VIIRS Brightness Temperature Difference (between 11.35 and 3.74 µm) and DayNight Band (DNB) at 0902 UTC 6 August 2012 (click image to play animation)

The Visible-Infraread Imager Radiometer Suite (VIIRS) instrument on Suomi/NPP samples longwave radiation emitted at 11.45 µm and shortwave radiation emitted at 3.74 µm. Because liquid water clouds have different emissivity properties at those two wavelengths, the difference in brightness temperature can be used to determine the presence of clouds (such as fog and stratus) made up of liquid water droplets. (This figure shows the channels on VIIRS; the ‘fog product’ is the difference in brightness temperature between Band I4 and I5; both channels have nadir resolutions of about 400m). The fog product in the above image has a very strong signal over the Wisconsin and Kickapoo river valleys of southwest Wisconsin, the Upper Iowa River valley in northeast Iowa, and the Root River valley of southeast Minnesota as well as the Mississippi River and other smaller tributaries. There is also a signal over southern Iowa, western Illinois and central Missouri.

The Day-Night Band (DNB) on VIIRS senses reflected (and emitted) radiation at 0.7 µm with a native resolution of about 800 m. During times near the full moon, such as 6 August, there is sufficient reflected moonlight that the DNB can identify the regions of fog in the river valleys, and there is excellent validation between the DNB and the brightness temperature difference in the river valleys. However, the DNB does not show fog over southern Iowa, western Illinois or central Missouri. The brightness temperature difference there is caused by stray light; although the satellite on the dark side of the planet, it is high enough in space that it is illuminated by the Sun, and some of that solar radiation makes its way to the sensor, contaminating the signal. The Equatorward edge of the stray light zone is the obvious zone from southwestern Missouri westnorthwestward into northeast Colorado. The stray light zone is a region where a fused fog product, such as that developed for GOES-R, that uses numerical model output, can de-emphasize positive fog signals from satellite data in regions where the numerical output does not show high boundary layer relative humidity. Some examples of the GOES-R Fog Products using GOES-East and GOES-West are shown here.

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Tropical Storm Ernesto as seen by VIIRS Day-Night Band on Suomi/NPP

Tropical Storm Florence as seen by VIIRS Day-Night Band on Suomi/NPP

The Day-Night Band from VIIRS on the Suomi/NPP has been giving excellent imagery of Tropical Storms Ernesto and Florence over the weekend. The views are especially crisp because of the near fullness of the moon. For example, the sheared nature of Ernesto (top) is evident, and the overshooting tops in Florence are distinct. Infrared imagery at the same times show the extreme height (and coldness) of the overshooting tops in both Ernesto and Florence.

Suomi/NPP Visible and Infrared Imagery of Tropical Storm Ernesto

The Day-Night Band (DNB) has a nadir resolution of approximately 800 m. Visible resolution in daytime is 400 m, and toggle from 1739 UTC on 4 August between visible and infrared imagery (both with native 400-m resolution) shows very cold overshooting tops (temperatures as cold as -91 C) and evidence of gravity waves propagating outward from the overshoots. Details on the different VIIRS bands are available here.

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MODIS 0.65 µm visible channel image + Land Surface Temperature product

A comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data and the corresponding MODIS Land Surface Temperature (LST) product (above) showed nearly cloud-free conditions and very hot LST values of 120 – 140 F (darker red color enhancement) across much of southern Oklahoma at 19:48 UTC or 2:48 PM local time on 03 August 2012. On this particular day, two notable temperature records were set at Oklahoma City:

RECORD EVENT REPORT
NATIONAL WEATHER SERVICE NORMAN OK
1145 PM CDT FRI AUG 3 2012

…NEW RECORD MAXIMUM TEMPERATURE SET AT OKLAHOMA CITY…
…NEW RECORD WARM MINIMUM TEMPERATURE AT OKLAHOMA CITY…

TODAYS MAXIMUM TEMPERATURE AT WILL ROGERS WORLD AIRPORT IN OKLAHOMA CITY WAS 113 DEGREES. THIS BREAKS THE PREVIOUS RECORD MAXIMUM
TEMPERATURE OF 109 DEGREES, SET ON THIS DATE IN 2011.

THIS ALSO TIES THE WARMEST MAXIMUM TEMPERATURE ON RECORD. THE RECORD WAS LAST REACHED ON AUGUST 11TH 1936.

THE MINIMUM TEMPERATURE OF 84 DEGREES IS ALSO THE WARMEST MINIMUM ON RECORD. THE PREVIOUS RECORD WARM MINIMUM OF 83 DEGREES WAS LAST REACHED ON AUGUST 13TH 1936.

TEMPERATURE RECORDS FOR OKLAHOMA CITY DATE BACK TO 1891.

The hot LST and air temperature values combined with dry fuels due to ongoing drought conditions created a very favorable environment for wildfire activity — and several fire smoke plumes were noted on GOES-13 0.63 µm visible channel images (below; click image to play animation). With the largest of the fires located east of Norman, Oklahoma (station identifier KOUN), a number of brighter white pyrocumulus clouds could be seen popping up through the lighter gray smoke plume.

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

The dense smoke plume from the Norman fire showed up very well on the 19:52 UTC (2:52 pm local time) Aqua MODIS true color Red/Green/Blue (RGB) image (below, displayed using Google Earth).

Aqua MODIS true-color Red/Green/Blue (RGB) image

The large fire east of Norman continued to burn into the the following night, exhibiting a pronounced “hot spot” (black to yellow to red color enhancement) on the 07:57 UTC (2:57 AM local time) Suomi NPP VIIRS 3.74 µm shortwave IR image (below). The flames from this fire also had a distinct bright signature on the corresponding 0.7 µm Day/Night Band image.

Suomi NPP VIIRS 11.45 µm IR, 3.74 µm shortwave IR, and 0.7 µm Day/Night Band images

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Overshooting Tops detected over the central Atlantic

A strong tropical wave near 50 W Longitude in the central Atlantic has become a Tropical Depression. The enhanced infrared image above shows a region of active convection (as signified by the overshooting tops designated by yellow circles) near the center of the system. (Current imagery of overshooting tops can be found here). This convection is aided by abundant moisture as shown in the animation of Total Precipitable Water (TPW), below, taken from the MIMIC TPW website.

Morphed Total Precipitable Water over the north Atlantic Basin

Diagnostics from the CIMSS Tropical Weather Website (below) suggest that an inhibiting factor to rapid strengthening may be wind shear, as analyses show the storm near a region of significant westerly shear. Factors favoring slow intensification are warm sea surface temperatures and a moist surrounding atmosphere.

Infrared imagery and diagnosed wind shear over Tropical Depression #5

Analysis of dry air near Tropical Depression #5

Sea-surface temperature analysis over the central Atlantic

The National Hurricane Center forecasts slow strengthening. Should the system become a tropical storm, it will take the name Ernesto. The current projected path has the storm in the central Caribbean Sea by the weekend.

===== 02 August Update =====

The Suomi NPP VIIRS 0.7 µm Day/Night Band offered a “night-time visible image” of Tropical Depression #5 at 05:23 UTC on 02 August (below; image courtesy of William Straka, CIMSS).

Suomi NPP VIIRS 0.7 µm Day/Night Band image

During the afternoon hours, the system was upgraded to Tropical Storm Ernesto. GOES-13 0.63 µm visible channel images (below; click image to play animation) showed that while Ernesto was producing a few convective bursts near its center (IR image animation with tropical overshooting tops product), it was also exhibiting a number of well-defined surface outflow arc clouds along the northern and western periphery of the circulation. This arc cloud signature often indicates that the storm is ingesting dry air which results in the production of dry thunderstorm downdrafts — and Ernesto was both relatively close the the continent of South America, and was surrounded by a dry Saharan Air Layer.

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

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