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Akio Arakawa

Abstract

A review of the cumulus parameterization problem is presented with an emphasis on its conceptual aspects covering the history of the underlying ideas, major problems existing at present, and possible directions and approaches for future climate models. Since its introduction in the early 1960s, there have been decades of controversies in posing the cumulus parameterization problem. In this paper, it is suggested that confusion between budget and advection considerations is primarily responsible for the controversies. It is also pointed out that the performance of parameterization schemes can be better understood if one is not bound by their authors' justifications. The current trend in posing cumulus parameterization is away from deterministic diagnostic closures, including instantaneous adjustments, toward prognostic or nondeterministic closures, including relaxed and/or triggered adjustments. A number of questions need to be answered, however, for the merit of this trend to be fully utilized.

Major practical and conceptual problems in the conventional approach of cumulus parameterization, which include artificial separations of processes and scales, are then discussed. It is rather obvious that for future climate models the scope of the problem must be drastically expanded from “cumulus parameterization” to “unified cloud parameterization,” or even to “unified model physics.” This is an extremely challenging task, both intellectually and computationally, and the use of multiple approaches is crucial even for a moderate success. “Cloud-resolving convective parameterization” or “superparameterization” is a promising new approach that can develop into a multiscale modeling framework (MMF). It is emphasized that the use of such a framework can unify our currently diversified modeling efforts and make verification of climate models against observations much more constructive than it is now.

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Donald R. Johnson
and
Akio Arakawa

Abstract

Professor Yale Mintz's contributions in combining theory, diagnostic analysis, and modeling in scientific studies across a broad range of interests over more than four decades are reviewed. His studies include diagnostic analysis of the general circulation and modeling of atmospheric circulation, planetary atmospheres, stratospheric ozone transport, ocean circulations, and hydrospheric and biospheric processes. The focus of the review is to examine some of the early interests of this illustrious individual during the formative year of his career in atmospheric science, to document Mintz's creative insight concerning the development of the Mintz-Arakawa General Circulation Model (GCM), and to summarize briefly his scientific contributions. Much of his scientific work involved collaborations with an unusually talented array of younger scientists.

His descriptions of the field of mean motion, the zonally averaged state, and poleward angular momentum flux must be regarded as classic contributions to meteorology. The Mintz-Arakawa GCM was also a remarkable contribution to atmospheric science, both with respect to the development of early general circulation models and its range of applications to varied scientific challenges.

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Rafael Terra
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

This paper examines the impact of orographically induced mesoscale heterogeneities on the macroscopic behavior of planetary boundary layer (PBL) stratiform clouds, and implements and tests a physically based parameterization of this effect in the University of California, Los Angeles (UCLA), atmospheric general circulation model (AGCM). The orographic variance and associated thermal circulations induce inhomogeneities in the cloud field that can significantly alter the PBL evolution; an effect that has been largely ignored in existing climate models. The impact of this effect on AGCM simulations is examined and the mechanisms at work are studied by analyzing a series of Cloud System Resolving Model (CSRM) simulations.

Both the CSRM and AGCM results show that, in the absence of the orographic effect, the continental PBL tends to be in one of two regimes: the solid regime characterized by a cold and overcast PBL and the broken regime characterized by a low time-mean cloud incidence and a large-amplitude diurnal cycle. Without the orographic effect, the PBL may lock in the convectively stable solid regime, with deep convection displaced to the surrounding oceans and subsidence induced over land further contributing to the persistence of the cloud deck. The inclusion of the orographic effect weakens the feedback between the cloud's albedo and the ground temperature responsible for the existence of the two regimes and, therefore, conspires against the persistence of the solid regime rendering the behavior of the PBL–ground system less bimodal. The parameterization featured in this paper also increases the amplitude of the diurnal cycle in the AGCM and reduces the excessive seasonality in PBL cloud incidence, resulting in an improved simulation of convective precipitation over regions where the solid regime was spuriously dominating.

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Yongkang Xue
,
Fernando De Sales
,
Ratko Vasic
,
C. Roberto Mechoso
,
Akio Arakawa
, and
Stephen Prince

Abstract

A global and seasonal assessment of regions of the earth with strong climate–vegetation biophysical process (VBP) interactions is provided. The presence of VBP and degree of VBP effects on climate were assessed based on the skill of simulations of observed global precipitation by two general circulation models of the atmosphere coupled to three land models with varying degrees of complexity in VBP representation. The simulated VBP effects on precipitation were estimated to be about 10% of observed precipitation globally and 40% over land; the strongest impacts were in the monsoon regions. Among these, VBP impacts were highest on the West African, South Asian, East Asian, and South American monsoons. The specific characteristics of vegetation–precipitation interactions in northern high latitudes were identified. Different regions had different primary impact season(s) depending on regional climate characteristics and geographical features. The characteristics of VBP effects on surface energy and water balance as well as their interactions were also analyzed. The VBP-induced change in evaporation was the dominant factor in modulating the surface energy and water balance. The land–cloud interaction had substantial effects in the feedback. Meanwhile, the monsoon regions, midlatitudes lands, and high-latitude lands each exhibited quite different characteristics in circulation response to surface heating changes. This study is the first to compare simulations with observations to identify and assess global seasonal mean VBP feedback effects. It is concluded that VBPs are a major component of the global water cycle.

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Chung-Chun Ma
,
Carlos R. Mechoso
,
Akio Arakawa
, and
John D. Farrara

Abstract

The sensitivity of a coupled ocean–atmosphere general circulation model to parameterizations of selected physical processes is studied. The parameterizations include those of longwave radiation and surface turbulent fluxes in the atmospheric model, and those of vertical turbulent mixing and penetration of solar radiation in the ocean model. It is shown that the performance of the coupled model is highly sensitive to the parameterization of longwave radiation. This sensitivity is not solely due to the difference in surface radiative flux but involves interactions among radiation, convection, and large-scale dynamics of the atmosphere and ocean. It is concluded that differences in parameterizations can have large impacts on the performance of the coupled model, and these impacts can be very different from what may be expected from uncoupled model simulations.

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Chung-Chun Ma
,
Carlos R. Mechoso
,
Andrew W. Robertson
, and
Akio Arakawa

Abstract

Extensive and persistent stratus cloud decks are prominent climatic features off the Peruvian coast. They are believed to play a key role in the coupled atmosphere-ocean processes that determine the sea surface temperature (SST) throughout the eastern tropical Pacific. This notion is examined and further developed using a coupled ocean-atmosphere general circulation model (GCM): a control simulation, in which the simulated amount of Peruvian stratus clouds is unrealistically low, is compared with an experiment in which a stratus cloud deck is prescribed to persistently cover the ocean off the Peruvian coast.

Beneath the prescribed cloud deck SSTs are reduced by up to 5 K, as expected from decreased solar radiation reaching the surface. In addition, there is significant cooling over much of the eastern tropical Pacific south of the equator, and even along the equator well into the central Pacific. The prescribed stratus deck largely alleviates the coupled GCM's warm bias in SST in the southeastern Pacific, which is common to most contemporary coupled GCMS, and produces a distribution of SST with more realistic interhemispheric asymmetries.

Examination of differences between SST evolutions in the enhanced stratus experiment and the control circulation reveals that the remote ocean cooling is not due to a single mechanism. The cooling immediately to the west and north of the region with the prescribed stratus deck is primarily associated with increased evaporation as the southeast trades strengthen. The cooling along the equator in the central Pacific is mainly due to increased oceanic cold advection.

The results of this study suggest that the Peruvian stratus clouds are important in modulating the circulation of the tropical Pacific. The “double ITCZ” syndrome of the coupled GCM, however, does not appear to be solely due to underpredicted stratus cloud cover and requires consideration of other processes in the coupled GCM.

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