|Volume 56(3) — July 2007
Collaborative research at the intersection of weather and climate
Fundamental barriers to advancing weather and climate diagnosis and prediction on time-scales from days to years are partly attributable to gaps in knowledge and the limited capability of contemporary operational and research numerical prediction systems to represent precipitating convection and its multi-scale organization, particularly in the tropics. In this regard improvements in convective parameterization have not kept pace with improvements in knowledge gained from process studies of convective organization. As convective organization is not represented by contemporary parameterizations, the large-scale effects of convective organization have, therefore, yet to be properly assessed. Examples of tropical phenomena in which the multi-scale organization of convection is a key process are:
Common to these examples is the multiplicity of interacting scales associated with precipitating systems and the accompanying distinctive three-dimensional transports of mass, momentum and energy. The ubiquity of organized phenomena under-scores the necessity for representing dynamical coherence, upscale evolution, and regime-dependent trans-port in global models, since these aspects are not captured by contemporary parameterizations.
The effects of phase changes of water within organized convective systems are manifested on various temporal scales: from convective turnover and diurnal time-scales (hours to a day), to the time-scale of meso-convective organization (~days), to the residence time of water in the atmosphere (~2weeks). Thus, the behaviour and effect of phase changes of water and their association with convective organization is a fundamental research challenge affecting weather and also longer-term climate processes through the effects of humidity and clouds on cloud-radiation interaction.
Although the MJO is not the only manifestation of tropical multi-scale convective organization, it represents a critical mode of atmospheric variability that straddles the intersection of weather and climate (Shapiro and Thorpe 2004; THORPEX/ICSC 2005). The MJO dominates tropical variability on subseasonal time-scales. It has global influences through tropical/extra-tropical interactions and is directly involved with breaks in the Asian, Australian and African monsoons. The MJO is increasingly recognized as influencing high-impact weather events and climate variability on seasonal-to-interannual time-scales. Yet, adequate knowledge of the processes contributing to the initiation and maintenance of the MJO and realistic simulations and predictions remain major scientific challenges for the weather/climate community.
Observations, parameterized and explicit representation of convection, theoretical and idealized models
Precipitating tropical convection is organized across a wide range of spatial-temporal scales, specifically, four key categories of multi-scale organization:
A crucial unknown is how the smaller scales interact to form self-reinforcing larger-scale organized systems, e.g. MJO, monsoons. It is recognized that tropical synoptic/meso-convective activity is often coupled to meridionally trapped modes of atmospheric variability idealized as Rossby gravity waves and Kelvin waves. Issues have been raised regarding:
Field campaigns document regional to mesoscale structures and associated physical processes, e.g. coupled atmosphere-ocean boundary layer processes. Two regional campaigns of direct relevance to MJO genesis in the western Indian Ocean were conducted recently:
The fact that the MJO spans a large range of spatial and temporal scales, from convective to planetary, means that traditional regional campaigns per se are no longer sufficient to fully document and/or predict the scope of MJO episodes. It is necessary to engage the full global observation and prediction systems along with the traditional regional campaigns such as the those described above. This observational prediction requirement was the basis of a recommendation of the THORPEX-WCRP international workshop reported on below; namely, to “develop an internationally coordinated, virtual computational-observational laboratory”. This concept has developed into a WCRP-THORPEX initiative: “Year of tropical convection (YOTC)”. It is summarized in Waliser and Moncrieff (2007). A draft of the details of the concept is described in hydro.jpl.nasa.gov/imp/WCRP.THORPEX.YOTC.pdf.
Parameterized and explicit representation of convection
A long-standing shortcoming of weather forecast and climate prediction systems is their inadequate representation of subgrid-scale physics and the MJO is no exception. Figure 2 shows an example where an MJO disintegrates from a robust system in the analysis to a nonentity in the forecast in ~5 days. This issue is widely thought to be due to deficiencies in convective parameterization, although there is no fundamental proof of this conjecture. Simulations with Aqua-Planet models experience similar difficulties. Figure3 shows the minimal consistency of simulated convective organization in Aqua-Planet climate models having different convective parameterizations. Simulated systems propagate eastward, others westward and, most likely, none is truly an MJO. Moreover, the disparate spatial scales of the simulated convective organization shows that the simulations do not provide a correct scale selection.
Modern computers enable cloud-system resolving models (CRM; presently, non-hydrostatic models operating with a grid spacing of order 1km originally developed for process studies and idealizations) to have progressively larger computational domains and be run for progressively longer times. At a grid-spacing of a few kilometres, CRMs may represent mesoscale circulations explicitly but imperfectly represent cumulonimbus convection. In other words, CRMs incompletely capture the spatial hierarchy identified in the opening paragraph of the section “Observations, parameterized and explicit representation of convection, theoretical and idealized models”, specifically, the first of four key categories of convective organization. The effects of this truncation are not fully known and constitute an important research area.
The state-of-the-art is the global CRM, e.g. Tomita (2005). CRMs are applied in place of contemporary convective parameterization in an approach called cloud-system resolving convection parameterization or superparameterization, originally developed in an Aqua-Planet model (Grabowski, 2001) and recently applied in full climate models (e.g.Khairoutdinov et al., 2005). Interestingly, MJOs in superparameterized models are usually too intense and persistent—the opposite from MJOs in conventional parameterized models. This exaggerated behaviour poses new questions that are, arguably, more amenable to solution than those associated with contemporary parameterization.
Theoretical and idealized models
The parameterization problem is unlikely to be solved through enhanced-resolution simulation alone. Idealized dynamical models quantify important issues, such as upscale energy transport associated with meso-convective organization and mechanisms at work in numerically simulated systems. For example, the non-linear mechanistic model of Moncrieff (2004) in which meso-convective organization interlocks with Rossby-gyre dynamics quantifies upscale transport and super-rotation properties of MJO-like systems generated by the Grabowski (2001) superparameterized simulation. The quasi-linear multi-scale model of Biello et al. (2006), based on the asymptotic perturbation theory of Majda and Klein (2003) and representing three categories of heating (deep convection, stratiform and congestus (Jonnson et al., 1999), shows that MJO-like systems can be generated by organized upscale momentum flux and heating. Figure4 shows characteristic signatures of MJOs in this model:
Another idealized approach to quantifying large-scale convective organization involves models with a dynamically active troposphere, a passive planetary boundary layer, and analogue parameterizations of deep convection, surface heat exchange, and radiative cooling, and crude vertical resolution, i.e. the first and second baroclinic velocities proportional to sin πz and sin 2πz , respectively. Multi-scale convective organization occurs in the presence of the first baroclinic mode but not MJO-like coherence (Yano et al., 1995). More realistic MJO-like systems occur when the second baroclinic mode is introduced (Khouider and Majda, 2006).
MJO and ENSO
The El Niño-Southern Oscillation (ENSO) is mostly driven by large-scale atmosphere-ocean coupling in the Pacific region. Convective organization strongly affects atmosphere-ocean coupling by modifying the surface radiation budget, evaporation and wind stress and hence slow-manifold interaction between oceanic and atmospheric boundary layers. Strong surface eastward-travelling equatorial westerly wind bursts associated with the MJO excite oceanic Kelvin waves that affect the onset of El Niño by reducing the equatorial zonal gradient of sea-surface temperature. This inter-action involves three distinct mechanisms:
Improved predictive skill for the MJO will ultimately need to be incorporated into next-generation ENSO prediction models. The fact that large-scale convective organization is significantly different in coupled atmosphere-ocean models compared to atmosphere-only models suggests that incomplete formulations of how boundary layers of the ocean and atmosphere interact lie at the heart of the MJO-ENSO interaction issue.
MJO and the extra-tropics
The influence of multi-scale convective organization is distinctly non-local. The subseasonal to interannual varia-bility of tropical convection has a profound influence on synoptic-scale Rossby-wave dispersion into the extra-tropics and on planetary-scale circulation anomalies such as the Pacific North-American oscillation (PNA), the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). Studies such as Ferranti et al. (1990) suggest that a successful representation of tropical convection in weather and climate models will lead to improved mid-latitude predictive skill at week-2 and beyond. Global CRMs and superparameterization can address a primary objective of THORPEX/WCRP: the two-way interaction between the tropics and the higher-latitudes sketched in Figure5.
The initiation and maintenance of planetary waves by organized convection, as a two-way interaction between tropical and extra-tropical circulation, are crucial to more skilful prediction in the week-2 timeframe. For example, tropical cyclones influence the extra-tropics through their direct migration polewards into the mid-latitude storm tracks and/or poleward Rossby-wave dispersion. Similarly, Kiladis (1998) has shown that Rossby waves propagating from higher latitudes can excite tropical convection. The challenge of improving the representation of convection, its organization and interaction with the regional-to-global scale circulations for weather and climate prediction systems cannot be over-emphasized.
THORPEX and WCRP convened an international workshop on the organization and maintenance of tropical convection and the Madden Julian Oscillation at the International Centre for Theoretical Physics in Trieste, Italy, 13-17 March 2006. The objective was to assess the current state of knowledge and predictive skill of multi-scale organized tropical convection, and set priorities for collaborative research leading to advanced knowledge and predictive skill of organized tropical convection and the MJO. The workshop brought together experts in tropical convection and two-way interaction between the tropics and higher latitudes. The participants were charged with formulating recommendations and fostering opportunities that address key challenges to advancing knowledge and predictive skill of tropical convection and its large-scale organization and two-way interaction with the extra-tropics, that would emerge from:
The workshop reviewed current knowledge of organized tropical convection, with a specific focus on the MJO. This included the identification of issues to be addressed as a collaborative THORPEX/WCRP effort to improve knowledge, numerical simulation and prediction of tropical organized convection and the MJO, as well as socio-economic research and applications. An important issue was the two-way interaction between the tropics and the higher latitudes. Specifically, how are extra-tropical synoptic and planetary waves modulated by organized tropical convection and vice versa? Knowledge of the diagnosis, genesis and maintenance of organized tropical convection were discussed, e.g.:
The workshop break-out groups identified the following two major objectives as a basis for advancing observing, modelling and predicting the MJO and its socio-economic implications, and the design of forecast demonstration projects:
Recommendations for collaborative research
The following items were identified as collaborative THORPEX/WCRP activities:
There has been considerable activity since the THORPEX-WCRP workshop, as follows:
Position papers under development
Two position papers have been commissioned by THORPEX and WCRP to address the broader aspects of collaborative research of weather/climate and its intersection with the Earth system. The first paper, referred to as THORPEX/WCRP White Paper 1, is directed toward the weather/climate/socio-economic communities and their supporting agencies. This effort will propose specific collaboration between THORPEX and WCRP involving high-priority issues on numerical prediction and modelling, data assimilation, observations from weeks to seasons and socio-economic assessments and applications.
A second paper being prepared to inform policy-makers, national academies of science and users of weather, climate and environmental information of the urgent necessity for establishing an international multidisciplinary research agenda to accelerate advances in predicting high-impact weather and climate events, knowledge of complex interactions in the biological-chemical Earth System, and hence improved decision-making.
This paper summarizes the challenge of advancing the knowledge of tropical convection, multi-scale convective organization and mechanisms of two-way interaction with the extra-tropics, within the framework of a THORPEX/WCRP collaborative research initiative. Meeting this challenge is a critical step toward improving present medium-range numerical prediction skill and its extension into subseasonal time-scales and beyond. The activities described show timely progress towards this lofty goal.
We thank Gilbert Brunet, Duane Waliser and Huw Davies for their helpful comments.
Biello, J.A., A.J. Majda and M.W. Moncrieff, 2007: Meridional momen-tum flux and superrotation in the multiscale IPESD MJO model. J. Atmos. Sci. (in press).
CLIVAR Exchanges 2006: Special edition on Indian Ocean climate, 39, Vol. 11, 31 pp.
Ferranti, L., T.N. Palmer, F. Molteni and E. Klinker, 1990: Tropical-extra-tropical interaction associated with the 30-60 day oscillation and its impact on medium and extended range prediction. J. Atmos. Sci., 47, 2177-2199.
Grabowski, W.W., 2001: Coupling cloud processes with large-scale dynamics using the Cloud-Resolving Convection parameterization (CRCP). J. Atmos. Sci., 58, 978-997.
Johnson, R.H., T.M. Rickenbach, S.A.Rutledge, P.E. Ciesielski and W.H. Schubert, 1999: Trimodal characteristics of tropical convection. J. Climate, 2397-2407.
Khairoutdinov, M. and D. Randall, 2007: Evaluation of the simulated interannual and subseasonal variability in an AMIP-style simulation using the CSU Multiscale Modeling Framework. J. Climate, submitted.
Khouider, B. and A.J. Majda, 2006: Multicloud convective parameterizations with crude vertical resolution. Theoretical and Computational Fluid Dynamics. Special issue: Theoretical Developments in Tropical Meteorology, 20, 351-375.
Kiladis, G.N., 1998: Observations of Rossby waves linked to convection over the eastern tropical Pacific. J. Atmos. Sci., 55, 2321-339.
Madden, R.A. and P.R. Julian, 1972: Description of global-scale circulation cells in the Tropics with a 40–50 day period. J. Atmos. Sci., 29, 1109–1123.
Majda, A.J. and R. Klein, 2003: Systematic multiscale models for the tropics. J. Atmos. Sci., 60, 393-408.
Moncrieff, M.W., 2004: Analytic repre-sentation of the large-scale organization of tropical convection. J. Atmos. Sci., 61, 1521-1538.
Newman M., P.D. Sardeshmukh, C.R.Winkler and J.S. Whitaker, 2003: A study of subseasonal predictability. Mon. Wea. Rev., 131, 1715–1732.
Shapiro, M.A.. and A.J. Thorpe, 2004: THORPEX International Science Plan. WMO/TD-No.1246, WWRP/THORPEX, No.2, 51 pp.
THORPEX/International Core Steer-ing Committee (ICSC), 2005: THORPEX International Research Implementation Plan, WMO/TD-No.1258, WWRP/THORPEX, No.4, 95pp.
Yano, J.-I., J.C. McWilliams, M.W.Moncrieff and K.A. Emanuel, 1995: Hierarchical tropical cloud systems in an analog shallow-water model. J. Atmos. Sci., 52, 1723-1742.