Evaluation of moisture in the Hadley Centre climate model using simulations of HIRS water-vapour channel radiances

It is important to establish that climate models can accurately simulate the observed present-day fluctuations of water vapour. In particular, water-vapour and cloud-radiative feedbacks are intrinsically linked to processes governing relative-humidity distribution and variability. To explore these issues, clear-sky radiances, sensitive to upper-tropospheric relative humidity, are simulated within the Hadley Centre atmospheric climate model, version HadAM3, allowing direct comparison with High Resolution Infrared Sounder (HIRS) observations. The model is forced by observed sea surface temperatures and sea-ice fields over the period 1979-98. Evaluation of the simulated distribution and variability of water vapour is undertaken utilizing the HIRS 6.7 mum brightness temperature (T-6.7) and satellite measurements of column-integrated water vapour and clear-sky outgoing long-wave radiation (OLR). Modifications are made to the clear-sky OLR and T-6.7 HadAM3 diagnostics to reduce sampling inconsistencies with the observed products. Simulated T-6.7 over subtropical dry zones are higher than T-6.7 from observations, particularly in the southern hemisphere, and is symptomatic of an overactive circulation. The observed spatial signature of the T-6.7 interannual variability is dominated by El Nino and is captured well by HadAM3. Interannual variability of the tropical ocean mean T-6.7 is consistent between HadAM3 and the HIRS observations, suggesting that the small simulated decadal changes in relative humidity are realistic.