Nighttime Fog Monitoring

Satellite fog monitoring during the nighttime can be a challenge since geostationary datasets are limited to infrared imagery. However, with the new GOES-16/17 and JPSS datasets users can employ polar-orbiting and geostationary imagery to identify and monitor areas of fog and low stratus (a.k.a liquid water clouds). As meteorologists, we know that fog can significantly reduce ‘near-surface’ visibilities affecting aviation and shipping industries along with the general public. Below is a static comparison over eastern Kansas and western Missouri highlighting the SNPP – VIIRS NCC, the GOES-16 Night Fog Difference and Nighttime Microphysics RGB products; all imagery has hourly METAR surface observations overlaid (i.e. shown in green). Note the hourly surface observations are at 0800Z, 24 April 2019, while satellite observations are at ~0747Z, 24 April 2019.

SNPP VIIRS Near-Constant Contrast (NCC) at 0747Z, 24 April 2019

NCC, a derived product of the Day/Night Band (DNB), illuminates atmospheric features and can sense emitted light sources (i.e. city lights) and reflected light sources (i.e cloud cover) during the nighttime. NCC is known as ‘nighttime visible’ imagery that appears similar to 0.64μm daytime visible imagery. In the imagery below, NCC observes emitted lights from cities and towns that reside along the interstates and in rural areas of Kansas and Missouri. Various levels of cloud cover encompass south-central and eastern Kansas along with western Missouri, where areas of fog are not conspicuous without the assistance of surface observations (i.e. fog indicated by parallel, horizontal green lines). In contrast, in northwestern Kansas, NCC observes fairly clear skies.

 

GOES-16 Night Fog Difference (10.3μm- 3.9μm) at 0746Z, 24 April 2019

Using an approximate time stamp, the GOES-16 Night Fog Difference is utilized. The Night Fog Difference product employs a channel difference of the 10.3μm Brightness Temperatures (BT) minus the 3.9μm BT to identify the fog and low stratus. Liquid water clouds are depicted as positive Brightness Temperature Differences (BTD) (i.e. seen in blue in the imagery below) since liquid water droplets do not emit radiation at 3.9μm but do at 10.3μm; employing the channel difference computes to a positive BTD. Conversely, ice crystals that are embedded in high clouds exhibit a negative BTD (i.e. in grey, refer to the bottom-right of the image).

Note there is an ellipse in western Kansas that observes positive BTD indicating fog. But is it really fog or low stratus we are seeing? The answer is ‘No’. Remember, the NCC product (above) observed clear skies in this area, where the surface observations validate the NCC. This is a false alarm that is produced by the Night Fog Difference product, and it is critical for users to validate this product with surface observations.

 

GOES-16 Nighttime Microphysics RGB at 0746Z, 24 April 2019

For further differentiation between fog and other types of clouds, look to the Nighttime Microphysics RGB (seen below) that employs the 10.3μm-3.9μm BTD and the 12.4μm-10.4μm BTD. Notice within the same ellipse in western Kansas the RGB observes clear skies (light pink) similarly to NCC and the surface observations.

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NCC monitoring severe weather during the nighttime

Monitoring severe weather during the nighttime can be challenging since GOES-16/17 is limited to infrared imagery during the overnight hours. In complement to geostationary data sets, polar-orbiting satellite data can be utilized, specifically the Near-Constant Contrast (NCC) product.

For unfamiliar readers, NCC is a derived product of the Day/Night Band (DNB) that utilizes a sun/moon reflectance model that illuminates atmospheric features and senses emitted (e.g. lights from lightning, fires, city lights) and reflected (e.g. clouds) light sources during the nighttime. The product is considered ‘nighttime visible’ imagery that looks very similar to 0.64μm visible imagery that forecasters use during the daytime. Now NCC also has its limitations, since it is dependent on the lunar phase (i.e. full moon compared to new moon) and moon elevation angle (i.e. the moon position above or below the horizon). NCC imagery can range in texture, varying from ‘crisp and clear’ imagery to ‘fuzzy and non-conspicuous’ imagery depending upon the lunar phase and moon elevation angle. NCC is at 0.7µm, exhibiting a 750-m spatial resolution.

NCC observed severe weather over the southern United States during the early morning hours of 18 April 2019. Severe weather was experienced in several states: Texas, Oklahoma, Arkansas, Louisiana and Mississippi. The NCC and GOES-16 infrared imagery (seen below) observed severe weather in the forms of convective cloud tops (i.e. very cold brightness temperatures), lightning, cloud cover and emitted lights from cities.  Imagery is taken at ~0800 UTC on 18 April 2019, where NCC imagery is seen during the full moon phase of the lunar cycle. Notice in the NCC, the lightning that is observed via horizontal white streaks. The white streaks are due to the time discontinuity between the lightning strike (i.e. on the order of milliseconds) and the satellite overpass (i.e. on the order of seconds).

NCC at 0759 UTC, 18 April 2019 – Nighttime Visible Imagery

GOES-16 10.35μm at 0801 UTC, 18 April 2019 – Infrared imagery

The Geostationary Lightning Mapper (GLM) (seen below at the same timestamp) is also used in complement to NCC, in identifying where the high density lightning strikes are observed within the line of storms (red dots); it matches up quite well with NCC. Note, GLM is overlaid onto GOES-16 10.35μm.

GLM at 0801 UTC, 18 April 2019 – Group Flash Counts Density (via CIRA SLIDER)

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High Plains Snowstorm

A strong extratropical cyclone moved through the Rocky Mountains and western high plains over the course of 10 April 2019. The low-pressure system produced heavy precipitation in the forms of rain and snow, along with blustery winds.

The system produced heavy snow over a large areal extent spanning from Colorado, Wyoming, portions of Nebraska and South Dakota. Below are surface observations captured at ~20 UTC, 10 April 2019. Note the areas of snow designated by pink asterisks, where the increase in the number of asterisks corresponds with the increase in snow intensity. Northerly and northeasterly high winds gusting over 40 mph were observed as well.

Surface Observations – at 1958 UTC, 10 April 2019

In complement to the surface observations are microwave observations from polar-orbiting satellites, notably, in the form of the Blended Snowfall Rate (SFR) product exhibiting a liquid equivalent snowfall rate. Image below is a static SFR product from the Advanced Technology Microwave Sounder (ATMS) instrument on-board the Suomi-National Polar-orbiting Partnership (S-NPP) satellite. Note high liquid equivalent snowfall rates are observed in north-central Colorado, southeast Wyoming, north-central Nebraska and South Dakota, where heavy snowfall rates in South Dakota ranged from 1.5-4 mm/hr or 0.06-0.16 inches/hr. Another benefit of SFR is the product can observe snowfall rates over large domains, where in comparison to radar, radar coverage can be limited due to data gaps or beam blockages.

S-NPP ATMS -Liquid equivalent snowfall rate at 2020 UTC, 10 April 2019.

In the following two images, notice the National Weather Service (NWS) ‘preliminary’ high snow totals observed over western South Dakota and north-central Colorado are located over the same domains as the high snowfall rates observed by the SFR product.

NWS Preliminary Snow Totals as of o1oo UTC, 11 April 2019 over Western South Dakota

 

NWS Preliminary Snow Totals as of o1oo UTC, 11 April 2019 over the Front Range of Colorado

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2-3 April 2019 East Coast Low – ALPW analysis

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Nebraska flooding

The past two weeks Nebraska has been inundated with heavy precipitation, in the forms of rain and snow. Nebraska was significantly affected by the ‘record-breaking’ mid-latitude cyclone that past through the area from 13-15 March 2019.  Refer to the GOES-16 10.3um infrared satellite imagery below, seen from 5Z, 13 March 2019 to 22Z, 14 March 2019. Throughout the animation, notice the large areal extent of the cyclone and the cold convective cloud tops (i.e. cold brightness temperatures) indicating varying levels of precipitation in Eastern Nebraska.

 

From this storm, plus subsequent storms thereafter, extreme flooding has transpired throughout eastern Nebraska. The image below displays the observed precipitation values over Nebraska the last 14 days, valid at 12Z, 22 March 2019. Product is provided from the National Weather Service (NWS) – Advanced Hydrologic Prediction Services (AHPS). Precipitation values ranged from 1-5 inches throughout Nebraska.

Coinciding with the precipitation values is polar-orbiting satellite data that observes the magnitude of flooding via Suomi-National Polar-orbiting Partnership (S-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) Flood Areal Extent product. The product (seen in animation below) shows the areas of flooding (i.e. yellow, orange, red colors) in Eastern Nebraska and Iowa between 15-21 March 2019. 17 and 19 March were omitted due to cloud obscuration observed over the area. VIIRS Flood Areal Extent calculates the floodwater fraction percentage of a pixel (i.e. from 0-100%, green-to-red colors), and discriminates between different scene types. Note in the legend: LD = land (brown), SI = supra/snow ice (mixed water and ice, or water over ice, seen in purple), IC = ice (river or lake ice, seen in aqua), CL = clouds (grey), CS = cloud shadows (dark grey), and WA = open water (blue). Spatial resolution of the product is at 375-m resolution. Within the animation, see the evolution of the floodwaters as they increase in width or move along the rivers.

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