The diurnal cycle : a theme for GMPPJan Polcher, Paul Dirmeyer, Peter Gleckler, Bert Holtslag, Steve Krueger
August 11, 2003

The GEWEX Modelling and Prediction Panel (GMPP) was founded some 3 years ago to coordinate groups working on the numerical representation of key aspects of the water and energy
exchanges in the atmosphere and over continents. The fields covered are : cloud processes with theGEWEX Cloud System Study (GCSS), land surface processes with the Global Land/Atmosphere
System Study (GLASS) and the planetary boundary layers with the GEWEX Atmospheric Boundary Layer Study (GABLS). Although these groups deal with different processes in the system, theircommon aim is to improve the representation of the energy and water cycles in large scale models
using similar methodologies. In all cases we deal with macroscopic processes for which physics only
proposes microscopic theories. The macroscopic interpretation of the physical laws known at themolecular level is not unique and as a consequence GMPP will always be faced with a large varietyof models. They will differ in the scales which are explicitly represented and those which are treatedby conceptual representations of varying complexity.
All three GMPP groups have learned to take advantage of the diversity of models and have madesubstantial progress in the understanding of the various conceptual representations by performing
model inter-comparisons. These comparisons have occurred at locations with sufficient data to ensure
validation, or where phenomena occur which were believed to be poorly represented by the models. In
all cases the comparisons involved models designed to represent a variety of spatial scales or designed
for various applications. This approach was able to attract large numbers of participants as for eachinter-comparison scientific questions existed and triggered important discussions. These exchanges
and collaborations between modellers have resulted in general progress within the community. The
challenge for GMPP now, is to extend the methodology to improve the representation of the interactions between the surface, the atmospheric planetary boundary layer and the cloud processes inlarge scale models.
In order to achieve this we propose to choose a theme for GMPP which will guide the activities inall three groups and encourage a closer study of the couplings which exist. Through the collaboration
with AMIP, the theme will also strengthen the the link with global climate models. The theme wewould like to propose is the diurnal cycle. This aspect of climate has not received sufficient attention
over the past few years although it is one of the most prominent cycles in the water and energy
exchanges within the system and is considered to be an ideal test bed for general circulation models
and their parametrisations [Lin et al., 2000]. A number of reasons have motivated this choice :• AMIP has shown that the annual cycle is well represented in many respects in the current
generation of general circulation models (GCMs). The next natural cycle which should beevaluated and probably requires the most improvements in GCMs is the diurnal one. Regional
climate models as well as numerical weather prediction models will probably show similar
deficiencies in the representation of the diurnal cycle and also benefit from the proposed theme.
• Systematic errors in the representation of the diurnal cycle have been documented in a numberof large scale atmospheric models and could be attributed to the representation of the water andenergy exchanges by turbulence, surface processes, convection and radiation [Viterbo et al., 1999,
Yang and Slingo, 2001, Betts and Jakob, 2002].
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• The diurnal cycle is the dominant fluctuations in the interactions between the surface, the atmospheric planetary boundary layer and cloud processes in many regions of the world [Ek and Holtslag, 2003,
Nesbitt and Zipser, 2003, Yang and Slingo, 2001]. Thus, a better representation of the diurnal
cycle by models will be the result of a better understanding of these interactions.
• A correct representation of the diurnal cycle in models is essential for comparing models toobservations. If the atmospheric model does not represent the diurnal variations well, thensampling it in the same way as the observations used for validation will induce systematic
biases and render the comparisons difficult even at longer time scales. Such problems have for
instance been encountered in the validation of simulated clouds with observations from polar
orbiting satellites [Salaby and Callaghan, 1997, Fowler et al., 2000].
• To make global atmospheric models more relevant for regional applications the diurnal cycle
needs to be well represented. The diurnal cycle forces land/sea and mountain/valley circulations
which are important at regional scales [Nesbitt and Zipser, 2003, Yang and Slingo, 2001].
GMPP proposes to evaluate the ability of our models to represent the diurnal cycle using modelinter-comparisons as it has proven its value in previous GMPP activities. The strategy envisaged isto diagnose the diurnal cycle in parallel on the processes with which GMPP deals and on the global
climate scale before moving onto the analysis of the interactions between clouds, the atmosphericboundary layer and land-surfaces. Three phases can thus be identified in the implementation of this
GMPP proposal :1. In a first phase GCSS, GLASS and GABLS will evaluate in off-line model inter-comparisons (i.e.
with limited feedbacks) currently underway or planned, the ability to reproduce the diurnal
cycle. This effort should allow us to identify problems areas but also explore the limits of theoff-line approach for the evaluation of the associated processes. The planned evaluation by
GMPP of single-column models should be able to determine if the deficiencies in the diurnal
cycle are specific to the parametrizations for large-scale atmospheric models or if they also exist
in cloud resolving models. The Global Soil Wetness Project (GSWP) could for instance identify
the regions in which land-surface schemes most consistently fail to reproduce the diurnal cycle
despite an acceptable forcing. This phase should not change the way GMPP projects work but
only refocus some of the diagnostics being done.
2. During this period AMIP should identify the geographical regions, seasons and weather regimes
for which atmospheric models show the most significant deficiencies in the diurnal cycle. This
could either be done with the 6 hourly output available in the current AMIP database orby requesting from participants higher frequency output for one representative year from theAMIP period (1979-1996). The results from this analysis should be compared to the problem
areas identified in off-line inter-comparisons in order to evaluate to role of the feedbacks inamplifying or damping the deficiencies in the representation of the various processes.
This type of analysis could be fostered by climate and weather forecasting centres reporting
problems in the diurnal cycle they have identified in their models to GMPP. The ubiquity ofthe deficiency could then be verified in the AMIP database.
To make these evaluations more systematic, GMPP will propose diagnostics which will beadded to the AMIP standard output list.
3. Once the problems have been identified in the off-line mode and the large scale atmosphericmodel environment then attempts should be undertaken to reproduce them in a limited coupledenvironment by progressively introducing feedbacks. For example, this could be done by using
a common land-surface model instead of prescribing the surface fluxes in a single column model(SCM). The SCM’s convection scheme would then affect the diurnal cycle of the surface fluxes
2but the errors of the land-surface model would also come into play. The same process will need
to be repeated with the convection and atmospheric boundary layer parameterizations and withthe land-surface and atmospheric boundary layer parameterizations. Through the progressive
re-introduction of the feedbacks the problem areas identified in large scale atmospheric models
will be reproduced in a simplified environment and the key interactions identified. This should
allow us to propose efficient remedies for the representations of these physical processes inatmospheric models which would improve the representation of the diurnal cycle.
4. To obtain the same synoptic evolution and large scale weather patters in the AMIP runs, newruns should include gentle relaxation towards analysed fields above the atmospheric boundary
layer (say 1000 m), as well as prescribed values for the sea surface temperature. Such a procedure follows Viterbo et al. [1999] in their study of the coupled land surface-atmosphere system
and allows to cut-off the dynamical feedbacks on errors in the diurnal cycle. An alternate solution would be to perform dynamical compositing of the output of GCMs in order to eliminate
in a first approximation the dynamical feedbacks.
With this approach we hope to foster the interactions between the various GMPP activities andensure that the progress made in our conceptual models of moist processes translate into improved
general circulation models. This effort should allow GMPP to connect more efficiently with thedevelopment work underway in numerical weather prediction centres as well as climate modelling
institutions.

Timeline
To be discussed with the three GMPP panels.

References[Betts and Jakob, 2002] Betts, A. and Jakob, C. (2002). Study of the diurnal cycle of convective
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3The dirunal cycle in the tropics.