Fog/stratus along the central California coast

April 30th, 2013
GOES-15 IR brightness temperature difference "Fog/stratus product" (click image to play animation)

GOES-15 IR brightness temperature difference “Fog/stratus product” (click image to play animation)

The San Francisco Bay Area National Weather Service forecast office made use of a variety of satellite images and products to monitor the development of fog and stratus clouds along the central California coast during the pre-dawn hours of 30 April 2013. From their forecast discussion:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
451 AM PDT TUE APR 30 2013

.DISCUSSION…AS OF 3:20 AM PDT TUESDAY…SATELLITE AND SURFACE OBS INDICATE ONLY SOME MINOR POCKETS OF LOW CLOUDS AND PATCHY FOG.
RIGHT NOW HALF MOON BAY IS THE ONLY SPOT REPORTING REDUCED VISIBILITIES WHILE MONTEREY NOW HAS A CEILING. DETAILS FROM THE OVERNIGHT
SATELLITE FOG PRODUCT ARE A BIT DIFFICULT TO MAKE OUT ALTHOUGH LOOKS LIKE THE AREAS ARE VERY SMALL IN NATURE.

Using the 4-km resolution GOES-15 11-3.9 µm IR brightness temperature difference (BTD) “fog/stratus product” with the default gray-scale enhancement applied (above; click image to play animation), it was indeed difficult to unambiguously determine the motion and areal coverage of the darker fog and stratus features (especially in the Monterey Bay area). Applying a tailored color enhancement to that same GOES-15 BTD fog/stratus product does help a little to highlight some of the more well-defined fog and stratus features (with a darker orange color), but there is also a tendency to produce large areas of noisy “false alarm” fog/stratus signals (yellow color enhancement) over much of the inland areas of central California.

In the Aviation section of their forecast discussion, mention was made of the GOES-R Marginal Visual Flight Rules (MVFR) product, which is produced using an algorithm designed for the upcoming GOES-R satellite (but applied to current GOES-15 data):

MONTEREY BAY AREA TERMINALS…SHORT LIVED MVFR THIS MORNING. VIIRS DAY NIGHT SATELLITE AND GOES-R MVFR PROBABILITIES INDICATE CLOUD DECK IS PATCHY.

This GOES-15 product (below; click image to play animation) did indeed show that the areas of 50-80% MVFR Probability were patchy in nature in the vicinity of Monterey Bay.

GOES-15 MVFR Probability product (click image to play animation)

GOES-15 MVFR Probability product (click image to play animation)

Other GOES-R Fog/Low Stratus products (produced as part of the GOES-R Proving Ground activities at CIMSS/ASPB) include an Instrument Flight Rules (IFR) Probability product, a Low Instrument Flight Rules (LIFR) Probability product, and a Low Cloud Thickness product.

Mention was also made of the Suomi NPP VIIRS 0.7 µm Day/Night Band, which provided a high spatial resolution (750 meters, re-mapped onto a 1-km AWIPS grid) “visible image at night” (below), helping to confirm the spotty nature of the coverage of fog/stratus in the Monterey Bay area.

Suomi NPP VIIRS 0.7 µm Day/Night Band image

Suomi NPP VIIRS 0.7 µm Day/Night Band image

Meanwhile, fog and stratus were more widespread across the southern California coast and adjacent offshore waters – for more details, see the GOES-R Fog Product Examples site.

 

Night-time VIIRS Day/Night Band images during the full “Pink Moon”

April 26th, 2013
Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference “Fog/stratus product”

A number of interesting night-time images were available using the Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) on 26 April 2013 — since there was abundant illumination from the full “Pink Moon”, this allowed the DNB to provide “visible images at night” which showed city lights as well as a number of meteorological phenomena. We’ll begin our tour in the northeastern US, where a comparison of the VIIRS DNB with the corresponding IR brightness temperature diference (BTD) “Fog/stratus product” (above) showed narrow fingers of valley fog from northwestern Pennsylvania to eastern Ohio at 06:52 UTC or 2:52 AM local time.

Farther to the south, a narrow band of thick cirrus cloud (which exhibited IR brightness temperatures as cold as -40º C) was casting a shadow on the ground from far southeastern Kentucky to northeastern Tennessee (below).

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

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

Over the north-central US, the VIIRS DNB showed areas that still had substantial snowpack remaining across parts of North Dakota, South Dakota, and Minnesota at 08:34 UTC or 3:34 AM local time (below). Even though there were patches of cirrus cloud over North Dakota (which appear black on the Fog/stratus product), features such as the ice-covered Lake Sakakawea, the Souris River (parts of which were in flood stage and producing moderate flooding), and the edge of the snow cover could still be identified through these high-level clouds on the DNB image.

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR BTD

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR BTD “Fog/stratus product” images

To the south, clouds bands associated with an undular bore could be seen on the DNB image over the Texas Panhandle region (below).

Suomi NPP VIIRS 0.7 µm Day/Night Band image with METAR surface reports

Suomi NPP VIIRS 0.7 µm Day/Night Band image with METAR surface reports

Moving toward the western US, images of the DNB and the 11.45 µm IR channel centered over the Salt Lake City, Utah area (below) revealed colder (violet color enhancement on the IR image) snow-covered mountain ranges (most notable were the Wasatch Range in Utah and the Wind River Range in Wyoming), as well as the large, bright area of the Bonneville Salt Flats in the Great Salt Lake Desert.

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

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

Finally, some interesting views along the West Coast of the US, beginning with a comparison of the VIIRS DNB and BTD Fog/stratus product centered over the Seattle and Vancouver Island region at 10:12 UTC or 3:12 AM local time (below). Fingers of marine layer stratus clouds could be seen moving inland across the Olympic Peninsula of far northwestern Washington State — and the snow-covered Olympic Mountains (which includes Mount Olympus at an elevation of 7,962 ft or 2,427 m) were plainly visible on the DNB image. Stratus clouds were also seen moving inland through the Strait of Juan de Fuca, as well as into the southern portion of Puget Sound.

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference “Fog/stratus product”

Farther to the south along the southern Oregon and northern California coasts, the inland extent of fingers of marine layer stratus clouds could again be seen on both the VIIRS DNB and BTD Fog/stratus product images (below).

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference Fog/stratus product

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference Fog/stratus product

Finally, a comparison of VIIRS DNB and BTD Fog/stratus product images centered over the San Francisco area (below) showed where marine layer stratus clouds covered portions of the San Francisco Bay, San Pablo Bay, and adjacent inland valleys. Other features of interest included a small eddy circulation just offshore of Monterey (KMRY), as well as an aircraft dissipation trail slicing southwest-to-northeast through the patch of offshore stratus cloud located just to the northwest of the eddy (likely caused by a jet either ascending or descending through the cloud flying outbound or inbound from San Francisco International airport KSFO).

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference “Fog/stratus product” images

Diagnosing areas of light winds over water

April 25th, 2013
Suomi NPP VIIRS 0.65 µm visible channel image with overlays of surface reports and RTMA surface winds

Suomi NPP VIIRS 0.65 µm visible channel image with overlays of surface reports and RTMA surface winds

An AWIPS image of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel data (above) revealed a large patch of water within the eastern Gulf of Mexico “sun glint” region that exhibited a much darker appearance than the surrounding waters off the west coast of Florida on 25 April 2013. This patch of darker water generally corresponded to a region of very light to calm winds, as verified by an overlay of the Real-Time Mesoscale Analysis (RTMA) surface winds. As explained in a previous blog post, there is often a significant amount of sun glint off the wind-driven rough water surfaces below a polar-orbiting satellite overpass — due to scattering of light these areas of sun glint make the rougher water surfaces appear brighter on visible imagery. However, in an area of calm winds, the water surface becomes very flat; this flat water surface then reflects incoming sunlight like a mirror (with all the light being reflected back in one direction — but in this case, that one direction was not directly back toward the satellite).

Another interesting signature of the flat water surface is seen in a comparison of 1-km resolution Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or “IR window” channel images (below). The shortwave IR channel is very sensitive to reflected solar radiation, and will often exhibit a much warmer, darker signal over areas of sun glint. However, note that the areas of darker water seen in the visible image above appear significantly cooler (lighter gray enhancement) on the shortwave IR image. Again, in the case of smooth, flat water in light wind regions, the incoming solar radiation is reflected back in a direction that happens to be away from the satellite sensors. Since the 11.45 µm IR channel is not sensitive to reflected solar radiation, no such signature was seen in that particular image.

Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or

Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or “IR window” channel images

A night-time vs daytime comparison of the 1-km resolution MODIS Sea Surface Temperature (SST) product (below) showed that the SST values over the patch of calm water increased from the low to middle 70s F at 06:58 UTC (2:58 AM local time) to the upper 70s to 80º F at 16:22 UTC (12:22 PM local time). Such an increase in SST within a relatively short 10-hour period was possible due to the fact that the presence of very light winds also allowed the “skin temperature” of the water surface to warm very quickly (as we have prevously seen over Lake Michigan).

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product

Hail-producing thunderstorm at Kandahar, Afghanistan

April 23rd, 2013
EUMETSAT Meteosat-7 0.7 µm visible channel images (click image to play animation)

EUMETSAT Meteosat-7 0.7 µm visible channel images (click image to play animation)

A thunderstorm produced golf-ball-size hail that reportedly lasted up to 30 minutes at Kandahar, Afghanistan on 23 April 2013. This hail was responsible for 3 fatalities, and damaged dozens of US and British military helicopters and aircraft stationed at Kandahar International Airport. McIDAS images of EUMETSAT Meteosat-7 0.7 µm visible channel images (above; click image to play animation) and 11.5 µm IR channel images (below; click image to play animation) showed the rapid development of the relatively compact thunderstorm as it passed over Kandahar International Airport (denoted by the “*” symbol). Shadow-casting overshooting tops were evident on the visible imagery, and the cloud-top IR brightness temperature cooled to -66 C (dark red color enhancement) at 08:30 UTC.

EUMETSAT Meteosat-7 11.5 µm IR channel images (click image to play animation)

EUMETSAT Meteosat-7 11.5 µm IR channel images (click image to play animation)