Subsidence and Upper-Tropospheric Drying along Trajectories in a General Circulation Model

A trajectory analysis of the CCM3 moisture simulation is used to show that the model simulates uppertropospheric moisture observations better than would be inferred from a traditional geographical comparison. The upper-tropospheric moisture simulation is compared to upper-tropospheric moisture derived from GOES 6.7-micron observations for September 1992. Trajectories start in convective regions of the tropics and are followed into non-convective subsidence regions. Moisture and pressure along the trajectories are determined for both the model and observations. Humidity values as a function of subsidence agree much better between observations and model than do geographical grid box comparisons, because the model does not simulate details in the large-scale flow pattern precisely. The relative humidity decreases slightly more slowly with subsidence along trajectories in the CCM3 simulation than in observations.