Fog Monitor Overview. Theory and Method Localization Customizing Your Monitoring Area Customizing Your Monitor Thresholds The Fog Monitor Customizing Your Display Thresholds The Fog Monitor Display. Fog Monitor Overview Theory and Method. separate presentation by Qin Zeng.
Fog monitoring is a kind of object-oriented processing
All we have to know is how to describe a fog object: its properties and its behaviors
Spectral feature of fog in multiple satellite channels
1. Scattering radiation feature (Daytime)
(a) VIS Channel
(b) 10.7 (3.9) μm Channel
2. Emitting radiation feature (Nighttime)
(a) 3.9 μm Channel
(b) 10.7 μm Channel
Kinematic feature of fog
size vs wavelength
According to Mie scattering theory:
In the VIS channel, reflectivity mainly is related to the depth of the cloud/fog.
In channel 3.9µm, fog/stratus has maximum scatter rate with its size equivalent to 3.9 µm.
Data Normalization of the VIS satellite data
Assumed that all objects are black bodies:
so TBemit3.9 = TB11 = Treal , and according to the Planck radiation law,
This is how reflective product of RAMSDIS ONLINE is generated . Because of the assumption of black body and TBemit3.9 = TB11 , it is an approximated reflective product and it does not take the angular correction into consideration
So in our algorithms, it is not adopted for fog detection but for sea ice/snow cover exclusion.
Allen, R.C., P.A. Durkee, and C.H. Wash, 1994: Snow-cloud discrimination with multispectral satellite images. J. Appl. Meteor
VIS (0.4 - 0.7μm)
3.9 μm IR
10.7 μm IR
Radiation received by satellite
( Mix together)
0 means “can be ignored” 1 means “major”
Without the presence of sun light, in fog region, a positive value with
[10.7 μm channel - 3.9 μm channel]. This difference is caused by the emissivity difference of fog between 10.7 um and 3.9 μm.
Above will be good only during the night time:
Other water/ice clouds like AC/AS also look similar to Fog in fog product . But in 10.7 μm channel , the brightness temperature of fog is higher than that of higher water/ice cloud (AC/AS). This can in a degree help distinguish them.
R(satellite)= R(ground) - R(absorbed) + R(cloud)
Different emissivities of different cloud types
Absorptivity is related to emissivity and the depth of the cloud/fog
Planck radiation law
Thin cloud (night) : TB(10.7 μm ) - TB(3.9 μm ) < 0
Thick cloud (night) : TB(10.7 μm ) - TB(3.9 μm ) ~ 0
Fog (night) : TB(10.7 μm ) - TB(3.9 μm ) > 0
ice cloud (night) : TB(10.7 μm ) - TB(3.9 μm ) < 0
Water/ice cloud (night) : TB(10.7 μm ) - TB(3.9 μm ) > 0
Comment: So fog product should be checked to eliminate other higher water/ice cloud (AC/AS) by using TB derived from 10.7 μm channel.
Sea Fog Discrimination Chart from Gary Elrod
use 3.9 and 10.7 um infra-red images to produce night time fog product.
Apply feature extraction to identify fog area based on night time fog product thresholdSAFESEAS Fog Monitor
Smoothness = (1- deviation/mean) * 100%
X denotes the gray value at the VIS pixel
and n is the number of pixels
At very low solar elevation, in the clear air region , there will still be small brightness values. After normalization, they will be enhanced to look like fog areas and will be detected by the Fog Monitoring Processor.
Radiation from the ground goes through thin cirrus/high cloud ,which makes cirrus look like fog in VIS
Use fractal dimension to try to reduce the detection of the irregular area
This is still not a good solution but supplied as an optional filter for user
FD = 2ln(P/4)/ln(A)
P= perimeter A= Area
Application of Fractal Dimension to filter out irregular area
An office’s area of responsibility (AOR) will consist of all the zones and counties for which it normally has the responsibility of issuing forecasts, watches, and warnings.
An office’s monitoring area (MA) will consist of its AOR plus the AOR of each adjacent office.
Channel 2 refers to the 3.9 micrometer infra-red satellite image.
Channel 4 refers to the 10.7 micrometer infra-red satellite image.
The fog product is the image derived by subtracting 10.7 micrometer image brightness temperatures from 3.9 micrometer image brightness temperatures.
* which zones and counties are included in the MA.
* (OB-6) which fixed stations are in the MA.
* (OB-6) in which zone/county is each fixed station located.
For the alpha test, the default MA will be based on the same “shape” files used for OB-4 SAFESEAS. It is the responsibility of each office to make sure its systems have the current versions of these files before localization is run.
The Fog Monitor will provide a GUI for customizing the MA. Specifically, the office will be able to:
The Fog Monitor will provide a GUI that will enable offices to
customize the range of:
The customization GUI will also allow offices to enable/disable filters in the
fog recognition algorithm:
In OB-6, a GUI will also be provided that will enable offices to customize the thresholds used for monitoring point observations (METARs, C-MAN reports, ship reports, buoy reports, and mesonet reports) for fog. We will “negotiate” details for this tomorrow!
The fog monitor will be a persistent background process, much like the SAFESEAS, SCAN, and
FFMP processors. The alpha test version will monitor:
In all cases, channel 2 images are used for certain kinds of filtering (unless the office has disabled
The OB-6 fog monitor will also monitor point observations. The details of this will be “negotiated”
The fog monitor will determine a fog threat level for each zone/county in the MA, and an overall
fog threat level for the entire MA. The threat level is the most severe of all the individual pixel
(OB-6: and point observation fog) threat levels for the past two hours.
The fog monitor generates a fog threat level image and a zone fog threat file for use by the Fog
Monitor display. Finally, the Fog Monitor threat level indicator and the Fog Monitor display are
signaled to update themselves.
* black = this location is not within the MA.
* gray = insufficient data is available for determining
the fog threat level for this location.
* green = there probably is not fog at this location.
* yellow = there may be fog at this location.
* red = there probably is fog at this location.
* only reports from within the MA are plotted.
* variables to be plotted will be “negotiated” tomorrow.
* two columns: zone ID and fog threat level (color).
* click column header to sort by that column.
* except for variables, same as SAFESEAS.
* variables to be included will be “negotiated” tomorrow.