DATA DESCRIPTION

GOES Data:

• GOES West, 4 km half-hourly visible data
• June 29 - July 19, 1987 (Days 180 - 200)
• Observations ~ 18 & 48 minutes after GMT hour
• Data averaged on two grids centered around tower
  • " Big Grid" 0.33° x 0.33°
  • " Fine Grid" 0.11° x 0.11°

ERBE Data:

• Data from the ERBS satellite
• July 1 - July 19, 1987
• Used to produce narrowband - broadband albedo conversion
• Data averaged on standard 2.5° x 2.5° ERBE Grid

NARROWBAND-BROADBAND CORRELATIONS

• GOES data are averaged on the ERBE 2.5° grid
• ERBE data are matched to nearest GOES hourly data
• No data are used with SZA > 85°
• GOES counts, D, are converted to narrowband reflectance using the Whitlock radiance calibration:

  

  where

  

  is the cosine of the solar zenith angle and

  

  is the narrowband visible solar constant.
• Narrowband albedos are calculated using ERBE bidirectional models. A composite anisotropic correction is calculated based on the GOES angles and the ERBE scene fractions
• A regression of the form:

  

  where

  

  is performed between the broadband (ERBE) and narrowband (GOES) albedos shown in this scattergram of the albedos.
• The coefficients are from data in the region 105°W > longitude > 100°W and 45°N > latitude > 35°N:

  

  

  

  

  where

  

  rms of fit = 12.5%

DIRECTIONAL MODELS

A clear-sky directional model (a model of the variation of albedo with solar zenith angle) was produced for each of the regions of the "Big" and "Fine" grids

• Created using GOES data converted to broadband albedos
• For each local hour, the day with the lowest average visible counts for the entire region was selected as clear

• Data in the region 105°W > Longitude > 100°W, 45°N > Latitude > 35°N were used
• The albedos for each scene type were averaged for each local hour
• For a given time and set of angles, these models can be converted to equivalent GOES counts
• GOES observations are compared against these models to enable the selection of the proper anisotropic factor

CALCULATION OF ALBEDO TIME SERIES

• The average visible count, D, is calculated for each observation time in each region of the GOES grids
• Data with D < 8 counts or SZA > 85° are excluded
• The counts are converted to narrowband reflectances using the Whitlock radiance calibration:

  

  as above.
• The cloud amount is estimated by comparing the squared GOES counts to the average ERBE Clear, PC, MC, and Overcast directional models. This cloud amount is used to select the proper anisotropic factor
• The ERBE bidirectional models are used to convert reflectance to narrowband albedo
• The narrowband-broadband correlation is then applied to produce broadband albedos
• A sample albedo time series from the central region of the "Fine" grid can be found here
• ASCII text files of the time series can be accessed for either the
  • " Big Grid" or the
  • " Fine Grid"

  where each data record contains:

  1. Region number
  2. Day of the year
  3. GMT
  4. Latitude of center of grid box
  5. Longitude of center of grid box
  6. Solar zenith angle
  7. Satellite viewing zenith angle
  8. Azimuth angle (0° = backscatter)
  9. Mean GOES observed temperature
  10. Mean GOES observed visible count (8-bit)
  11. Number of GOES pixels
  12. Narrowband reflectance
  13. Narrowband albedo
  14. Derived broadband albedo
  15. Anisotropic factor
  16. Cloud amount used to select anisotropic factor