The CO2 slicing technique has been used to process cloud parameters with GOES/VAS data for four years prior to the start of the NOAA/HIRS analysis reported here (see Wylie and Menzel, 1989, and Menzel et al., 1992). The GOES/VAS algorithm uses the same equations as shown in Appendix A. However, there are differences in the data and methods used in handling of the data. (1) The GOES/VAS has three bands in the CO2 absorption region of the spectrum whereas the HIRS has four bands; the VAS does not have the 13.7 micron band which has additional mid-level sensitivity. (2) The GOES/VAS FOV (10 X 10 km) is smaller than the HIRS FOV (20 X 20 km); the VAS algorithm averages three FOVs for each cloud determination representing a 300 km-squared observation area and a sample noise of roughly 0.8 mW/m2/ster/cm-1, while the HIRS algorithm uses a single FOV representing a 400 km2 observation area with sample noise roughly 0.2 mW/m2/ster/cm-1. (3) Surface topographic heights are used in the HIRS algorithm, while they are not in the VAS algorithm; the surface pressure in Equation (A5) of Appendix A is determined from topography for the HIRS solutions while all VAS solutions assume a surface of 1000 hPa. This affects the VAS results over the Rocky Mountains by 50 hPa or less. (4) A separate sea surface temperature analysis is used in the HIRS processing while the VAS processing uses the same MRF model surface temperature analysis both over land and water. The VAS land surface temperatures are corrected using the SVCA hourly reports, whereas the HIRS are not. This will affect the determination of low cloud with the window band, but it will have almost no effect on CO2 algorithm solutions since only one of the bands sees the ground.
Single FOV comparisons of HIRS and VAS cloud parameter determinations are attempted. For several days during the 1986 FIRE (First International Satellite Cloud Climatology Project Experiment) in transmissive cloud conditions, over 100 collocated single FOV observations within 15 minutes were accomplished. CO2 slicing cloud heights determined by the HIRS are 20 hPa larger (lower altitude) on the average than those from the VAS in the single FOV comparison; effective emissivities average 0.05 higher on the VAS than the HIRS. Fluctuations are on the order of 100 hPa for cloud top pressure and 0.30 for effective emissivity. In this single FOV study, the VAS and HIRS cloud parameters compare within the estimated errors (Menzel et al., 1992). However, these results, while reassuring, must be viewed with some caution. Collocation over the same cloud element is very difficult as the satellite sensors have different FOVs, viewing times, and viewing angles.
A more meaningful comparison of HIRS and VAS cloud analyses is the seasonal average of the frequencies of cloud observations over North America for the four years of the HIRS study (June 1989 to May 1993). Table C1 shows the summary of the four winter seasons (December, January, February) and four summer seasons (June, July, August) of the HIRS cloud observations covering the North American region (29 N to 49 N and 70 W to 130 W) as well as the corresponding seasonal summary of the VAS over the same area and time period. Both HIRS and VAS find roughly the same amount of seasonal cloud cover (70% HIRS and 73% VAS in the summer and 76% HIRS and 78% VAS in the winter). However, HIRS reports more tranmissive cirrus in both winter and summer, 45% HIRS to 33% VAS in winter and 35% HIRS to 27% VAS in summer. In particular, HIRS has 10 to 15% more mid-level (400 to 700 hPa) transmissive observations than the VAS. On the other hand VAS has 10% more low level opaque cloud observations. This is caused primarily by the larger noise in the VAS sensor which inhibits CO2 slicing solutions for smaller cloudy versus clear radiance contrasts and identifies the cloud as low opaque in the infrared window solution.
The geographical distribution of the probabilities of high clouds above 500 hPa in each season computed from VAS and HIRS are presented in Figure C1. The cloud cover patterns are similar. In the winter, both the HIRS and VAS show higher probability of cloud cover over the Pacific northwest, the Rocky Mountains, and along the Gulf of Mexico and the eastern shore (40 to 60%), while they indicate a lower probability (20 to 40%) in the southwest over Baja Mexico. There is some disagreement in the northeast where VAS sees 20% more clouds than HIRS; this may be attributed to the large viewing angle of the VAS. In the summer, the cloud cover reduces (down by 20%) along the west coast and in the southern states, but it persists in the Rocky Mountains and along the eastern shore in both HIRS and VAS. Again there is disagreement in the northeast, where HIRS now sees 20% more cloud. Some of the moderate disagreement in both seasons over the oceans can be attributed to the difference in the sea surface temperature analysis used for HIRS versus VAS (as mentioned above).
In summary, the HIRS finds more mid-level (400-700 hPa) transmissive clouds and fewer low level opaque clouds than the VAS, but agrees with it in overall cloud reports. The geographical distribution of cloud reports from HIRS is similar to that from VAS, especially when a small VAS view angle is maintained.
Table C1a: The HIRS cloud cover in the BOREAL SUMMER
from all four years (June 1989 - May 1993) over North America
(29 to 49 N, 70 to 130 W). NE refers to effective emissivity
and T refers to the corresponding infrared optical
depth. Numbers are frequency of cloud cover; over 600,000
observations are included.
EFFECTIVE EMISSIVITY (IR OPTICAL DEPTH)
None Thin Thick Opaque
NE<0.50 0.5<NE<0.95 NE>0.95
LEVEL T<0.7 0.7< T<3.0 T>3.0
Hi < 400 hPa 9 9 6
Mid < 700 hPa 9 8 8
Low <1000 hPa 30 0 0 21
Total 30 35 35
(clear) (cirrus) (opaque)
Table C1b: The VAS cloud cover in the BOREAL SUMMER
from all four years (June 1989 - May 1993) over North America
(29 to 49 N, 70 to 130 W). NE refers to effective emissivity
and T refers to the corresponding infrared optical
depth. Numbers are frequency of cloud cover; over 2,200,000
observations are included.
EFFECTIVE EMISSIVITY (IR OPTICAL DEPTH)
None Thin Thick Opaque
NE<0.50 0.5<NE<0.95 NE>0.95
LEVEL T<0.7 0.7< T<3.0 T>3.0
Hi < 400 hPa 10 12 3
Mid < 700 hPa 2 3 10
Low <1000 hPa 27 0 0 33
Total 27 27 46
(clear) (cirrus) (opaque)
Table C1c: The HIRS cloud cover in the BOREAL WINTER
from all four years (June 1989 - May 1993) over North America
(29 to 49 N, 70 to 130 W). NE refers to effective emissivity
and T refers to the corresponding infrared optical
depth. Numbers are frequency of cloud cover; over 600,000
observations are included.
EFFECTIVE EMISSIVITY (IR OPTICAL DEPTH)
None Thin Thick Opaque
NE<0.50 0.5<NE<0.95 NE>0.95
LEVEL T<0.7 0.7< T<3.0 T>3.0
Hi < 400 hPa 9 12 5
Mid < 700 hPa 9 15 12
Low <1000 hPa 24 0 0 14
Total 24 45 31
(clear) (cirrus) (opaque)
Table C1d: The VAS cloud cover in the BOREAL WINTER
from all four years (June 1989 - May 1993) over North America
(29 to 49 N, 70 to 130 W). NE refers to effective emissivity
and T refers to the corresponding infrared optical
depth. Numbers are frequency of cloud cover; over 2,200,000
observations are included.
EFFECTIVE EMISSIVITY (IR OPTICAL DEPTH)
None Thin Thick Opaque
NE<0.50 0.5<NE<0.95 NE>0.95
LEVEL T<0.7 0.7< T<3.0 T>3.0
Hi < 400 hPa 10 15 4
Mid < 700 hPa 2 6 20
Low <1000 hPa 22 0 0 21
Total 22 33 45
(clear) (cirrus) (opaque)