Monthly Archives: June 2015

Dense Fog over Iowa and Illinois

Front page of the National Weather Service in the Quad Cities, Monday morning 29 June 2015 (Click to enlarge)

Dense fog developed over the mid-Mississippi Valley early on Monday, 29 June 2015, and Dense Fog Advisories were hoisted by the DVN WFO, as shown above. How did the GOES-IFR Probabilities (and other products) capture this event? The animation below shows the evolution of surface visibilities at 0600, 0800 and 1000 UTC. IFR Conditions have developed by 0600 UTC and they subsequently expand.

Surface Visibilities (Statute Miles) over Iowa, 0600, 0800 and 1000 UTC, 29 June 2015. IFR Conditions are highlighted in white (Click to enlarge)

The Day Night band suggests clouds are present over parts of Iowa at 0707 UTC, but the waxing gibbous moon has set by 0848 UTC (below), and the lack of reflected moonlight at the later time precludes cloud detection. The brightness temperature difference (11.45µm – 3.74µm) from Suomi NPP can detect the tops water-based low clouds and it does confirm that the clouds have not vanished at 0848 UTC despite the lack of signal in the Day Night band. The brightness temperature difference field includes signals (black in the enhancement used) that suggest the presence of cirrus clouds.

VIIRS_DNB__REF_20150629_toggle_0707_0848

Visible Imagery from the Suomi NPP Day Night Band, 0707 and 0848 UTC on 29 June 2015 (Click to enlarge)

Suomi NPP Brightness Temperature Difference (11.45µm – 3.74µm) at 0707 UTC and 0848 UTC (Click to enlarge)

MODIS data can be used to compute IFR Probability fields. These fields are not available frequently, although they do present a high-resolution view of events when available. Two overpasses, at 0408 (Terra) and 0817 (Aqua), provided imagery early on 29 June. The MODIS data suggests the development of a large area of fog. What does GOES data show? Click here for a comparison of MODIS and GOES at 0408 UTC, and here for a comparison of MODIS and GOES at 0817 UTC). The chief difference between MODIS and GOES is somewhat higher values at the earlier time, and sharper edges (as might be expected given the resolution differences) at both times.

MODIS-based GOES-IFR Probabilities, 0408 and 0817 UTC on 29 June 2015 (Click to enlarge)

GOES-R IFR Probabilities computed from GOES-13 have a temporal resolution that allows for monitoring of fog development, and values increase rapidly after 0500 UTC over eastern Iowa, in accord with the development of IFR observations shown above. Brightness Temperature Difference fields (bottom) also suggest the development of low clouds over Iowa. However, there are places where high clouds prevent a signal and the rising sun (and its 3.9 µm radiation) mean the signal is reduced at the end of the animation. GOES-R IFR Probabilities maintain a coherent signal through sunrise.

GOES-based GOES-R IFR Probabilities, 0400-1215 UTC on 29 June 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm), 0400-1215 UTC, 29 June 2015 (Click to enlarge)

Parallel Lines in IFR Probability over the Great Plains

GOES-R IFR Probabilities over the Great Plains. Notice the north-south lines/artifacts in the field over Kansas and Nebraska (Click to enlarge)

GOES-R IFR Probability fields over the Plains can sometimes include structures as shown above. These are related to the sloped topography of the Great Plains. They occur because of interpolation between model layers and the lowest 1000 feet that are examined for saturation. In a sloping region, quick changes in saturation amount can occur where changing topography changes which model levels are used in the examination of those lowest 1000 feet.

In other words, satellite pixels that are very close horizontally may nevertheless have different surface elevations that cause different profile levels to be analyzed for the maximum relative humidity (RH). If the RH drastically changes at the bottom or top of the profile being analyzed then differences will emerge as shown above. Extra interpolation before the profile is analyzed may mitigate this issue and could be incorporated into the algorithm in the future.

Resolution Matters

GOES-R IFR Probabilities computed from MODIS data and from GOES-15 data, ~1000 UTC on 22 June 2015 (Click to enlarge)

GOES-R IFR Probability was created with an eye towards using data from GOES-R (currently scheduled for launch at the end of March 2016). GOES-R will have better spatial, temporal and spectral resolution than the present GOES. A benefit of better spatial resolution is shown in the toggle above between present GOES (nominal 4-km resolution — vs. the nominal 2-km resolution that will be on GOES-R) and MODIS (1-km resolution). The small valleys along the northern California coastline are far better resolved. The fog/low clouds over San Francisco bay is also better resolved (and the same could be said for the Salinas Valley, south of Monterey Bay if this scene were shifted slightly south). (You might notice a slight 1-pixel shift between MODIS and GOES-15 IFR Probabilities. GOES-15 navigation is compromised by the lack of star-tracking data, so MODIS data are probably better navigated.)

GOES-15 Brightness Temperature Difference (10.7 µm – 3.9 µm) and GOES-R IFR Probabilities, 1000 UTC on 22 June 2015 (Click to enlarge)

IFR Probabilities are derived from GOES-15 brightness temperature difference fields, and a benefit of the IFR Probabilities is obvious above. Brightness Temperature Differences can be driven by emissivity differences in soil. These false positives over Nevada (from the point of view of fog detection) are easily removed if the Model Data does not show low-level saturation.

Suomi NPP Day/Night band imagery, Brightness Temperature Difference Fields (11.45 µm – 3.74 µm), and 3.74 µm Image, 0921 UTC on 22 June 2015 (Click to enlarge)

Suomi/NPP’s early morning overpass also detected the presence of fog/low stratus over the valleys along the northern California coast. The Brightness Temperature Difference field shows things distinctly. The Day-Night Visible imagery shows little in the way of fog on this day, as the waxing crescent moon had already set so no lunar illumination was present. The Day Night band is included here because it shows a very bright wildfire south of Lake Tahoe. That feature is also present in the 3.74 µm imagery. Fog and stratus is also evident in the 3.74 µm imagery, detectable based on its very smooth appearance.

IFR Conditions over eastern Massachusetts and the Cape

GOES-R IFR Probability, 0315 through 1215 UTC on 22 June 2015 (Click to enlarge)

Sea fog penetrated inland over eastern New England overnight. How did GOES-R IFR Probabilities depict the event, and how did those fields compare to the traditional fog-detection method, brightness temperature differences between the 10.7 µm and 3.9 µm channels? The animation above shows the IFR Probabilities, and they neatly outline the regions of low ceilings and reduced visibilities.

In contrast, Brightness Temperature Difference fields, shown below, are troubled by two different factors in these loops. Around 0700-0800 UTC, thin cirrus over southern Cape Cod impedes the satellite view of low clouds (Click here for a toggle between the two fields at 0722 UTC); brightness temperature difference fields yield little information when that happens. (GOES-R IFR Probability values drop when the Satellite component cannot be used; make certain when interpreting the values that you are aware of the presence/absence of high clouds!) In addition, the brightness temperature difference field loses features around sunrise, when solar radiation with a wavelength around 3.9 µm increases. GOES-R IFR Probability fields maintain a coherent signal through sunrise, however.

Careful inspection of the animation above does reveal some stations where IFR Conditions occur and IFR Probabilities are low. For example, Lebanon NH in the Connecticut River Valley reports IFR Conditions intermittently. Small-scale valley fog is a challenge for both GOES detection and for Rapid Refresh detection.

GOES-13 Brightness Temperature Difference Fields (10.7 µm – 3.9 µm), ~0315-1215 UTC on 22 June 2015 (Click to enlarge)

Dense Fog over eastern Maine

GOES-R IFR Probability Fields, hourly from 0315 to 1215 UTC on 10 June 2015 (Click to enlarge)

A cold front that moved across Maine early in the morning on 10 June 2015 was accompanied by dense fog. Dense Fog Advisories were hoisted over eastern Maine (screenshot from weather.gov; screenshot from the Caribou ME National Weather Service). The hourly imagery of IFR Probabilities, above, showed high probabilities over eastern Maine (where surface observations are scant). Note how the back edge of the high IFR Probabilities, after the frontal passage, correlates will with the timing of rising ceilings and reduced visibilities. This occurs at Augusta (KAUG), Bangor (KBGR) and Millinocket (KMLT), for example.

The traditional method of detecting fog and low cloud, the brightness temperature difference field that compares values at 10.7 µm and 3.9 µm , had difficulty indicating regions of fog for two reasons on the morning of 10 June. An animation of the product is below. There were high clouds present that interfered with the satellite’s view of low clouds. (IFR Probability can still give a useful signal in this case because of information that comes from the Rapid Refresh Model). In addition, the ever-earlier sunrise in early June supplies enough 3.9 µm radiation at the end of the animation to flip the sign of the brightness temperature difference field and the distinct signal of low water-based clouds is lost.

Brightness Temperature Difference Fields (10.7 µm – 3.9 µm), hourly from 0315 through 1215 UTC on 10 June 2015 (Click to enlarge)

Lead Time with GOES-R IFR Probabilities and Brightness Temperature Difference

A small region of dense fog developed over northeast Colorado and western Nebraska during the early morning on June 1st 2015. How did the GOES-R and traditional products handle this event? The animation below shows IFR Probabilities from 0730-0800 UTC on 1 June. Probabilities jump from <10% to about 20% at 0800 UTC in a region centered on Holyoke, CO, just south of I-76 in northeast Colorado. The Brightness Temperature Difference Field for the same 3 times, below the IFR Probabilities, shows a signal moving over the region but not substantially changing. (From this, one could conclude that the Rapid Refresh model data might be driving increase in the IFR Probability field)

GOES-R IFR Probabilities, 0730-0800 UTC on 1 June (Click to enlarge)

GOES-East Brightness Temperature Difference fields (10.7 µm – 3.9 µm) at 0730, 0745 and 0800 UTC, 1 June 2015 (Click to enlarge)

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 0915 UTC on 1 June (Click to enlarge)

By 0915 UTC (above), IFR Probabilities and GOES-13 Brightness Temperature Difference fields show a strong signal over NE Colorado where IFR Conditions occur/are developing. IFR Probability fields have provided more lead-time in the development of this region of low ceilings and visibilities. By 1100 UTC (below), a stronger, more widespread signal is apparent in both fields. At 1230 UTC (bottom), the rising sun has altered the brightness temperature field so it gives no useful information on low clouds; this highlights an advantage of GOES-R IFR Probability fields: A consistent signal through sunrise.

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 1100 UTC on 1 June (Click to enlarge)

Toggle between GOES-R IFR Probabilities and GOES-East Brightness Temperature Difference Fields, 1230 UTC on 1 June (Click to enlarge)

GOES-R Fog Product Examples

Fog detection fusing GOES, Terra/Aqua or Suomi/NPP Satellite data with Model output