GOES-13 fog/stratus product (click image to play animation)

AWIPS images of the 4-km resolution GOES-13 10.7 µm – 3.9 µm “fog/stratus product” (above; click image to play animation) showed a large area of fog and/or stratus (yellow to orange color enhancement) that was increasing in areal coverage during the pre-dawn hours on 06 February 2012. Although the fog/stratus product is useful for locating the presence and temporal trends of such features, it does not offer any reliable indication of whether it is fog on the ground or stratus cloud aloft.

One product that attempts to give the forecaster some quantitative information is the GOES Low CLoud Base (LCB) prodcut (below; click image to play animation), which attempts to blend surface observations with satellite data to indicate whether the cloud base is above or below the threshold of 1000 feet.

GOES-13 Low Cloud Base product (click image o play animation)

With 1-km resolution data, the MODIS instrument aboard the polar-orbiting Terra and Aqua satellites offers a similar “fog/stratus product” (below) that provides better clarity, especially regarding the exact location of the edges of the fog and/or stratus.

MODIS fog/stratus product images

In this particular case, a number of locations beneath the western and southern edge of the fog/stratus feature were expereincing freezing fog (below) and visibilities of 1/4 mile or less, which was creating hazardous road conditions and prompting the issuance of Freezing Fog Advisories.

MODIS fog/stratus product with METAR surface reports

As part of CIMSS participation in GOES-R Proving Ground activities, products are being developed which can provide more quantitative information about such parameters as Fog Depth and the Probability of Marginal Visual Flight Rules (MVFR) or Instrument Flight Rules (IFR) conditions (below). In this case, across the southwestern part of Iowa (where widespread freezing fog was being reported), the fog depth was as high as 1400-1500 feet, with probabilities of MVFR and IFR conditions as high as 75-90% and 60-75%, respectively.

MODIS Fog Depth, MVFR Probability, and IFR Probability products

Shortly after sunrise, it is interesting to note that a comparison of 1-km resolution POES AVHRR 0.63 µm visible channel, 3.74 µm “shortwave IR” channel, and 10.8 µm channel “IR window” channel images (below) revealed that part of the swath of fresh snow cover (as deep as 4-6 inches) across western Iowa could be seen through the translucent western edge of the fog/stratus deck that was beginning to burn off during the morning hours. The fog/stratus deck appears warmer (darker gray enhancement) om the 3.74 µm image, due to the sensitivity of that channel to the reflection of solar radiation off the tops of supercooled water droplet clouds.

Farther to the south, note the presence of narrow fingers of valley fog in the Ozark Mountains and surrounding regions in Oklahoma, Arkansas, and Missouri.

POES AVHRR 0.63 µm visible, 3.74 µm "shortwave IR", and 10.8 µm "IR window" images

GOES-13 6.5 µm water vapor channel images (click image to play animation)

AWIPS images of 4-km resolution GOES-13 6.5 µm water vapor channel data (above; click image to play animation) showed the middle-tropospheric circulation and cloud features associated with the large storm system which brought heavy snow, heavy rainfall, and severe thunderstorms to much of the central US on 03 February – 04 February 2012. Snowfall amounts included 51.1 inches at Pinecliffe, Colorado, 26.0 inches at Laramie, Wyoming, 17.0 inches at Tyron, Nebraska, and 11.5 inches at Cumberland, Iowa.

Denver received 15.9 inches of snow during 02/03/04 February, setting a new 3-day record accumulation for the month of February. Boulder also set a new single-storm snowfall record, with 22.7 inches of snowfall (NWS Denver/Boulder CO storm summary).

POES AVHRR 0.63 µm visible channel + 3.74 µm shortwave IR channel images

As the storm departed, a comparison of AWIPS images of 1-km resolution POES AVHRR 0.63 µm visible channel and 3.74 µm shortwave IR data (above) at 15:06 UTC (8:06 am local time) on 04 February showed that some low clouds persisted across much of northeastern Colorado, backed up against the highest terrain of the Continental Divide in some places. The low clouds showed up as darker gray features on the shortwave IR image, due to the sensitivity of reflection of solar radiation off of cloud top supercooled water droplets at the 3.74 µm wavelength.

At 17:47 UTC (10:47 am local time), a comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data with the corresponding MODIS false-color Red/Green/Blue (RGB) image (created using MODIS channel 01/07/07 as the red/green/blue components of the image) indicated that most of the low clouds (which appeared as varying shades of white on the false-color image) had dissipated, revealing a good deal of the snow cover (which appeared as darker shades of red on the false-color image). A few streaks of high-level cirrus clouds could also be seen over the snow cover. Bare ground appeared cyan on the false-color image.

MODIS 0.65 µm visible channel + False-color Red/Green/Blue (RGB) images

About 2 hours later, a more detailed example of using false color images to discriminate between snow cover and supercooled water droplet clouds can be seen with a 375-meter resolution Suomi NPP VIIRS Red/Green/Blue (RGB) image (below), created using Band I1 (0.64 micrometer visible) as the red component and Band I3 (1.61 micrometer near-IR) as the green and blue components of the image.

Suomi NPP VIIRS false color RGB image

Farther to the east and south, heavy rainfall amounts included 9.30 inches at Romayer, Texas, 5.69 inches at Alexandria, Louisiana, and 4.34 inches at Medicine Lodge, Kansas. Wichita, Kansas received 2.86 inches of rain — the wettest February day on record at that location. Severe thunderstorms produced one tornado and hail up to 2.0 inches in diameter in Texas (SPC storm reports). A McIDAS image of 375-meter resolution Suomi NPP VIIRS 11.45 µm IR channel data (below) showed very intricate detail to the cloud top IR brightess temperature structure associated with strong thunderstorms producing heavy rainfall and flash flooding across the Interstate 35 corridor in the Austin/San Antonio, Texas region during the pre-dawn hours on 04 February. VIIRS IR brightness temperatures were as cold as -81º C with the far southwestern storm — and rare “warm trench” signatures (a ring of warmer cloud top temperatures surrounding a well-defined cold overshootng top) were seen associated with the 2 storms located near Austin-Bergstrom International airport (KAUS) and Houston County Airport (KDKR).

Suomi NPP VIIRS 11.45 µm IR image + Station locations and Interstate highways

===== 05 February Update =====

A large portion of the resulting swath of snow on the ground across parts of Wyoming, Colorado, Nebraska, and Kansas could be seen on a 250-meter resolution MODIS true color RGB image from the SSEC MODIS Today site (below, viewed using Google Earth) at 20:17 UTC on 05 February 2012.

MODIS true color image (viewed using Google Earth)

POES AVHRR 12.0 µm and MODIS 11.0 µm IR images (with METAR surface reports)

Record cold continued across the interior of Alaska, with Fairbanks reaching a minimum temperature of -50º F on 28 January 2012 and -51º F on 29 January 2012. These were the first -50º F temperatures at Fairbanks since 2006 (NWS Fairbanks public information statements). The coldest temperature reported was -65º F at Galena and by a coopertive observer at Fort Yukon (Fairbanks region temperature and precipitation data).

A sequence of AWIPS images of 1-km resolution POES AVHRR 12.0 µm IR and MODIS 11.0 µm IR data (above) revealed the expansion of surface IR brightness temperatures of -50º C or colder (violet to white color enhancement) during the early morning hours on 28 and 29 January. The coldest surface air temperatures at the times of the IR images included -50º F at Fairbanks (station identifier PAFA) and -60º F at Fort Yukon (station identifier PFYU) and Tanana (station identifier PATA). The signature of cold air drainage into lower elevation terrain (such as the relatively narrow river valleys along the south side of the Brooks Range, and also the broad Yukon Flats) could be seen on the 1-km resolution IR images.

Suomi NPP VIIRS 11.450 µm (Band I5) IR image

The pattern of cold air drainage into lower elevations could be seen in even greater detail using McIDAS images of 375-meter resolution Suomi NPP VIIRS 10.450 µm IR data at 12:06 UTC on 28 January, over northwestern Alaska and the Yukon Territory of Canada (above), and also just to the southwest over the eastern interior of Alaska (below). These 2 VIIRS images use a different color enhancement, where the coldest surface IR brightness temperatures are darker blue.

Unfortunately, there was no surface air temperature report for Arctic Village (station identifier PARC) at this time, but the coldest surface IR brightness temperatures within some of the deeper valleys near that site was -58.4º C (-73.1º F).

To the south, a broad area of very cold (dark blue) surface IR brightness temperatures was seen across the Yukon Flats, with a minimum value of -58.3º C (-72.9º F). The hourly surface air temperature at the Fort Yukon (PFYU) reporting station close to the time of the satellite image was -56º F, while the surface IR brightness temperature at that location was -54º F. Although there is not always a direct 1:1 correspondence between satellite-sensed IR surface temperature values and the actual air temperature measured within an instrument shelter at a height of 5 feet above ground level, the IR satellite imagery can be used to located areas that might have the coldest surface air temperatures.

Suomi NPP VIIRS 11.450 µm (Band I5) IR image

POES AVHRR 12.0 µm IR images + surface reports

Unusually cold conditions were seen across the North Slope region of Alaska during the 22 January – 24 January 2012 period. A sequence of AWIPS images of 1-km resolution POES AVHRR 12.0 µm IR channel data (above) showed the expansion of a large area of surface IR brightness temperatures of -50 C and colder (violet to white color enhancement) across the interior portions of the North Slope. Nuiqsut (station identifier PAQT) was as cold as -62 F (-52 C) on 24 January, and Barrow (station identifier PABR) reached a low temperature of -45 F (-43 C) on 23 January (the record low temperature for the date was -47 F, and the normal low for the date is -20 F).

Another feature of interest over the Arctic Ocean was the appearance of a number of what resembled “warm cracks”  in the sea ice, where IR brightness temperatures were -30 C or warmer (yellow color enhancement) — significant amounts of thermal energy from the warmer waters below were able to “bleed up” through weaknesses and thinner areas of the sea ice, showing up as warm anomalies on the IR imagery.

A Public Information Statement was issued by the National Weather Service forecast office at Fairbanks:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
700 PM AKST TUE JAN 24 2012

…SEVERE COLD CONTINUES OVER THE NORTH SLOPE OF ALASKA…

A VERY COLD AIR MASS CONTINUES OVER THE NORTH SLOPE…COMBINED
WITH WINDS IN SOME AREAS. HERE ARE SOME LOW TEMPERATURES RECORDED SO FAR TODAY ACROSS THE NORTH SLOPE OF ALASKA.

NUIQSUT……………..62 BELOW
UMIAT……………….59 BELOW
INIGOK………………54 BELOW
ALPINE………………53 BELOW
ATQASUK……………..48 BELOW
DEADHORSE……………47 BELOW
WAINRIGHT……………44 BELOW
KAKTOVIK…………….40 BELOW
BARROW………………39 BELOW

TEMPERATURES OVER THE NORTH SLOPE WILL REMAIN IN THE 40S AND 50S BELOW WITH POCKETS NEAR 60 BELOW FOR THE NEXT FEW DAYS…AND POTENTIALLY INTO THE WEEKEND.

$$

JM

POES AVHRR 0.86 µm visible channel + POES AVHRR 12.0 µm IR channel images

A surge of cold air brought the first measurable snowfall to parts of the Pacific Northwest states on 14 January – 15 January 2012. The Seatle-Tacoma aiport received 2.4 inches of snow on 15 January. A comparison of 1-km resolution POES AVHRR 0.86 µm visible channel and 12.0 µm IR channel data (above) displayed a classic example of “open cell convection” — this type of open-cell mesoscale convective cloud pattern is a signature of strong instability (via boundary layer cold air advection over relatively warmer waters) in an environment of cyclonic flow.

A sequence of 1-km resolution POES AVHRR 12.0 µm IR channel images (below) showed the inland progression of the open cell convection, eventually producing snowfall at Seattle, Washington (station identifier KSEA) and Portland, Oregon (station identifier KPDX).

POES AVHRR 12.0 µm IR channel images

GOES-15 (GOES-West) and GOES-13 (GOES-East) 0.63 µm visible images (click image to play animation)

Snowfall amounts as high as 10-15 inches fell across parts of west Texas and southeast New Mexico on 09 January 2012 as a strong upper level disturbance moved across that region (NWS Lubbock TX storm summary). On the following morning, a comparison of GOES-15 (GOES-West) and GOES-13 (GOES-East) 0.63 µm visible channel images (above; click image to play animation) showed the areal coverage of the snow cover that remained on the ground. Note how the patch of snow began to melt from the outer edges inward as the full day of sunshine warmed the ground surface. Also note the curious “donut hole” of bare ground on the northern end of the main snow cover — this feature rapidly disappeared, as the snow depth associated with this feature was not very high.

A comparison of 250-meter resolution MODIS true color and false color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (below) showed greater detail in the snow cover (snow on the ground appears as darker shades of cyan on the false color image) at 18:02 UTC.

MODIS true color and false color Red/Green/Blue (RGB) images

A comparison of AWIPS images of MODIS 0.65 µm visible channel data and the corresponding false color RGB image (below) offered another tool that can be used to discriminate between snow cover (which in this example appears as darker shades of red on the false color image) and supercooled water droplet clouds (which appeared as varying shades of white).

MODIS 0.65 µm visible image + MODIS false color RGB image

A comparison of the MODIS 0.65 µm visible image with the corresponding MODIS Land Surface Temperature (LST) product (below) revealed how the deep snow cover was helping to keep surface air temperatures significantly colder than adjacent regions with bare ground. MODIS LST values were in the low to middle 30s F across the deeper snow cover, in the upper 40s to low 50s F in the “donut hole” region where the snow had just melted, and in the 60s F to the north over bare ground. Also note how the urban areas of Midland and Odessa stand out in the LST image, with LST values in the low to middle 40s F.

MODIS 0.65 µm visible image + MODIS Land Surface Temperature product

The mechanism for the creation of the “donut hole” snow cover feature is unclear at this point. A comparison the MODIS Land Surface Temperature product with the regional topography (below) seems to suggest that this feature was not topographically-driven.

MODIS Land Surface Temperature product + Topography

The MODIS true color image viewed using Google Earth (below) showed that the community of Brownfield (which did received about an inch of snowfall the previous day) was aptly named, being located within the brown-colored snow-free region at 18:02 UTC.

MODIS true color image (viewed using Google Earth)

MODIS Land Surface Temperature product

Much of the state of Florida experienced below-freezing temperatures on the morning of 04 January 2012. An AWIPS image of the 1-km resolution MODIS Land Surface Temperature (LST) product (above) showed that LST values of 32ª F and colder (blue color enhancement) extended far to the south at 07:27 UTC (02:27 am local time). There were even several pockets exhibiting LST values less than 20ª F (violet color enhancement), including a minimum of 10.7ª F just southwest of Gainesville (station identifier KGNV) in northern Florida, and 15.3ª F just to the west of Lake Okeechobee in southern Florida.

A map of minimum temperatures on the morning of 04 January (below) showed that the cold air had even moved as far south as Cuba, where lows in the 40s F were observed. In Florida, new record  low temperatures for the day included 18ªF at Tallahassee, 20ª F at Gainesville, 22ª F at Jacksonville, 33ª F at Melbourne, and 36ª F at Naples.

04 January 2011 minimum temperatures

MODIS true color RGB image (viewed using Google Earth)

An interesting pattern of “snow streaks” across parts of Illinois, Indiana, and Kentucky was seen on a MODIS true color Red/Green/Blue (RGB) image from the SSEC MODIS Today site on 03 January 2012 — this pattern was caused by narrow cells of convective snowfall which propagated southeastward across the region on the previous day. The amount of snow on the ground was only a Trace to 1 inch, which allowed the snow streaks to quickly melt during the day.

GOES-13 6.5 µm WV images (click image to play animation)

A potent winter storm moved into the southern Plains on 19 December 2011 as a cut-off circulation off the west coast of the US opened up and moved eastward. GOES water Vapor imagery (looped, above) shows a distinct dry slot. Stationary terrain features in the Cordillera of Northern Mexico are visible, suggesting that the surface radiation emitted at 6.5 micrometers is not absorbed by water vapor within the atmosphere (because of the extreme dryness). Cirrus that develops later in the loop along the southern border of the image does obscure some of the surface features. Note also how a strong moisture signal develops over New Mexico. Rising motion over that state moves water vapor to higher and higher levels in the atmosphere. At the start of the loop, most of the water vapor exists below the mid-tropospheric level where the Imager Sensor is detecting water vapor. Persistent rising motion allows the moist layer to deepen, and the imager starts detecting this higher, colder moisture. The loop in the computed weighting function for Albuquerque at 00 UTC and at 12 UTC on 19 December is here. Note that the amount of water vapor in the atmosphere increases between 00 UTC and 12 UTC as indicated on the linked-to charts. The weighting function describes the relative importance of emitted radiation from different levels in the atmosphere.

Lack of moisture in mid-levels (the peak response is around 500 hPa) at 00 UTC means the water vapor signal is being emitted from farther down in the troposphere, where it is warmer. As moisture deepens, the water vapor signal is emitted from colder regions. The water vapor detector on the imager shows the temperature at the top of the moist layer. It does not reveal the total moisture content in the column.

GOES-13/MODIS 6.5 µm WV images (click image to play animation)

MODIS and GOES-13 water vapor imagery (above) from between 1630 and 1700 UTC (that is, just after the loop at the top), show significant brightness temperature differences between sensed water vapor. Values from the MODIS instrument shows water vapor brightness temperatures that are uniformly colder than the GOES-13 values. Why? The Spectral Response Functions below (courtesy of Mat Gunshor, SSEC/CIMSS), for GOES-12 (the imager on GOES-12 is similar to that on GOES-13) and for the MODIS WV Channel suggest a possible reason. The Imager water vapor detection (in blue) spans a larger part of the electromagnetic spectrum, including regions at longer wavelengths. (The MODIS water vapor channel is a single sharply defined peak (shown in red)). As the wavelength increases, the level sensed decreases, so a broader spectrum that includes longer wavelengths will show warmer temperatures because it is detecting more energy from lower in the atmosphere where temperatures are warmer. At Nadir, GOES-13 will be about 1 K warmer than the MODIS Brightness temperature. (Use this website to show different weighting functions.

GOES-12/MODIS Spectral Response Functions for Water Vapor Channel

It is common to relate features in the water vapor imagery to structures in the atmosphere. The figure below shows a 325-K Jet maxima aligned, as expected, with the dry slot in the WV imagery. The dry slot is a region of sinking motion. Warm brightness temperatures develop in the dry slot because water vapor is confined to the lowest levels of the atmosphere, so the emitting surface is warm. A cross-section that is nearly orthogonal to the jet (here) shows an isentropic structure that is characteristic of an intrusion of stratospheric air into the mid-troposphere. It also shows extreme dryness in the middle of the tropopshere.

GOES-13 WV/GFS 325K Winds

This storm brought needed precipitation to parts of the southern Plains that have been plagued by drought all year.

MODIS true color RGB images (viewed using Google Earth)

Two days after the storm, the clouds had cleared to reveal the large swath of fresh snow cover on 21 December 2011, as seen on a composite of MODIS true color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above; viewed using Google Earth). Across the Southern Plains, the highest storm total snowfall amounts (in inches) in Texas, Oklahoma, Colorado, and Kansas are highlighted on the image.

A comparison of AWIPS images of the 1-km resolution MODIS 0.65 µm visible channel and the corresponding MODIS false color RGB image created using the visible and “snow/ice” channels 01/07/07 (below) revealed the swath of snow cover (red on the RGB image) on the 16:40 UTC overpass of the Terra satellite. Note the darker red appearance along the far southeastern edge of the snow cover on the false color image — this is a signature of areas where there was a significant accrual of ice due to freezing drizzle. Near Pratt, Kansas (station identifier KPTT) the thickness of the ice accrual was around 0.25 inch. Since ice is a stronger absorber of radiation than snow cover at the 2.1 µm wavelength, this leads to a darker appearance on a single-channel MODIS Band 7 imagery.

MODIS 0.65 µm visible channel + MODIS false color RGB image

MODIS true color RGB image + MODIS false color RGB image

A comparison of a MODIS true color Red/Green/Blue (RGB) image with the corresponding MODIS false color RGB image of Hudson Bay, Canada on 15 December 2011 (above) revealed the following: (1) most of the northern and western portions of Hudson Bay were ice-covered (the ice appeared bright white on the true color image, and darker shades of red on the false color image) and (2) thee false color image made it easier to discriminate between the ice (darker red) and a diverging pattern of supercooled water cloud streets (white to cyan colored features) which were aligned in the direction of strong boundary layer winds in the wake of a cold frontal passage. The horizontal convective rolls that created these cloud streets may have also contained significant amounts of blowing snow.

AWIPS images of MODIS false color RGB images with overlays of METAR surface reports (below) showed that winds were gusting as high as 33 to 38 knots at stations along the western edge of Hudson Bay.

MODIS Red/Green/Blue (RGB) false color images

McIDAS images of GOES-13 0.63 µm visible channel data (below) showed that the strong winds were acting to move large portions of the ice, which according to the Canadian Ice Service was still fairly young “gray” (5-15 cm thick) to “gray-white” (15-30 cm thick) ice.

GOES-13 0.63 µm visible channel images