) were able to portray that the air mass in the warm sector of the low ahead of the strong cold front was was both unstable — LI values of -4 to -8º C (yellow to red color enhancement) and CAPE values of 3000-4000 J/kg (yellow to red color enhancement) — and rich in moisture, with TPW values of 30-40 mm or 1.2 to 1.6 inches (yellow to red color enhancement).

GOES-13 Sounder Lifted Index derived product images (click to play animation)

GOES-13 Sounder CAPE derived product images (click to play animation)

GOES-13 Sounder Total Precipatable Water (TPW) derived product images (click to play animation)

On the following day (10 April), it was cloud-free as the Landsat-8 satellite passed over northern Illinois at 16:41 UTC or 11:41 AM local time — and the 30.2 mile long southwest-to-northeast oriented tornado damage path that produced EF-4 damage and was responsible for 2 fatalities and 22 injuries (NWS Chicago event summary) was evident on 15-meter resolution Band 8 0.59 µm panchromatic visible images viewed using the SSEC RealEarth web map server (below). An aerial survey of part of the tornado damage path can be seen here.

Landsat-8 0.59 µm panchromatic visible image of southwestern portion of tornado damage track (click to enlarge)

Landsat-8 0.59 µm panchromatic visible image of northeastern portion of tornado damage path (click to enlarge)

A Landsat-8 false-color image (using Bands 6/5/4 as Red/Green/Blue) is shown below. The 2 tornado-related fatalities occurred in Fairdale.

Landsat-8 false-color image (using Bands 6/5/4 as R/G/B)

On a side note, in the cold (northwestern) sector of the low it was cold enough for the precipitation type to be snow — and up to 4 inches of snow fell in western Iowa. GOES-13 0.63 µm visible channel images (below; click image to play animation) showed the swath of snow cover as it rapidly melted during the daytime hours on 10 April.

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

In fact, the swath of snow cover across eastern Nebraska and western/northern Iowa was also evident on a Suomi NPP VIIRS Day/Night Band (DNB) image at 08:49 UTC or 3:39 AM local time (below), highlighting the “visible image at night” capability of the DNB (given ample illumination from the Moon).

Suomi NPP VIIRS 0.7 µm Day/Night Band image

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GOES-15 6.5 µm water vapor channel images (click to play animation)

GOES-15 (GOES-West) 6.5 µm “water vapor channel” images (above; click image to play animation; also available as an MP4 movie file) revealed an interesting transition in the signal displayed by the 2 summits (Mauna Kea and Mauna Loa) on the Big Island of Hawai’i on 06 April 2015 — beginning as a pair of colder (darker blue color enhancement) areas during the nighttime hours, becoming a pair of warmer (brighter yellow color enhancement) areas as daytime heating warmed the land surfaces.

As was discussed in a previous blog post, the water vapor channel is essentially an Infrared (IR) channel that senses the mean temperature of a layer of moisture — usually a layer which is located in the middle troposphere. However, if the middle troposphere is dry, the water vapor detectors are able to “see” lower into the atmosphere and detect radiation from the lower atmosphere (or even high-elevation terrain features, such as Mauna Kea and Mauna Loa). A comparison of the 00 UTC and 12 UTC rawinsonde profiles from Hilo (below) showed that the middle troposphere was indeed quite dry, with the typical tropical moisture residing below the 700 hPa pressure level.

Hilo, Hawai’i rawinsonde data profiles (00, 12 UTC)

The altitude (and depth) of the layer being sensed by a water vapor channel is defined by its weighting function, which depends on (1) the temperature and moisture profile of the atmosphere, and (2) the satellite viewing angle or “zenith angle”. This site allows you to select a rawinsonde site of interest, and the GOES Imager (and Sounder) water vapor channel weighting functions are calculated and plotted. The GOES-15 Imager water vapor channel weighting functions for the 2 Hilo soundings are shown below (along with the weighting function for the US Standard Atmosphere). It can be seen that the peak of the weighting function response is at a lower altitude for both Hilo soundings than it would be for the US Standard Atmosphere, which in part allows the strong cold/warm thermal signatures of the two Big Island summits to be seen on the GOES-15 water vapor imagery.

Hilo, Hawai’i GOES-15 imager water vapor weighting functions, compared with the US Standard Atmosphere

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