1. Intercomparison of Polar Cloud Climatology: APP-x, ERA-40, Ground-based Observations Xuanji Wang Cooperative Institute for Meteorological Satellite Studies University of Wisconsin Jeff Key Office of Research and Applications, NOAA/NESDIS Madison, Wisconsin Sea Ice Cover Change from 1979 to 2003 2003 1979 (From Earth Science News at NASA, Nov. 30, 2003)

2. APP-x Dataset:  Extended AVHRR Polar Pathfinder (APP-x) Dataset Time period : 0400 & 1400 Local Solar Time (LST) for the period of 1982 - 2004. Spatial resolution : 25 x 25 km 2 on EASE-grid.

3. APP-x vs SHEBA : Cloudiness  Surface Heat Budget of the Arctic Ocean (SHEBA) Time period : Oct. 1997 – Oct. 1998, monthly mean; Spatial resolution : SEHBA ship observation. Comparison was made on 14:00 LST at SHEBA ship site, APP-x box size is 25x25 km 2 ( 5km spatial resolution APP-x data was used.) MONTH APP-x SHEBA BIAS(A-S) RMSS 1 0.6152 0.3802 0.2350 0.2835 2 0.7280 0.5509 0.1771 0.3022 3 0.8372 0.5907 0.2466 0.3643 4 0.8029 0.6991 0.1037 0.2034 5 0.8209 0.7997 0.0212 0.0880 6 0.6892 0.7460 -0.0568 0.1908 7 0.8895 0.8445 0.0451 0.1931 8 1.0000 0.8960 0.1040 0.1040 10 0.1667 0.1250 0.0417 0.0417 11 0.6640 0.6750 -0.0110 0.3324 12 0.5787 0.2124 0.3663 0.3677 ---------------------------------------------------------------- Average 0.7291 0.5970 0.1322 0.2637

4. APP-x vs Surface : Cloudiness  Surface-based Observations by Hahn (1995) Time period : monthly mean over 1982-1991; Spatial resolution : Arctic area north of 80 o N. Comparison was made on the monthly mean basis, APP-x was averaged for 14:00 LST ( 25km spatial resolution APP-x dataset was used.)

5. APP-x vs ERA-40 : Cloudiness  ECMWF 40 Year Re-analysis (ERA-40) Dataset Time period : mid-1957 to mid-2002, four times per day; Spatial resolution : 2.5 o x 2.5 o grid. Comparison was made on the hourly and monthly mean basis, APP-x was averaged for box size of 225 km x 225 km with 14:00 and 04:00 datasets ( 25km spatial resolution APP-x dataset was used.) MONTH(NO) APP-x | ERA-40 | Bias (A-E) ------------------------------------------------------------ 1(61) 0.47 0.63 -0.16 2(56) 0.46 0.75 -0.29 3(62) 0.30 0.53 -0.24 4(59) 0.56 0.82 -0.27 5(62) 0.60 0.60 0.00 6(59) 0.57 0.72 -0.15 7(62) 0.72 0.80 -0.08 8(62) 0.70 0.75 -0.05 9(60) 0.75 0.75 0.00 10(60) 0.66 0.70 -0.04 11(60) 0.59 0.71 -0.12 12(62) 0.57 0.64 -0.07 ------------------------------------------------------- (730) 0.58 0.70 -0.12

6. Diurnal cycles of cloudiness at Kougarok site, Alaska from APP-x (black cross) and ERA-40 (blue curve) for January, April, July, and October, 1999. APP-x ERA-40

7. APP-x at 14:00 LST for January, 1999 APP-x at 14:00 LST for July, 1999 ERA-40 at 14:00 LST for January, 1999 ERA-40 at 14:00 LST for July, 1999 ERA-40 minus APP-x at 14:00 LST for January, 1999 ERA-40 vs APP-x : Cloudiness

8. ERA-40 at 04:00 LST for January, 1999 ERA-40 at 04:00 LST for July, 1999 APP-x at 04:00 LST for January, 1999 APP-x at 04:00 LST for July, 1999 ERA-40 minus APP-x at 04:00 LST for January, 1999 ERA-40 vs APP-x : Cloudiness

9. Trends in the Arctic Cloudiness from APP-x, North of 60 o N, 14:00 LST

10. Trends in cloudiness from APP-x for winter, spring, summer, and autumn.

11. Summary  Satellites tend to overestimate cloudiness in comparison to the ground-based naked eye observation during the Arctic darkness, and underestimate cloudiness during the Arctic brightness to less extent than the Arctic darkness.  ERA-40 tends to overestimate cloudiness in comparison to APP-x for the high- latitude area, i.e., north of 60 o N, and underestimate cloudiness for the lower-latitude area, i.e., south of 60 o N.  Arctic cloudiness has generally decreased or not changed in winter and at night, but increased during the day in spring. Overall, an annual weak decreasing trend in cloudiness over 1982-2004 was found. But if the data of 2001-2004 was excluded from the analysis, there would be no annual trend in Arctic cloudiness. There is a suspect that calibration of NOAA-16 is problematic, especially for AVHRR visible channels, though it was solved more or less in standard APP dataset.  Trends in cloudiness for different surface types of the Arctic area north of 60 o N are consistent.