Category Archives: Multiple Cloud Layers

Central Valley fog underneath high clouds

MIMIC Total Precipitable Water, 1300 UTC on 30 December 2020 (Click to enlarge)

A large storm moving ashore in British Columbia (0900 UTC Map), shown above in MIMIC Total Precipitable Water (from this site), was accompanied by widespread high clouds over much of the Pacific Coast of the United States. The 1511 UTC image, below, shows GOES-16 “clean window” (10.3 µm) infrared imagery, with high clouds apparent.

GOES-16 “Clean Window” infrared imagery (10.3 µm) at 1511 UTC on 30 December 2020

Satellite-only detection of fog/low clouds will be challenged on this day by the abundance of high clouds that block the satellite’s view of low stratus decks. Indeed, the ‘Night Fog’ brightness temperature difference field, below, allows for only periodic glimpses of what is happening near the surface. There are indications of fog — but it is challenging even in the animation to determine the horizontal extent of the fog regions.

GOES-16 ‘Night Fog’ Brightness Temperature Difference (BTD, 10.3 µm – 3.9 µm), 1111 – 1516 UTC on 30 December 2020, along with surface observations of ceilings and visibilities

GOES-16 Low IFR Probability fields, below (note: GOES-17 IFR Probability fields are still undergoing testing in preparation for their being deemed operational) highlight two regions of visibility restrictions: One is off the coast of central California, and a another is a narrow ribbon of reduced visibilities in the Central Valley. This case highlights a strength of IFR Probability fields: You get a useful and consistent signal even if high clouds are blocking the satellite view of low clouds. This is because Rapid Refresh Model estimates of low-level saturation are incorporated into the Probability fields.

GOES-16 Low IFR Probability fields, 1116 – 1511 UTC, 30 December 2020, along with surface observations of ceilings and visibilities (Click to enlarge)

IFR Conditions over Virginia

GOES-16 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) and IFR Probability at 0901 UTC, along with 0900 UTC plots of surface ceilings and visibility (Click to enlarge)

GOES-16 IFR Probability fields, above, from 0901 UTC on 27 October 2020 (in a toggle with the Night Fog Brightness Temperature Difference), outline two regions of likely IFR conditions over Virginia and surrounding states. Southern Virginia shows restricted conditions, as does the high terrain along the spine of the Appalachians, extending up to Johnstown PA. (Surface observations largely agree with IFR Probability fields: IFR Probability is high where ceilings are low and visibilities are restricted; IFR Probability is low in regions where IFR conditions are not occurring).

Night Fog Brightness Temperature Difference fields (10.3 µm – 3.9 µm), struggle to identify regions of IFR conditions — elevated stratus (over western Virginia and western Maryland, for example) has a similar signal to fog over southeastern Virginia. There are also regions of high clouds above the low stratus over Maryland and Virginia surrounding middle Chesapeake Bay. In such regions, high IFR Probability is being driven by Rapid Refresh model predictions of low-level saturation.


Visible imagery (0.64 µm) toggled with the IFR Probability fields at 1301 UTC, below, show how IFR Probability fields can better discriminate between stratus and fog than visible imagery alone.

GOES-16 Band 2 Visible (0.64 µm) and IFR Probability, 1301 UTC on 27 October 2020 (Click to enlarge)

Use IFR Probability fields to identify regions that have low ceilings and reduced visibilities when Visible imagery (in the day) and Night Fog Brightness Temperature difference (at night). During the day time, this can be achieved by displaying both images at once.

GOES-16 IFR Probability underneath the Visible (0.64 µm) imagery, 1301 UTC, 27 October 2020 (with a transparency alpha of 0.9 applied; click to enlarge)

IFR Probability as a Heads Up in Indiana

Because IFR Probability includes both model information on low-level saturation and satellite information on low-level clouds, it can give an advanced warning on the development of fog when high clouds — at might occur behind departing convection, for example — prevent the satellite from viewing the low cloud beneath high clouds.  Consider the animation below, that shows IFR Probability fields along with surface observations of ceilings and visibilities plotted so that regions with IFR conditions can be identified.  IFR probability fields develop over northwest Indiana over the course of the night, especially over extreme northern Indiana where the east-west Indiana Toll road sits.

GOES-R IFR Probability fields, along with observations of ceilings and visibility, 0051 – 1321 UTC (every 10 minutes) on 3 August 2020 (Click to enlarge)

Compare the IFR Probability fields above to the Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) field below. The field below historically has been used at night to identify regions of low clouds/fog — regions that are cyan in the enhancement are low clouds, regions that are black are high clouds. As the sun rises, the Night Fog brightness temperature difference field will struggle to identify regions of low clouds (although a different enhacement can be used to highlight the low cloud regions after sunrise). The satellite is not able to view low clouds over northern Indiana until the high clouds move out. Thus, a distinct indication of low clouds over the Indiana Toll road lags the suggestion of low clouds given by IFR Probability fields.

GOES-16 ‘Night Fog’ Brightness Temperature Difference (10.3 µm – 3.9 µm), every ten minutes from 0051 to 1321 UTC, 3 August 2020 (click to enlarge)

Consider the toggle below of GOES-R IFR Probability, Night Fog Brightness Temperature Difference and the Nighttime Microphysics RGB at 0601 UTC on 3 August.. IFR Probability fields show a developing region of IFR conditions (in yellow) over extreme NW Indiana. The two satellite-only detectors show only a scant suggestiong that fog is present. (A similar scenario is occurring near the bootheel of Missouri)

GOES-R IFR Probability, Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), and Nighttime Microphysics RGB, 0601 UTC on 3 August 2020 (Click to enlarge)

At 0901 UTC, below, all three indicators of low clouds are in better agreement over northwest Indiana (and over the Missouri Bootheel) as high clouds move from those areas.

GOES-R IFR Probability, Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), and Nighttime Microphysics RGB, 0901 UTC on 3 August 2020 (Click to enlarge)

Fields at 0931 and 1001 UTC continue this trend of stronger signals in the regions initially highlighted at 0601 UTC by the IFR Probability fields.

GOES-R IFR Probability, Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), and Nighttime Microphysics RGB, 1031 UTC on 3 August 2020 (Click to enlarge)

By 1131 UTC, below, increasing solar reflectance over the eastern part of the domain has changed the signal in both the Nighttime fog and Nighttime microphysics RGB over northern Indiana and Ohio. The GOES-R IFR Probability signal however is maintained.

GOES-R IFR Probability, Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm), and Nighttime Microphysics RGB, 1131 UTC on 3 August 2020 (Click to enlarge)

IFR conditions over the Great Plains

Surface Analysis at 0900 UTC on 13 May 2020

Much of the central Plains of the United States was under easterly flow during the night on 12-13 May 2020, as suggested by the surface analysis above. Strong High Pressure over the Great Lakes and lower pressures over the Rocky Mountains drove that upslope flow. Consequently, IFR conditions were common over much of the Plains, as shown below.

The animation, below, of IFR Probability (overlain with surface observations and ceilings and visibilities) shows high probabilities. There is good correlation between regions with High IFR Probabilities — red and orange — and regions with low ceilings and reduced visibilities. The highest probabilities, dark red in the enhancement, occur where satellite data shows low clouds and where model data shows low-level saturation. Note that there is a trend towards less satellite information in the IFR Probability field with time over central Oklahoma and south-central Kansas because of developing convection.

GOES-16 IFR Probability fields, 0800-1136 UTC on 13 May 2020

The night time microphysics RGB, shown below, is also used to detect regions of low clouds; they typically appear yellowish at this time of year. The challenge with the RGB use in highlighting IFR conditions is two-fold: high clouds will mask the low cloud signal (as in the case of developing convection over Oklahoma, or with cirrus over Colorado and Nebraska). The RGB can also struggle to differentiate between fog and low stratus. Note also in this animation how the effect of the rising sun becomes apparent at the end of the animation over the eastern third of the image.

Night time Microphysics RGB, 0801-1136 UTC on 13 May 2020

In the toggle below, the signal of low clouds in the RGB, yellow/cyan over the Texas panhandle (for example) has an echo in the IFR Probability field. In the absence of high clouds, both products will show regions of low clouds. The Night Time Microphysics RGB gives extra information about the cloud type in regions where high clouds are blocking the satellite view of low clouds (the purple region over Colorado/Nebraska and the reddish regions over Kansas and Oklahoma). In contrast, the IFR Probabilty field in those regions gives no information on the character of the higher cloud; the IFR Probability field in those regions is flat (i.e., not pixelated), the hallmark of IFR Probability that is derived chiefly from model fields, so all an analyst can tell is that a high cloud is present.

GOES-16 IFR Probability and Night Time Microphysics RGB, 1001 UTC on 13 May 2020

IFR Probability with a frontal passage over Pennsylvania

GOES-16 ABI Band 2 (0.64 µm) Visible Imagery, 1000-1700 UTC on 11 May 2020

Frontal passage with extratropical cyclones will frequently be accompanied by low ceilings and reduced visibilities, that is, IFR conditions. A loop of visible imagery, above, suggests a surface cyclone (surface analyses are shown below) but it is difficult to determine from the imagery where low ceilings and reduced visibilities are present. Continue reading

How to tell at a glance that IFR Probability is model-driven: Pacific Northwest version

GOES-16 IFR Probability at 10-minute timesteps, 1051 to 1721 UTC on 6 February 2020 (Right click to enlarge)

The animation above, of GOES-16 IFR Probability, shows high IFR values in regions over western Washington and Oregon, and those high values correlate spatially very well with surface observations of low ceilings and reduced visibilities.b You will note a couple things in the animation: The field is mostly stationary, with slight adjustments on every hour. Those small changes reflect the change in the model fields (hourly Rapid Refresh) that are used to complement satellite estimates of low clouds in the computation of IFR Probabilities.

On this day, the satellite did not view many low clouds (some are apparent in southwestern Oregon). The flat field (vs. the more pixelated view over southwest OR, and also occasionally in the west-northwest flow coming in off the Pacific) suggests only model data are being used. The stepwise changes on the hour also suggest that.

The animation of the Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) field, below, has a signal that is consistent with the lack of observed stratus over the coastal Pacific Northwest. Note also how the Night Fog Brightness temperature difference field flips sign as the Sun comes up and the 3.9 µm signal becomes larger due to reflected solar radiance with a wavelength of 3.9 µm.

GOES-16 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) at 10-minute timesteps, 1051 to 1721 UTC on 6 February 2020 (Right click to enlarge)

GOES-16 views of the Pacific Northwest show fairly large pixel sizes. NOAA/CIMSS scientists have been creating GOES-17 IFR Probability for the past couple weeks, and this product will become available via an LDM field in the near future.

Note that GOES-17 IFR Probability products are available online at https://cimss.ssec.wisc.edu/geocat.

IFR Probability, Brightness Temperature Differences and Nighttime Microphysics RGB estimates of Fog

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), Nighttime Microphysics RGB and IFR Probability Fields, 1126 UTC on 17 December 2019. Also included are sky conditions, ceilings and visibilities in black or green.

Fog and low clouds were widespread over the eastern half of the United States on 17 December 2019. In this example over ArklLaTex the Brightness Temperature Difference suggests low stratus clouds over the region, and the Nighttime Microphysics shows a signal congruent with low clouds. Note, however, that observations over much of the region do not suggest IFR conditions are present. Accordingly, IFR Probability shows fairly low probabilities in this region, with values increasing to the north where visibilities decrease. IFR Probability fields screen out regions of elevated stratus because the Rapid Refresh model in this region does not suggest low-level saturation. Over northeast Oklahoma and northwest Arkansas, however, saturation at low levels is more likely and IFR Probabilities there are larger.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), Nighttime Microphysics RGB and IFR Probability Fields, 1126 UTC on 17 December 2019. Also included are sky conditions, ceilings and visibilities in green.

At the same time, high clouds overspread most of the east coast as a storm moved through the area. The high clouds prevent the satellite from seeing low clouds, so both the Brightness Temperature Difference and the Nighttime Microphysics RGB will not have a signal that comports with low stratus detection. However, IFR Probability includes a signal from the Rapid Refresh model if that model shows low-layer saturation in a region of multiple cloud layers; IFR Probability has a strong signal on this date over the east coast were clouds and fog are widespread. IFR Probability also shows IFR Conditions under the low clouds that the satellite does detect over eastern Ohio, and correctly notes the a region of higher ceilings over West Virginia, western Virginia and eastern Tennessee.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), Nighttime Microphysics RGB and IFR Probability Fields, 1126 UTC on 17 December 2019. Also included are sky conditions, ceilings and visibilities in green.

When high clouds are not present, there are different equally good ways to estimate low clouds, and that’s shown above. The Brightness Temperature Difference fields, the Nighttime RGB and the IFR Probability fields all tell a similar tale: Much of Iowa and regions to the south and northeast have low ceilings and reduced visibility.

Careful observers to the toggle note that the RGB has a different color over Wisconsin compared to Iowa. In part this is because the Brightness Temperature Field has values that are smaller over Wisconsin. A bigger driver of the color difference, however, is the 10.3 µm brightness temperature — the blue component of the Nighttime Microphysics RGB. Values are around -25º C over Wisconsin, and closer to -10º C over Iowa!

Nighttime Microphysics RGB and Band 13 10.3 µm Brightness Temperature, 17 December 2019

Fog detection when Cirrus is present: Southern Plains edition

GOES-16 Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) at 0906, 1006, 1106, 1206, 1306 and 1401 UTC on 8 November 2019 (Click to enlarge)

Consider the stepped 1-hour images of the ‘Night Fog’ Brightness Temperature Difference (10.3 – 3.9) field, above. (Here’s one with full temporal resolution) Extensive jet stream cirrus is preventing GOES-16 from seeing evidence of low clouds, and the satellite alone is therefore failing to identify regions of low ceiling and fog over much of Texas.  (The Nighttime Microphysics RGB that depends on the Night Fog brightness temperature difference to detect low clouds suffers from a similar shortcoming here : link )

When Cirrus covers an area of low clouds, GOES-R IFR Probability does allow for low cloud/fog detection because it incorporates low-level saturation information from the Rapid Refresh to delineate regions of fog.  Similarly, it can screen out regions of mid-level stratus that can appear as fog in both the Night Fog brightness temperature difference and the Nighttime Microphysics RGB: Note the region around Childress TX, for example, where IFR Probability is low, but where the Night Fog Brightness Temperature Difference has a strong signal in the presence of elevated stratus.  Regions around Midland are highlighted in IFR Probability — Rapid Refresh data there are correctly diagnosing low ceilings and reduced visibilities.  (Here’s IFR Probability with an animation at full temporal resolution).  Because Rapid Refresh model data input into the IFR Probability computation changes every hour, you will sometimes see a pulsing in the imagery.

Use IFR Probability to detect regions of low ceilings and reduced visibility when cirrus has overspread your area.

GOES-R IFR Probability at 0906, 1006, 1106, 1206, 1306 and 1401 UTC on 8 November 2019 (Click to enlarge)

 

Fog Detection when Cirrus is present

GOES-16 NightTime Microphysics, Night Fog Brightness Temperature Difference (10.3 µm – 3.9 µm) and IFR Probability at 1206 UTC on 4 November 2019, along with 1200 UTC observations of ceilings and visibilities (Click to enlarge)

Fog can be a challenge to detect from satellite for two reasons: High clouds can block the satellite’s view of low clouds, or low clouds (stratus) can be detected but the satellite cannot accurately determine the cloud base. The toggle above cycles between the Night Fog Brightness Temperature difference (10.3 µm – 3.9 µm), the Nighttime Microphysics RGB (the Night Fog Brightness Temperature Difference is the green component of that RGB), and GOES-R IFR Probability.

The Night Fog Brightness Temperature Difference can be used to detect low stratus clouds because the small water droplets that make up those clouds have different emissivities at 10.3 µm and 3.9 µm: water droplets do not emit 3.9 µm radiation as a blackbody would, but they do emit 10.3 µm radiation more nearly like a blackbody. Thus the amount of energy at 3.9 µm collected by the satellite is smaller than it would be above a blackbody emitter. The computation of the blackbody temperature associated with the collected energy assumes a blackbody emission. Because that’s not the case at all wavelengths above clouds made of water droplets, 3.9 µm brightness temperatures are colder than those computed from observed 10.3 µm energy.

The Night Fog brightness temperature difference field allows an observer to infer the presence of low clouds over the Pacific Ocean, in and around Portland OR on the Columbia River to the ocean, and also northward towards Puget Sound, and in the Willamette Valley between Portland and Eugene OR. There is also a signal over north-central Oregon that is likely unrelated to clouds — variable surface soil emissivity in that region is affecting this brightness temperature difference (The GOES-R Cloud Phase product and Cloud Mask both show no cloud signal in that region).

Note that a cirrus shield is apparent over the northeast half of the image. The cirrus covers most of Puget Sound in Washington, and the Brightness Temperature Difference field can therefore not give information about low clouds (except where breaks in the cirrus occur — such as in the Strait of Juan de Fuca).

The Nighttime Microphysics RGB is a way to identify low clouds — but its information is coming from the Night Fog brightness temperature difference and it has limitations that are similar to those with the brightness temperature difference, namely: what is going on under cirrus clouds, and how close to the ground are the low clouds that are detected? In the RGB, low clouds will have a yellowish hue (red — the split window difference and green — the night fog brightness temperature difference — make yellow). The RGB shows different values over Portland than over the Willamette Valley because the clouds are warmer over Portland (as detected by the clean window 10.3 as the blue part of the RGB) and the split window difference field (the red part of the RGB) shows larger values over the Willamette Valley.

IFR Probability shows a a strong and consistent signal from Portland south to Eugene, and also over the Pacific, and north of Portland into the southern Puget Sound. In these regions, Rapid Refresh Model data are showing the presence of low-level saturation, a good indicator that IFR conditions may be present. IFR Probability shows small values under the region of low clouds detected by satellite over the Strait of Juan de Fuca; the Rapid Refresh model there is not showing low-level saturation. There is also a small signal in the clear skies over north-central Oregon where a brightness temperature difference is occurring because of soil-driven emissivity differences.

Use IFR Probability to ascertain the likelihood of fog underneath cirrus decks, or to determined the likelihood of stratus decks reaching the surface (that is, if stratus is actually fog).

Fog in the Mid-South on August 8th

GOES-R IFR Probability, 1002 UTC on 8 August 2019, along with surface reports of ceilings and visibility (Click to enlarge)

GOES-R IFR Probability fields from 1002 UTC on 8 August 2019, above, show a region of high probabilities mostly colocated with surface observations of ceilings and visibilities.  IFR conditions are widespread from north of Memphis TN to southern Illinois.  IFR Probability is a fused product, using both satellite imagery to detect low clouds and Rapid Refresh model data to identify regions of low-level saturation.  Where both indicators are present (for example, over extreme western Kentucky and adjacent regions of southern Illinois and northwest Tennessee), IFR Probabilities are very high and IFR conditions are observed.  There are also regions where only model data can be used — because satellites are not detecting low clouds (because high clouds are blocking the view);  such a case is over northwest Arkansas, where IFR Probabilities are high (albeit not as high as over western Kentucky) and the field is not pixelated like a satellite image can be.  IFR Conditions are observed there as well.  (Click here for a screen shot from later in the morning of the Paducah National Weather Service office front page showing Dense Fog Advisories in Tennessee, issued by the Memphis Forecast Office).

The toggle below compares the IFR Probability field with the GOES-16 ‘Night Fog’ brightness temperature difference.  There is no consistent signal in the brightness temperature difference field to indicate fog on the ground.  The color enhancement for the brightness temperature difference is created so that teal to blue is positive.  Clouds made of liquid water droplets — such as stratus, or fog — will have a strong positive value because of the emissivity properties of small water droplets.  (That is, those droplets emit 10.3 µm radiation mostly as a blackbody would, but do not emit 3.9 µm radiation as a blackbody would).  Note in particular the very dark enhancement over northwest Arkansas (suggestive of cirrus there that will block any satellite view of low clouds) and the grey enhancement over west central Tennessee, north of Memphis, where mid-level clouds are similarly blocking a good view of the stratus at the surface.

GOES-16 IFR Probability and GOES-16 ‘Night Fog’ Brightness Temperature Difference field (10.3 µm – 3.9 µm) at 1002 UTC on 8 August 2019 (Click to enlarge)

The Nighttime Microphysics RGB can also be used to alert forecasters to fog — but a main component of that detection is the Night Fog brightness temperature difference field, and where the Night Fog brightness temperature difference fails to identify fog, as above, Nighttime Microphysics will as well, as shown in the toggle below.  Which color is useful for identifying fog in the RGB below?  Almost all of them!