Scientific Theme: Advanced Modeling and Observing Systems

Theme report: Climate System Variability
MILESTONE PSD03.1:
Continue improving the representation of physical processes and the Madden-Julian Oscillation
(MJO) in the NCEP Global Forecasting System (GFS).
ACCOMPLISHMENTS FOR PSD03.1:
To put the MJO errors of the NCEP/GFS in context, a comprehensive study of the fidelity of the MJO in fourteen
coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC)
Fourth Assessment Report (AR4) was conducted. Eight years of daily precipitation from each model‘s 20th-century
climate simulation were analyzed and compared with daily satellite-retrieved precipitation. Space-time spectral
analysis was used to obtain the variance and phase speed of the dominant convectively coupled equatorial waves.
The results show that all current state-of-the-art GCMs have significant problems in simulating tropical intraseasonal
variability. The total intra-seasonal (2-128 day) variance of precipitation is too weak in most of the models.
About half of the models have signals of convectively coupled equatorial waves. However, their variances are
generally too weak. The MJO variance approaches the observed value in only two of the fourteen models, and is less
than half of the observed value in the other twelve models. Moreover, the MJO variance in thirteen of the fourteen
models does not come from a pronounced spectral peak, but rather from part of an over-reddened spectrum, which
in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at
simulating the MJO are the only ones having convective closures/triggers linked to moisture convergence.
MILESTONE PSD03.2:
Diagnose the physical mechanisms behind the tropical biases in current GCMs, such as double-
ITCZ, ENSO, and MJO.
ACCOMPLISHMENTS FOR PSD03.2:
A study examining the double-ITCZ problem in the coupled general circulation models (CGCMs) participating in
the IPCC‘s Fourth Assessment Report (AR4) was completed. Twentieth century climate simulations of 22 IPCC
AR4 CGCMs were analyzed, together with the available ―AMIP‖ (i.e., prescribed SST) runs from 12 of them. To
understand the physical mechanisms contributing to the double-ITCZ problem, the main ocean-atmosphere
feedbacks, including the zonal sea surface temperature (SST) gradient−trade wind feedback (or Bjerknes feedback),
the SST−surface latent heat flux (LHF) feedback, and the SST−surface shortwave flux (SWF) feedback, were
studied in detail. The results show that most of these CGCMs have a double-ITCZ problem, which is characterized
by excessive off-equatorial and insufficient equatorial precipitation over much of the tropics. The excessive offequatorial
precipitation causes overly strong trade winds, excessive LHF and insufficient SWF, leading to
significant cold SST bias in much of the tropical oceans. Most of the models also simulate insufficient latitudinal
asymmetry in precipitation and SST over the eastern Pacific and Atlantic Oceans. The ―AMIP‖ runs also produce
excessive precipitation over much of the tropics, which also lead to overly strong trade winds, excessive LHF and
insufficient SWF. This suggests that the excessive tropical precipitation is an intrinsic error of the atmospheric
component of the coupled models, and that the insufficient equatorial Pacific precipitation in the coupled models is
due to errors in ocean-atmosphere feedbacks. These feedback errors are of three types: (1) excessive Bjerknes
feedback, which is caused by excessive sensitivity of precipitation to SST and overly strong time-mean surface
winds; (2) overly positive SST-LHF feedback, which is caused by excessive sensitivity of surface air humidity to
SST; and (3) insufficient SST-SWF feedback, which is caused by insufficient sensitivity of cloud amount to
precipitation. Off the equator over the eastern Pacific stratus region, most of the models produce insufficient stratus-
SST feedback associated with insufficient sensitivity of stratus cloud amount to SST, which may contribute to the
insufficient latitudinal asymmetry of SST in their coupled runs. These results suggest that the double-ITCZ problem
in CGCMs may be alleviated by reducing the excessive tropical precipitation and the above-feedback-relevant errors
in the atmospheric models.
MILESTONE PSD03.3:
Investigate improved methods of representing subgrid-scale variability in clouds and radiative
transfer in weather and climate models, conceptually as a series of sub-columns within a GCM's
large-scale column. Explore connections with more traditional single-column parameterizations as
well as "super-parameterizations" being developed by other researchers.
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