Modelled and observed variability in atmospheric vertical temperature structure

Realistic simulation of the internal variability of the climate system is i mportant both for climate change detection and as an indicator of whether t he physics of the climate system is well-represented in a climate model. In this work zonal mean atmospheric temperatures from a control run of the se cond Hadley Centre coupled GCM are compared with gridded radiosonde observa tions for the past 38 years to examine how well modelled and observed varia bility agree. On time scales of between sir; months and twenty years, simul ated and observed variability of global mean temperatures agree well for th e troposphere, but in the equatorial stratosphere variability is lower in t he model than in the observations, particularly at periods of two years and seven to twenty years. We find good agreement between modelled and observe d variability in the mass-weighted amplitude of a forcing-response pattern, as used for climate change detection, but variability in a signal-to-noise optimised fingerprint pattern is significantly greater in the observations than in a model control run. This discrepancy is marginally consistent wit h anthropogenic forcing, but more clearly explained by a combination of sol ar and volcanic forcing, suggesting these should be considered in future 'v ertical detection' studies. When the relationship between tropical lapse ra te and mean temperature was examined, it was found that these quantities ar e unrealistically coherent in the model at periods above three years. Howev er, there is a clear negative lapse rate feedback in both model and observa tions: as the tropical troposphere warms, the mid-tropospheric lapse rate d ecreases on all the time scales considered.