Transcript Detection of Hazardous Fog at Night Using Data From

Analysis of Hazardous Fog
and Low Clouds Using
Meteorological Satellite Data
Gary P. Ellrod
NOAA/NESDIS, Camp Springs, MD
([email protected])
FRAM, Montreal, Que
15 June 2005
Outline
• Benefits/limitations of remote sensing
• Detection of low clouds
– Night: Longwave – Shortwave IR
– Day: Visible and Shortwave IR
•
•
•
•
Determination of low ceilings
Fog depth estimates
Technology upgrades needed
Summary
FRAM, Montreal, Que
15 June 2005
Nighttime GOES Infrared Fog
Detection Capabilities
• Advantages:
– High frequency (15-30 min)
– Good spatial coverage, resolution (4km)
• Limitations
– Obscuration by higher clouds
– Some fog too narrow, thin to detect
– False signatures (sandy soils)
– Is it fog or stratus?
FRAM, Montreal, Que
15 June 2005
Remote Sensing of Fog
• Radiative studies
(Hunt 1973)
• Experience with
AVHRR in U.K. (Eyre
et al 1984)
• GOES investigations
– Gurka 1978, 1980
– Ellrod 1991, 1994
– Lee (NRL) et al 1997
• METEOSAT
– Cermak, Bendix
Nighttime fog product from GOES
Sounder, June 1987
FRAM, Montreal, Que
15 June 2005
Radiative Properties of Clouds
FRAM, Montreal, Que
15 June 2005
Nighttime Fog Detection
Using GOES Multi-spectral Image Data
FRAM, Montreal, Que
15 June 2005
Features Observed in Nighttime Fog Images
Yellow = T4 – T2 > 2C
FRAM, Montreal, Que
15 June 2005
Fog-related Highway Accident
Windsor, Ont., 3 Sep 1999 (Pagowski et al 2004)
FRAM, Montreal, Que
15 June 2005
Spread of Lake Fog – Time Lapse
FRAM, Montreal, Que
15 June 2005
Daytime Fog Detection
• Visible images
– Smooth texture, sharply defined borders,
moderate brightness
• 3.9 mm IR (or 1.6mm AVHRR)
– Fog droplets are good reflectors at 3.9mm
• Result is relatively warm Tb
– Snow is poor reflector at 3.9mm
– Result: Good contrast with snow or cold
ground
FRAM, Montreal, Que
15 June 2005
Fog Clearing on 3 Sep 1999
FRAM, Montreal, Que
15 June 2005
Snow vs Fog
Using Visible and Shortwave IR
MODIS Visible CH1
MODIS 1.6mm CH6
FRAM, Montreal, Que
15 June 2005
Snow vs Fog
Using Visible and Shortwave IR
MODIS Visible CH1
MODIS 3.9mm CH6
FRAM, Montreal, Que
15 June 2005
RGB Depiction of Fog Over
Snow-Covered Ground (MODIS)
Red = Visible
Green= 1.6mm
Blue= 11mm IR
Fog is yellow
Snow is red
Bare surface
is green
FRAM, Montreal, Que
15 June 2005
Daytime Fog Discrimination
Using Visible and IR Data
FRAM, Montreal, Que
15 June 2005
Estimation of Low Cloud Base
Category from GOES
• When GOES IR cloud top is <4º K from surface
temperature, low clouds (<1000 ft) likely
Brown 1987
Ellrod 2003
FRAM, Montreal, Que
15 June 2005
Low Visibility Determination
FRAM, Montreal, Que
15 June 2005
GOES Low Cloud Base Product
Available for all regions of the U. S. and parts of southern Canada at:
http://www.orbit.nesdis.noaa.gov/smcd/opdb/fog.html
FRAM, Montreal, Que
15 June 2005
Verification of LCB Product *
Overall verification for low clouds detected but
not covered by cirrus clouds (N = 2381):
• POD = 72 %
• FAR = 11 %
Regional Statistics
* Completed in 2001-2002
San Francisco Fog Project (Terabeam Inc, 2001)
GOES Ceiling Categories
Categories created to compare satellite data with ceilometer data.
FRAM, Montreal, Que
15 June 2005
San Francisco Fog Project (Terabeam)
Brightness values plotted against ceilometer ceiling heights. Top-left and bottomright quadrants (separated by dashed lines) show category 1 and 2 agreement,
respectively. Top-right shows false alarms, bottom-left shows under-detection.
Estimation of Fog Depth
• Based on BTD
for 3.9mm and
10.7mm IR
• Developed
using cloud
top heights
from aircraft
pilot reports
(PIREPs)
Brightness count difference (GOES-7 Sounder) vs
fog depth estimated from PIREPs
FRAM, Montreal, Que
15 June 2005
Fog Depth Verification
FRAM, Montreal, Que
15 June 2005
Fog Depth Product – 3 Sep 99
FRAM, Montreal, Que
15 June 2005
Fog Depth Estimation
• Application of fog depth to forecast burnoff time
GOES Fog Depth, 1045 UTC
FRAM, Montreal, Que
15 June 2005
Results for 3 Sep 99 Case
GOES Fog Depth, 1045 UTC
GOES visible, 1415 UTC
FRAM, Montreal, Que
15 June 2005
Visible Brightness Differences
Fog vs Cloud-Free to Estimate Clearing Time
• Requires visible (CH1) imagery >1.5
hours after sunrise (Gurka 1974)
– Uses following data:
• Digital brightness count difference (fog vs
clear region)
• Obtain incoming solar radiation
– Larger brightness difference = longer
clearing time after sunrise
FRAM, Montreal, Que
15 June 2005
Depth Threshold for GOES Detection
270 m
~160 m
~100 m ?
FRAM, Montreal, Que
15 June 2005
Technology Upgrades
Needed for Better Fog
Detection from GOES
FRAM, Montreal, Que
15 June 2005
1. Optimal SWIR wavelengths
FRAM, Montreal, Que
15 June 2005
2. Improved Resolution
Based on AVHRR IR (3.7 mm and 11.0 mm)
FRAM, Montreal, Que
15 June 2005
3. Improved Signal to Noise
MODIS Fog Depth
GOES Fog Depth
FRAM, Montreal, Que
15 June 2005
Summary and Conclusions
• GOES can effectively detect fog/low
clouds and show areal extent
– Problems with small scale, shallow fog
• Able to estimate fog depth, ceilings
– Good correlation with SFO visibility data
• GOES needs to be complemented by
surface data to be most effective
• GOES-R will have major upgrades
http://www.orbit.nesdis.noaa.gov/smcd/opdb/fog.html
FRAM, Montreal, Que
15 June 2005
References
• Hunt, G. E., 1973: Radiative properties of
terrestrial clouds at visible and IR thermal
window wavelengths. QJRMS, 99, 346-369.
• Eyre, J. R., J. L. Brownscombe, and R. J.
Allam, 1984: Detection of fog at night using
AVHRR imagery. Meteor. Mag., 113, 266-271.
• Ellrod, G. P., 1994: Advances in the detectio of
fog at night using GOES multispectral IR
imagery, Wea. Forecasting, 10, 606-619.
• Pagowski, M., I. Gultepe, and P. King, 2004:
Analysis and modeling of an extremely dense
fog event in Southern Ontario. J. Appl.
Meteor., 43, 3-16.
FRAM, Montreal, Que
15 June 2005
References
• Brown, R., 1987: Observations of the structure of
a deep fog. Meteorological Magazine, 116, 329338.
• Ellrod, G. P., 2002: Estimation of low cloud base
heights at night from satellite infrared and surface
temperature data. Nat. Wea. Digest, 26, 39-44.
• Fischer, K. et al, 2003: Validation of GOES Imager
experimental low cloud data products for
terrestrial free space optical telecommunications.
12th AMS Conference on Satellite Meteor. and
Oceanography, Long Beach, California, 9-13 Feb
2003.
• Gurka, J., 1974: Using satellite data for
forecasting fog and stratus dissipation. Preprints,
5th Conf. on Weather Forecasting and Analysis,
March 4-7, 1974, St. Louis, MO, AMS, Boston, 5457.
FRAM, Montreal, Que
15 June 2005