Evaluating Tropical Upper-tropospheric Water in Climate Models Using Satellite Data

Doktorsavhandling, 2014

Measuring and simulating moist processes in the tropical upper troposphere are difficult tasks. Humidity in this region of the atmosphere is mainly supplied by deep convection and, problems with simulated convection are known to be a major contributor to uncertainties in climate model projections. Observations within this region of the atmosphere are hampered by the low absolute humidity as well as by the presence of clouds. This thesis examines the seasonal changes in and the effects tropical deep convection have on upper-tropospheric water, in addition to its effect on outgoing longwave radiation (OLR). Multiple satellite observations are assessed and used to evaluate the climate models EC-Earth, CAM5, and ECHAM6. The data are analysed using two main methods: longterm averages and compositing. Compositing represents an improvement over climatologies because it brings the comparison closer to the processes associated with deep convection. The compositing method is adapted from Zelinka and Hartmann [2009], improved, and applied for the first time to climate models. Upper-tropospheric humidity (UTH) undergoes large seasonal and regional changes in the tropics. Over land areas, convection is more intense, producing greater amounts of water at higher heights, and having a greater effect on the OLR. Corresponding model simulations capture the large-scale and seasonal changes, however there are significant inconsistencies when compared with the observations, especially over land regions. Simulated mean UTH in areas where DC systems develop are consistently higher than observed over both land and ocean. However, the direct response of UTH to DC systems is found to be similar to the observations. Modeled cloud fractions near the tropopause are tend to be overestimated, whereas ice water content is often too low. The observed OLR can, regionally, differ from the simulated results by as much as 20 W m −1 . Moreover, above and around deep convection systems, the local decrease of OLR is throughout underestimated. Further, the models all demonstrate a lack of spatial variability indicated by a diurnal repetition of convection at the same location over land. These results obtained by the composite method reveal details that could not have been obtained using a traditional climatology based comparison.