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Abstract: The role of Arctic clouds in the recent rapid Arctic warming has attracted much attention. However, Arctic cloud water paths (CWPs) from reanalysis datasets have not been well evaluated. This study evaluated the CWPs as well as LWPs (cloud liquid water paths) and IWPs (cloud ice water paths) from five reanalysis datasets (MERRA-2, MERRA, ERA-Interim, JRA-55, and ERA5) against the COSP (Cloud Feedback Model Intercomparison Project Observations Simulator Package) output for MODIS from the MERRA-2 CSP (COSP satellite simulator) collection (defined as M2Modis in short). Averaged over 1980–2015 and over the Arctic region (north of 60°N), the mean CWPs of these five datasets range from 49.5 g/m2 (MERRA) to 82.7 g/m2 (ERA-Interim), much smaller than that from M2Modis (140.0 g/m2). However, the spatial distributions of CWPs, show similar patterns among these reanalyses, with relatively small values over Greenland and large values over the North Atlantic. Consistent with M2Modis, these reanalyses show larger LWPs than IWPs, except for ERA-Interim. However, MERRA-2 and MERRA underestimate the ratio of IWPs to CWPs over the entire Arctic, while ERA-Interim and JRA-55 overestimate this ratio. ERA5 shows the best performance in terms of the ratio of IWPs to CWPs. All datasets exhibit larger CWPs and LWPs in summer than in winter. For M2Modis, IWPs hold seasonal variation similar with LWPs over the land but opposite over the ocean. Following the Arctic warming, the trends in LWPs and IWPs during 1980~2015 show that LWPs increase and IWPs decrease across all datasets, although not statistically significant. Correlation analysis suggests that all datasets have similar interannual variability. The study further found that the inclusion of re-evaporation processes increases the humidity in the atmosphere over the land and that a more realistic liquid/ice phase can be obtained by independently treating the liquid and ice water contents.
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Key words:
- Arctic /
- clouds /
- cloud water paths (CWPs) /
- reanalysis evaluation
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Figure 1. Climatologies of the cloud water path (CWP, left column), the cloud liquid water path (LWP, middle column), and the cloud ice water path (IWP, right column) from M2Modis and the five reanalyses during the period of January 1980 to December 2015.
Figure 2. Climatologies of standard deviations of the cloud water path (CWP, left column), the cloud liquid water path (LWP, middle column), and the cloud ice water path (IWP, right column) from M2Modis and the five reanalyses during the period of January 1980 to December 2015.
Figure 3. Distributions of the IWP/CWP from M2Modis and the five reanalysis datasets (a), and differences between the five reanalysis datasets and M2Modis (b).
4. Monthly means (first row) and monthly standard deviations (second row) of CWPs (left column), LWPs (middle column) and IWPs (right column) for different datasets.
5. Spatial distributions of the monthly mean (from December to November) LWPs (a) and IWPs (b) from M2Modis.
Figure 6. Four sub-regions of the Arctic (Mainland: dark blue; Greenland: light blue; sea-ice-covered region: yellow, where monthly mean SIC ≥15%; and ice-free ocean: dark red). a. March and b. September.
Figure 7. Climatological seasonal cycles of LWPs (left column) and IWPs (right column) in four sub-regions of the Arctic (rows from top to bottom: Mainland, Greenland, sea-ice-covered region, and ice-free ocean).
Figure 8. Annual mean anomalies of CWPs (a), LWPs (b) and IWPs (c) from M2Modis and reanalysis datasets for the period of 1980−l2015.
Table 1. Datasets used in this study
Variable ERA5 ERA-Interim JRA-55 MERRA-2 MERRA M2Modis Institution/project ECMWF ECMWF JMA NASA-GSFC NASA-GSFC NASA-GSFC/CFMIP COSP Resolution (lon. × lat.) 0.25°×0.25° 0.75°×0.75° 1.25°×1.25° 0.625°×0.500° 0.66°×0.50° 0.625°×0.500° Temporal range Jan. 1979 to present Jan. 1979 to present Jan. 1958 to present Jan. 1980 to present Jan. 1979 to Feb. 2016 Jan. 1980 to present Cloud property parameterizations Tiedtke (1993) Tiedtke (1993) Sommeria and Deardorff (1977) Bacmeister et al. (2006) Bacmeister et al. (2006) Note: See Section 2.5 for more details of M2Modis. 
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Table 2. Means and standard deviations of LWPs, IWPs, and CWPs and the ratios of average IWPs to average CWPs over the Arctic
Reanalysis datasets LWP/ (g·m–2) IWP/ (g·m–2) CWP/ (g·m–2) $\dfrac{\rm{IWP}}{\rm{CWP}} $/% Mean STD Mean STD Mean STD M2Modis 80.9 40.7 59.1 29.8 140.0 62.6 42 MERRA-2 58.8 24.0 13.1 5.5 71.8 25.8 18 MERRA 35.0 14.9 14.6 5.8 49.5 17.3 29 ERA5 49.0 31 22.4 8.5 71.4 35.6 31 ERA-Interim 39.6 31.0 43.1 16.3 82.7 37.9 53 JRA-55 29.7 23.7 25.1 11.6 54.8 31.1 46
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Table 3. Linear trends in M2Modis and five reanalysis datasets, as well as correlation coefficients using original and detrended time series (in brackets) between M2Modis and each reanalysis dataset
Reanalysis datasets CWP LWP IWP Trend/(g·m–2·century–1) Corr/10–2 Trend/(g·m–2·century–1) Corr/10–2 Trend/(g·m–2·century–1) Corr/10–2 M2Modis 22 42 –20 MERRA-2 8 94(91) 18 95(83) –9 86(87) MERRA 6 67(61) 11 86(68) –5 84(76) ERA5 5 60(52) 8 57(33) –2 74(73) ERA–I 9 74(68) 17 84(62) –7 82(75) JRA–55 13 69(55) 12 84(60) –8 40(57) Note: Red color means passing the 95% significance test.
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