Testing broadband radiation schemes for their ability to calculate the radiative forcing and temperature response to stratospheric water vapour and ozone changes

The radiative forcing and fixed dynamical heating temperature response is calculated for observed changes (1979-1997) in carbon dioxide, stratospheric ozone and stratospheric water vapour, using several radiation schemes. It is found that certain broadband schemes substantially underestimate the radiative forcing and absorption by stratospheric water vapour by up to 50%, for the instantaneous forcing, and 15% for the adjusted radiative forcing. This error in the water vapour absorption leads to a 30% smaller stratospheric temperature response and also causes an underestimate of the adjusted stratospheric ozone radiative forcing of up to 50%. It was found that this error could be corrected by splitting the wavelength band which accounts for strong water vapour absorption into two separate bands. This work illustrates the need to carefully test each new forcing introduced into a broadband scheme, as its designer may not have catered for such eventualities.