Evaluating Tropical Upper-tropospheric Water in Climate Models Using Satellite Data
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
, 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.
EB-salen, Hörsalsvägen 11, Chalmers University of Technology
Opponent: Prof. Richard P. Allan, Department of Meteorology, University of Reading, England