Results

Global annual mean values of shortwave absorption at the surface, within the atmosphere and in the entire surface-atmosphere system are shown in Table 1 for the GCMs investigated in the present study, both for all-sky and clear-sky conditions. Note that "all-sky" includes all types of weather conditions, i.e. from totally overcast to completely cloudless. Additional estimates of the TOA fluxes from ERBE (Barkstrom 1990) as well as two estimates of surface and atmospheric absorption (Ohmura and Gilgen 1993, Wild et al. 1998), which make use of the GEBA observations, are displayed in Table 1. Wild et al. (1998) provide separate estimates of surface and atmospheric absorption under both clear-sky and all-sky conditions. They are based on a blend of direct observations and model calculations with well-established bias structures.

Compared to these estimates, the GCMs show significantly higher absorption of solar radiation at the surface while significantly less in the atmosphere. On the other hand, at the TOA, the GCMs agree quite well with the (satellite) estimates in their net fluxes. This is found for both all-sky and clear-sky conditions.

To get more insight into these discrepancies, the (all sky) radiation climatologies of the GCMs are assessed in the following using the 720 surface sites and their collocated satellite-measured TOA climatologies. The radiation budgets of the models ECHAM3, ARPEGE and HadAM2b are discussed in Section 4.1, while the HadAM3 models with and without aerosol are discussed in Section 4.2.

Table 1. Global annual mean absorption of solar radiation at the surface, within the atmosphere and in the total system (TOA) in the GCMs ECHAM3, ARPEGE, HadAM2b, Had AM 3 with aerosol and withouth aerosol (Had AM 3 NA) Units Win"2_

Surface

ALL-SKY Atmosphere

TOA

Surface

CLEAR-SKY Atmosphere

TOA

GCMs:

ECHAM3

! 64

71

235

222

63

285

ARPEGE

174

68

242

215

67

282

HadAM2b

175

67

242

234

61

295

HadAM3

168

76

244

219

71

290

H ad AM 3 NA

174

71

245

228

66

294

Other estimates:

F.RBE

240

287

Ohmura+Gilgen (1993)

142

98

240

Wild et al (1989)

154

86

240

216

72

287

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