FRAM_ellrod_Jun05 by xiaopangnv

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									 Analysis of Hazardous Fog
   and Low Clouds Using
Meteorological Satellite Data

           Gary P. Ellrod
     NOAA/NESDIS, Camp Springs, MD
         (Gary.Ellrod@noaa.gov)




             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                     Nighttime fog product from GOES
                               Sounder, June 1987
  – Cermak, Bendix


                     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




       Ellrod 2003                         Brown 1987
                     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 bottom-
right 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      Brightness count difference (GOES-7 Sounder) vs
                     fog depth estimated from PIREPs
  (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, 329-
  338.
• 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, 54-
  57.                 FRAM, Montreal, Que
                       15 June 2005

								
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