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D11206 HALTHORE ET AL.: SHORTWAVE MODEL INTERCOMPARISON D11206 Figure 1b. For Case 1, direct-normal solar irradiance in the UV (0.2–0.35 mm) is plotted similar to that in Figure 1a to show the variability that is encountered in this wavelength range. Consequences for the integrated flux are negligible because of the small magnitude of the energy in this range. The solid symbols for the two atmospheres and 75° SZA fully overlap. imation to provide the necessary speed, not accuracy) being an outlier. This is because, in the absence of aerosols, the # main reason for the differences in E dir is the differences in treatment of atmospheric gaseous absorption. For most of the models the absorption at the surface is correlated with, and comparable to, the negative of the absorption in the atmosphere, with model 14 again being an outlier (R 2 = 0.66 with model 14, 0.88 without). This is perhaps not surprising, as most of the energy incident at the surface is in the primary beam, so a decrease in incident flux will result in a decrease in surface absorbed flux. Without model 14, the spread of the data for diffuse upwelling flux at the TOA, or equivalently total absorption by the Earth-atmosphere system, exhibits about one half the spread of the atmospheric absorption (standard deviation 2.5 W m 2 compared with 5.8 W m 2 ), indicating that the apparent good agreement in diffuse upwelling flux at the TOA is due in considerable part to compensating errors in the atmospheric and surface absorption. 2.2. Results for Case 2 (With Aerosols) [17] Results for Case 2 (with aerosols), summarized in Tables 3a and 3b, show that the best agreement among the models for the low AOT case (Table 3a) is obtained for total downward irradiance and upward flux at the surface in the broadband for the SAW atmosphere (Figure 2a) and the total downward irradiance at the surface for the TROP atmosphere. The worst agreement for the low AOT case is in the computation of absorptance for the SAW atmosphere (Figure 2b) and diffuse downward flux at the surface for the TROP atmosphere. For both atmospheres, the agreement in the computation of total irradiance at the surface is better than for its components of direct and diffuse irradiance because of the conservative nature of scattering for which photons not directly transmitted predominantly manifest as diffuse light (absorption being the other mechanism) so that the total irradiance remains approximately the same. Clearly, compared with values for the SAW atmosphere, differences in the parameterization of water vapor absorption for the TROP atmosphere account for the larger model disagreements among all components, except the absorptance (columns 8–9 compared with columns 6–7, Table 3a). For the absorptance, the standard deviation among models increases more as a function of SZA than water vapor absorption and therefore is seen to be relatively insensitive to the type of atmosphere present. For high AOT, the best agreement among models is in the computation of diffuse surface irradiance (Figure 2c) and the worst, absorptance (Figure 2d). An improvement in agreement among models for the computation of absorptance is obtained for the 75° SZA, TROP case by excluding the GCM, model 16 (from 3.4% to 2.7%, column 9, Table 3b), which appears to be an outlier; still, it is reassuring that the performance of the GCM is as good as it is. [18] One would expect that inclusion of aerosols in models with the attendant increase in uncertainties in specifying aerosol properties should increase the STDVM among models for most, if not all, of the irradiance quantities. However, we do not find this to be the case for all the irradiance quantities. Generally, 42 quantities out of 80 in Table 3a (Low AOT, columns 6–9) and 36 out of 80 in Table 3b (High AOT, columns 6–9) have a better agreement among models than for the aerosol-free case (Table 2, columns 6–9). Also, 27 quantities out of 80 in Table 3b (High AOT) have better agreement than in Table 3a (Low AOT). One possible explanation is that inclusion of aerosols in the atmospheres of the type used here decreases the fractional solar energy that is interacting with the constituents of the lower atmosphere with their associated uncertainties (such as uncertainties in aerosol and molecular optical properties), thus increasing the level Figure 1c. For Case 1, broadband direct solar irradiance at the surface is plotted for the different models as percent deviation from mean for the two atmospheres and two SZAs. Model 5 (GAME model with a wave number resolution of 100 cm 1 ) shows the most deviation and was subsequently discontinued from calculations in other cases. The results from GCM (Model 16) are not included here because the model does not calculate the direct and diffuse components of the global irradiance separately. Line-by-line models are identified by the box in the legend description. 6of18
D11206 HALTHORE ET AL.: SHORTWAVE MODEL INTERCOMPARISON D11206 Figure 1d. For Case 1 SAW atmosphere, SZA = 30°, comparison among the 15 models is shown for six flux quantities as deviation from the mean in W m 2 . The components are direct irradiance at the surface, negative of atmospheric absorption, absorption at the surface, diffuse upwelling flux at the TOA, negative of diffuse irradiance at the surface, and diffuse upwelling flux at the surface. The mean quantities respectively are, in W m 2 , 940.91, 147.03, 803.66, 227.40, 63.80, and 201.05. Table 3a. Case 2, Computations With Aerosols, Low AOT a Average, Irradiance in W m 2 Standard Deviation as a % of Mean Subarctic Winter Tropical Subarctic Winter Tropical 30.00° SZA 75.00° SZA 30.00° SZA 75.00° SZA 30.00° SZA 75.00° SZA 30.00° SZA 75.00° SZA Direct Down 0.28–5.00 mm 872.07 171.67 784.22 144.76 0.65 1.62 0.76 2.12 0.20–0.35 mm 10.00 0.27 10.29 0.27 2.91 11.95 2.95 12.21 0.35–0.70 mm 376.08 59.80 375.37 59.72 0.27 1.29 0.30 1.42 0.70–5.00 mm 485.84 111.58 398.82 84.76 1.03 1.95 1.45 2.80 Diffuse Down 0.28–5.00 mm 120.12 64.53 116.51 61.95 2.10 1.84 2.86 3.34 0.20–0.35 mm 9.96 2.80 10.46 2.95 2.27 3.70 2.29 4.05 0.35–0.70 mm 82.29 43.91 82.19 44.09 1.63 0.97 1.58 0.96 0.70–5.00 mm 27.88 17.84 23.84 14.91 5.50 5.67 10.69 13.57 Total Down 0.28–5.00 mm 992.19 236.20 900.73 206.71 0.50 0.93 0.50 1.02 0.20–0.35 mm 19.96 3.07 20.76 3.22 2.16 3.81 2.18 4.29 0.35–0.70 mm 458.38 103.71 457.56 103.80 0.38 0.80 0.38 0.90 0.70–5.00 mm 513.73 129.42 422.65 99.67 0.88 1.35 1.16 1.92 Diffuse Up 0.28–5.00 mm 198.61 47.41 181.22 42.36 0.45 0.72 1.12 4.79 0.20–0.35 mm 3.99 0.61 4.15 0.64 2.05 5.24 2.28 5.51 0.35–0.70 mm 91.68 20.74 91.51 20.76 0.36 0.83 0.38 0.90 0.70–5.00 mm 102.91 26.05 85.55 20.95 0.79 1.25 2.39 10.22 Diffuse Up TOA 0.28–5.00 mm 228.46 91.82 208.53 85.54 1.57 1.30 2.00 2.20 0.20–0.35 mm 9.66 4.00 10.16 4.17 5.07 4.31 4.53 4.08 0.35–0.70 mm 119.72 52.99 120.03 53.62 1.61 1.22 1.57 1.16 0.70–5.00 mm 99.08 34.83 78.33 27.76 2.15 2.59 3.45 6.39 Absorptance 0.28–5.00 mm 0.132 0.203 0.212 0.290 3.67 3.05 2.15 2.22 0.20–0.35 mm 0.481 0.562 0.458 0.543 2.96 2.90 2.84 3.11 0.35–0.70 mm 0.041 0.103 0.041 0.098 8.69 5.49 8.37 5.96 0.70–5.00 mm 0.187 0.263 0.338 0.432 4.37 4.02 2.60 2.84 a In this and all subsequent tables, ‘‘Total Down’’ refers to symbol, E S # . 7of18
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