Intercomparison of Polar Cloud Climatology: APP-x, ERA-40, Ground-based Observations

Xuanji WangCooperative Institute for Meteorological Satellite Studies

University of Wisconsin

Jeff KeyOffice of Research and Applications, NOAA/NESDIS

Madison, Wisconsin

Sea Ice Cover Change from 1979 to 2003

20031979

(From Earth Science News at

NASA, Nov. 30, 2003)

APP-x Dataset: APP-x Dataset:

Extended AVHRR Polar Pathfinder (APP-x) DatasetTime period: 0400 & 1400 Local Solar Time (LST) for the period of 1982 - 2004.

Spatial resolution: 25 x 25 km2 on EASE-grid.

APP-x vs SHEBA : Cloudiness APP-x vs SHEBA : Cloudiness

Surface Heat Budget of the Arctic Ocean (SHEBA)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 km2 ( 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

APP-x vs Surface : Cloudiness APP-x vs Surface : Cloudiness

Surface-based Observations by Hahn (1995)Surface-based Observations by Hahn (1995)Time period: monthly mean over 1982-1991; Spatial resolution: Arctic area north of 80oN.

Comparison was made on the monthly mean basis, APP-x was averaged for 14:00 LST ( 25km spatial resolution APP-x dataset was used.)

APP-x vs ERA-40 : Cloudiness APP-x vs ERA-40 : Cloudiness

ECMWF 40 Year Re-analysis (ERA-40) DatasetECMWF 40 Year Re-analysis (ERA-40) Dataset

Time period: mid-1957 to mid-2002, four times per day; Spatial resolution: 2.5o x 2.5o 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 0.0010(60) 0.66 0.70 -0.0411(60) 0.59 0.71 -0.1212(62) 0.57 0.64 -0.07------------------------------------------------------- (730) 0.58 0.70 -0.12

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

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 minus APP-x at 14:00 LST for

January, 1999

ERA-40 vs APP-x : Cloudiness

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 minus APP-x at 04:00 LST for

January, 1999

ERA-40 vs APP-x : Cloudiness

Trends in the Arctic Cloudiness from APP-x, Trends in the Arctic Cloudiness from APP-x, North of 60North of 60ooN, 14:00 LSTN, 14:00 LST

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

SummarySummary Satellites tend to overestimate cloudiness in comparison to the ground-based naked Satellites tend to overestimate cloudiness in comparison to the ground-based naked

eye observation during the Arctic darkness, and underestimate cloudiness during the eye observation during the Arctic darkness, and underestimate cloudiness during the

Arctic brightness to less extent than the Arctic darkness.Arctic brightness to less extent than the Arctic darkness.

ERA-40 tends to overestimate cloudiness in comparison to APP-x for the high-ERA-40 tends to overestimate cloudiness in comparison to APP-x for the high-

latitude area, i.e., north of 60latitude area, i.e., north of 60ooN, and underestimate cloudiness for the lower-latitude N, and underestimate cloudiness for the lower-latitude

area, i.e., south of 60area, i.e., south of 60ooN.N.

Arctic cloudiness has generally decreased or not changed in winter and at night, but 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 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 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 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, that calibration of NOAA-16 is problematic, especially for AVHRR visible channels,

though it was solved more or less in standard APP dataset.though it was solved more or less in standard APP dataset.

Trends in cloudiness for different surface types of the Arctic area north of 60Trends in cloudiness for different surface types of the Arctic area north of 60ooN are N are

consistent. consistent.