University of Wisconsin-Madison Space Science and Engineering Center

CIMSS Satellite Blog

Fog/stratus along the central California coast

April 30th, 2013

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)

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

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.

Posted in Aviation, Fog detection, GOES-15, GOES-R, Suomi NPP, VIIRS | No Comments »

“Southerly surge” of stratus along the coast of California

April 22nd, 2013

GOES-15 0.63 µm visible channel images (click image to play animation)

McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) revealed a well-defined “southerly surge” of stratus clouds, which were moving northward along the coast of the Big Sur region of central California on 22 April 2013.

AWIPS images of the GOES-R Marginal Visual Flight Rules (MVFR) product algorithm applied to GOES-15 data (below; click image to play animation) showed the northward progression of the narrow band of stratus clouds, as a mesoscale reversal in the boundary layer winds just off the coast (from northerly, to southerly) was seen in the buoy data.

GOES-15 Marginal Visual Flight Rules (MVFR) Probability product (click image to play animation)

A 1-km resolution MODIS 11-3.7 µm IR brightness temperature difference (BTD) “fog/stratus product” image at 05:34 UTC or 10:34 PM local time (below) showed the initial stages of the southerly surge stratus cloud feature, in the Santa Barbara Channel north of the Channel Islands off the coast of southern California (and south of Point Conception). Farther offshore, the BTD image also displayed a well-defined eddy and a few ship tracks embedded within the marine boundary layer stratus clouds.

MODIS IR brightness temperature difference “fog/stratus product”

Several hours later, a 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) image at 09:56 UTC or 2:26 AM local time (below) indicated that the leading edge of the southerly surge stratus cloud feature had moved to the north of Point Conception. This demonstrates that the VIIRS DNB can provide a “visible image at night”, given sufficient illumination by the Moon (or other light sources). On this night the Moon was in the waxing gibbous phase, at 95% full.

Suomi NPP VIIRS 0.7 µm Day/Night Band image

A Suomi NPP VIIRS 0.64 µm visible channel image at 21:13 UTC or 2:13 PM local time (below) showed leading edge of the southward surge stratus feature as it was curving eastward into the Monterey Bay area. Not far to the north, San Francisco reached a record high temperature of 83º F.

Suomi NPP VIIRS 0.64 µm visible channel image

===== 23 April Update =====

A comparison of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference (BTD) “Fog/stratus product” images at 09:28 UTC or 2:29 AM local time (below) showed that the leading edge of the southerly surge stratus clouds had progressed just to the north of Point Reyes. Once again, the BTD image revealed the presence of cloud eddies and ship tracks within the marine boundary layer stratus clouds.

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

Posted in Fog detection, GOES-15, GOES-R, Marine weather, MODIS, Suomi NPP, VIIRS | No Comments »

Fog and stratus deck over the North Slope region of Alaska

March 11th, 2013

The Area Forecast Discussion issued by the National Weather Service forecast office in Fairbanks mentioned the presence of a layer of fog and stratus over parts of the North Slope region of Alaska:

NORTHERN ALASKA FORECAST DISCUSSION NATIONAL WEATHER SERVICE FAIRBANKS AK 1258 PM AKDT MON MAR 11 2013

NORTH SLOPE…THE SUOMI NPP VIIRS SATELLITE FOG PRODUCT WAS INDICATING A DECENT LAYER OF STRATUS ALONG THE NORTH SLOPE. OBSERVATIONS ACROSS THE AREA GENERALLY INDICATED 1 TO 2 MILES IN VISIBILITY WITH FLURRIES AND FOG. THE IFR CONDITIONS ALIGN VERY WELL WITH THE HIGHER PROBABILITIES OF MODIS IFR PRODUCT. THERE ARE SOME VERY ISOLATED POCKETS OF HIGHER PROBABILITIES OF THE MODIS LIFR CONDITIONS. THESE CONDITIONS SHOULD REMAIN THROUGH TUESDAY EVENING OR WEDNESDAY MORNING AS THE SURFACE HIGH PRESSURE REMAINS WITHIN THE AREA. BY WEDNESDAY MORNING THE SURFACE PRESSURE GRADIENT BEGINS TO TIGHTEN…PROVIDING AN INCREASE IN WINDS AND PERHAPS A BREAK IN SOME OF THE FOG.

AWIPS images of the Suomi NPP VIIRS IR brightness temperature difference “fog/stratus product” (below) showed the coverage of the fog and stratus over the area.

Suomi NPP VIIRS IR brightness temperature difference “fog/stratus product”

The MODIS Instrument Flight Rules (IFR) Probability product (below) exhibited probabilities in excess of 90% (darker red color enhancement) over some areas. As mentioned in the forecast discussion, surface high pressure centered over the region was helping to maintain the presence of fog and stratus.

MODIS Instrument Flight Rules (IFR) Probability product

The corresponding MODIS Low Instrument Flight Rules (LIFR) Probability product (below) had some values greater that 70% (red color enhancement) over the eastern portion of the North Slope. However, you will notice that a large data gap appears in the LIFR product, in an area where IFR probabilities were high. Corey Calvert from CIMSS explained:

“Looks like the issue stems from the RAP model. The max RH in the lowest 1000′ layer is well over 90% for the area being discussed. However, the max RH in the lowest 500′ layer decreases significantly northward toward the water where is terrain is lower. Looks like the RAP is representing an elevated cloud layer with a ceiling between 500′-1000′ above sea level in that area. The high LIFR probs appear to coincide with higher terrain, therefore the lowest 500′ layer is elevated into the RAP cloud layer. The low LIFR probs (the hole in the data) match up with the lower RH values at lower terrain under the RAP cloud layer. The ceiling obs do indicate an elevated cloud layer (one station measuring 300′ and another 700′ in the area), but it’s hard to tell which is right due to the sparsity of obs. If the ceiling truly is hovering around 500′ though then this is believable representation from the products (high prob of IFR but low prob of LIFR)”.

MODIS Low Instrument Flight Rules (LIFR) Probability product

These examples of new satellite products are being evaluated in an operational environment as part of the GOES-R Proving Ground, in an effort to prepare forecasters for the types of satellite data that will be available from the next generation of meteorological satellites (JPSS and GOES-R)

Posted in Arctic, Aviation, Fog detection, GOES-R, MODIS, VIIRS | No Comments »