Model Intercomparison Project Research Articles

Improving land-surface parameterization schemes using stable water isotopes: Introducing the ‘iPILPS’ initiative

iPILPS, isotopes in the Project for the Intercomparison of Land-surface Parameterization Schemes (PILPS), is a new stage in one of the longest running atmospheric/climate model intercomparison projects (MIPs). Its goal is to provide a rigorous framework within which isotopic simulations of the continental surface can be inter-compared, evaluated, validated and/or improved. Operating under the auspices of the Global Energy and Water Cycle Experiment's (GEWEX) Land–Atmosphere System Study, PILPS is an off-line intercomparison and improvement framework offered for land-surface schemes used in weather forecast, climate and Earth System models. This paper describes the motivation for iPILPS, emphasising the energy and water budget difficulties of current land-surface schemes (LSSs) and underlining the need for new tools with which to probe “correctness” of evapotranspiration parameterization. It also explains and describes the likely beneficial outcomes anticipated from including isotopes in land-surface parameterization evaluation. These are the potential to: improve parameterization of surface energy partition by current LSSs; add information leading to refinement of basin-wide water modelling; reduce the range in simulated carbon uptake in the whole biomass and its subcomponents; and fingerprint sources and sinks involved in specific processes allowing evaluation of LSSs and, hopefully, their validation.

Intrabasin comparison of surface radiocarbon levels in the Indian Ocean between coral records and three‐dimensional global ocean models

Oceanic uptake and transport of bomb‐produced radiocarbon is used as a diagnostic in global ocean models to test parameterizations of mixing and air‐sea gas exchange between the ocean and atmosphere. A model's ability to simulate bomb‐produced 14C is also a good indicator of its ability to predict uptake of anthropogenic CO2. We have conducted a model‐data comparison of surface radiocarbon time series from coral records from the coasts of Kenya and Sumatra and a suite of dynamical three‐dimensional ocean models that were included in the second phase of the Ocean Carbon‐Cycle Model Intercomparison Project. The coral records comprise the first intrabasin record of surface water Δ14C variability in the equatorial Indian Ocean and provide an independent evaluation of model performance. Differing treatments of lateral subgrid scale mixing in different models appear to be less important than other, unknown factors in explaining differences in results between the models. Those models that include a dynamic vertical mixing scheme appear to be more capable of matching observed coral radiocarbon time series. However, among models with the same parameterization of lateral subgrid scale mixing, there is a large degree of variation, suggesting that at both sites, factors such as resolution, topography, physical forcing and horizontal advection are more important than mixing parameterization in explaining intermodel differences. None of the models reproduce the time lag in the rate of bomb 14C response between the Kenya and Sumatra coral sites. Future efforts are needed to improve model simulation of radiocarbon in surface waters in the equatorial Indian Ocean.

Seasonal variability of deep currents in the equatorial Atlantic: a model study

A suite of high-resolution models of the Atlantic Ocean circulation is used to study the deep seasonal current variability in the equatorial regime, with a particular emphasis on its manifestation in the variability of the interhemispheric transports near the western boundary. The basic experiment has a resolution of 1/3∘ horizontally and 45 vertical levels, and is subject to a monthly mean atmospheric forcing based on ECMWF flux fields. Sensitivity experiments explore the effects of higher horizontal resolution (1/12∘), and alternative mixing parameterizations. The model behavior near the equator confirms previous suggestions based on solutions of the WOCE Community Modelling Effort (“CME”) and the “DYNAMO” model intercomparison project, of the presence of a system of vigorous seasonal current oscillations, spanning the whole water column and nearly the whole zonal extent of the basin. The patterns of the primarily zonal current anomalies are fairly robust across the range of model cases investigated, i.e., show relatively little sensitivity to horizontal resolution/mixing, or to the different choices of vertical discretization and vertical mixing as in the DYNAMO cases. The amplitude of the seasonal variation exceeds 10cm/s in the surface layer, and decreases to about 5cm/s near 1000m and 2–3cm/s in the deep ocean in both the basic 1/3∘- and the 1/12∘-cases, thereby leading to seasonally reversing current signatures at all depths below the EUC.A particular aspect of the seasonal current variability concerns its manifestation in the southward transport of North Atlantic Deep Water (NADW) by the Deep Western Boundary Current (DWBC). The temporal characteristics of the DWBC variability are in agreement with moored current meter observations at 44∘W, with simulated DWBC transports varying between a maximum of more than 30Sv in January/February, and almost vanishing transport in September. However, in contrast to the annual-mean deep water transport which is confined to the DWBC and tight, O(100)km-recirculation cells, the seasonal cycle of transport is not trapped near the boundary: the simulations show that the zonal current variations of the equatorial wave guide, near the western boundary give rise to a broad system of seasonal recirculation cells of the DWBC. Calculations of the amplitude of the seasonal variability in the deep water transport near the equator are therefore strongly dependent of the spatial extent of the cross-section considered; in particular, for being approximately representative of low-frequency variations in the net, zonally-integrated meridional transport of deep water in the equatorial regime, transport sections would need to extend over nearly the whole western basin.

Evaluating global ocean carbon models: The importance of realistic physics

A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon‐cycle Model Intercomparison Project (OCMIP‐2). Model‐data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3‐D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP‐2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model‐model and model‐data differences can be related to variations in surface forcing, subgrid‐scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air‐sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model‐model ranges in OCMIP‐2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model‐model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport‐dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological‐chemical‐physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

Estimates of anthropogenic carbon uptake from four three‐dimensional global ocean models

We have compared simulations of anthropogenic CO2 in the four three‐dimensional ocean models that participated in the first phase of the Ocean Carbon‐Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr−1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present‐day air‐sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data‐based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data‐based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the data‐based estimates could be provided regionally.

Intercomparison of Three- and one-Dimensional Model Simulations and Aircraft Observations of Stratocumulus

As part of the EUropean Cloud REsolving Modelling (EUCREM) model intercomparison project we compared the properties and development of stratocumulus as revealed by actual observations and as derived from two types of models, namely three-dimensional Large Eddy Simulations (LES) and one-dimensional Single Column Models (SCMs). The turbulence, microphysical and radiation properties were obtained from observations made in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). The goal of the intercomparison was to study the turbulence and microphysical properties of a stratocumulus layer with specified initial and boundary conditions. The LES models predict an entrainment velocity which is significantly larger than estimated from observations. Because the observed value contains a large experimental uncertainty no definitive conclusions can be drawn from this. The LES modelled buoyancy flux agrees rather well with the observed values, which indicates that the intensity of the convection is modelled correctly. From LES it was concluded that the inclusion of drizzle had a small influence (about 10%) on the buoyancy flux. All SCMs predict a solid stratocumulus layer with the correct liquid water profile. However, the buoyancy flux profile is poorly represented in these models. From the comparison with observations it is clear that there is considerable uncertainty in the parametrization of drizzle in both SCM and LES.