Scientific papers

Below is the list of scientific and technical publications stemming out of PRIMAVERA, all published in open access journals.

Tropical cyclone interaction with the ocean: the role of high frequency (sub-daily) coupled processes

E. Scoccimarro, P.G. Fogli. K. Reed, S. Gualdi, S.Masina, A. Navarra • December 2016Journal of Climate • doi:10.1175/JCLI-D-16-0292.1 • View publication

Abstract. Through tropical cyclone (TC) activity the ocean and the atmosphere exchange a large amount of energy. In this work possible improvements introduced by a higher coupling frequency are tested between the two components of a climate model in the representation of TC intensity and TC–ocean feedbacks. The analysis is based on the new Centro Euro-Mediterraneo per I Cambiamenti Climatici Climate Model (CMCC-CM2-VHR), capable of representing realistic TCs up to category-5 storms. A significant role of the negative sea surface temperature (SST) feedback, leading to a weakening of the cyclone intensity, is made apparent by the improved representation of high-frequency coupled processes. The first part of this study demonstrates that a more realistic representation of strong TC count is obtained by coupling atmosphere and ocean components at hourly instead of daily frequency. Coherently, the positive bias of the annually averaged power dissipation index associated with TCs is reduced by one order of magnitude when coupling at the hourly frequency, compared to both forced mode and daily coupling frequency results. The second part of this work shows a case study (a modeled category-5 typhoon) analysis to verify the impact of a more realistic representation of the high-frequency coupling in representing the TC effect on the ocean; the theoretical subsurface warming induced by TCs is well represented when coupling the two components at the higher frequency. This work demonstrates that an increased horizontal resolution of model components is not sufficient to ensure a realistic representation of intense and fast-moving systems, such as tropical and extratropical cyclones, but a concurrent increase in coupling frequency is required.

High resolution model intercomparison project (HighResMIP)

R. J. Haarsma, M. Roberts, P. L. Vidale, C. A. Senior, A. Bellucci, Q. Bao, P. Chang, S. Corti, N. S. Fučkar, V. Guemas, J. von Hardenberg, W. Hazeleger, C. Kodama, T. Koenigk, L. R. Leung, J. Lu, J.-J. Luo, J. Mao, M. S. Mizielinski, R. Mizuta, P. Nobre, M. Satoh, E. Scoccimarro, T. Semmler, J. Small, J.-S. von Storch • November 2016Geoscientifc Model Development Discussions • doi:10.5194/gmd-2016-6 • View publication

Short summary. Recent progress in computing power has enabled climate models to simulate more processes in detail and on a smaller scale. Here we present a common protocol for these high-resolution runs that will foster the analysis and understanding of the impact of model resolution on the simulated climate. These runs will also serve as a more reliable source for assessing climate risks that are associated with small-scale weather phenomena such as tropical cyclones.

Impact of ocean resolution on coupled air-sea fluxes and large-scale climate

M. J. Roberts, H. T. Hewitt, P. Hyder, D. Ferreira, S. A. Josey, M. Mizielinski, A. Shelly • October 2016Geophysical Research Letters • doi:10.1002/2016GL070559 • View publication

Abstract. Air-sea fluxes are a crucial component in the energetics of the global climate system. The largest air-sea fluxes occur in regions of high sea surface temperature variability, such as ocean boundary, frontal currents and eddies. In this paper we explore the importance of ocean model resolution to resolve air-sea flux relationships in these areas. We examine the sea surface temperature-wind stress relationship in high-pass filtered observations and two versions of the Met Office climate model with eddy-permitting and eddy-resolving ocean resolutions. Eddy-resolving resolution shows marginal improvement in the relationship over eddy-permitting resolution. However, by focussing on the North Atlantic we show that the eddy-resolving model has significant enhancement of latent heat loss over the North Atlantic Current region, a long-standing model bias. While eddy-resolving resolution does not change the air-sea flux relationship at small scale, the impact on the mean state has important implications for the reliability of future climate projections.

Designing variable ocean model resolution based on the observed ocean variability

D. V. Sein, S. Danilov, A. Biastoch, J. V. Durgadoo, D. Sidorenko, S. Harig, Q. Wang • June 2016Journal of Advances in Modeling Earth Systems • doi:10.1002/2016MS000650 • View publication

Abstract. If unstructured meshes are refined to locally represent eddy dynamics in ocean circulation models, a practical question arises on how to vary the resolution and where to deploy the refinement. We propose to use the observed sea surface height variability as the refinement criterion. We explore the utility of this method (i) in a suite of idealized experiments simulating a wind-driven double gyre flow in a stratified circular basin and (ii) in simulations of global ocean circulation performed with FESOM. Two practical approaches of mesh refinement are compared. In the first approach the uniform refinement is confined within the areas where the observed variability exceeds a given threshold. In the second one the refinement varies linearly following the observed variability. The resolution is fixed in time. For the double gyre case it is shown that the variability obtained in a high-resolution reference run can be well captured on variable-resolution meshes if they are refined where the variability is high and additionally upstream the jet separation point. The second approach of mesh refinement proves to be more beneficial in terms of improvement downstream the midlatitude jet. Similarly, in global ocean simulations the mesh refinement based on the observed variability helps the model to simulate high variability at correct locations. The refinement also leads to a reduced bias in the upper-ocean temperature.

Characteristics and development of European cyclones with tropical origin in reanalysis data

M. M. Dekker, R. J. Haarsma, H. de Vries, M. Baatsen, A. J. van Delden • March 2017Climate Dynamics • doi:10.1007/s00382-017-3619-8 • View publication

Abstract. Major storm systems over Europe frequently have a tropical origin. This paper analyses the characteristics and dynamics of such cyclones in the observational record, using MERRA reanalysis data for the period 1979-2013. By stratifying the cyclones along three key phases of their development (tropical phase, extratropical transition and final re-intensification), we identify four radically different life cycles: the tropical cyclone and extratropical cyclone life cycles, the classic extratropical transition and the warm seclusion life cycle. More than 50% of the storms reaching Europe from low latitudes follow the warm seclusion life cycle. It also contains the strongest cyclones. They are characterized by a warm core and a frontal T-bone structure, with a northwestward warm conveyor belt and the effects of dry intrusion. Rapid deepening occurs in the latest phase, around their arrival in Europe. Both baroclinic instability and release of latent heat contribute to the strong intensification. The pressure minimum occurs often a day after entering Europe, which enhances the potential threat of warm seclusion storms for Europe. The impact of a future warmer climate on the development of these storms is discussed.

Resolution dependence of extreme precipitation and deep convection over the Gulf Stream

S. Scher, R. J. Haarsma, H. de Vries, S. S. Drijfhout, A. J. van Delden • April 2017Journal of Advances in Modeling Earth Systems • doi:10.1002/2016MS000903 • View publication

Abstract. Modeled wintertime precipitation over the Atlantic Gulf Stream region is shown to be sensitive to the horizontal resolution of the driving Global Circulation Model (GCM). By contrasting simulations with the EC-Earth GCM over a range of horizontal resolutions (T159, T319, T799), it is shown that especially the precipitation extremes become more populated if resolution is higher. Higher resolution also appears to strengthen the communication from the sea surface toward the troposphere. With increasing resolution, deep convection over the Gulf Stream region, diagnosed via wind-convergence and vertical motion, occurs more frequently and the former is in better agreement with observations. Likewise the frequency increase of the precipitation extremes over the region for increasing resolution makes them agree better with observations, despite large natural variability and discrepancies between different observational sources.

Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?

H. T. Hewitt, M. J. Bell, E. P. Chassignet, A. Czaja, D. Ferreira, S. M. Griffies, P. Hyder, J. L. McClean, A. L. New, M. J. Roberts  • November 2017Ocean Modelling • doi:10.1016/j.ocemod.201711002 • View publication

Abstract. As the importance of the ocean in the weather and climate system is increasingly recognised, operational systems are now moving towards coupled prediction not only for seasonal to climate timescales but also for short-range forecasts. A three-way tension exists between the allocation of computing resources to refine model resolution, the expansion of model complexity/capability, and the increase of ensemble size. Here we review evidence for the benefits of increased ocean resolution in global coupled models, where the ocean component explicitly represents transient mesoscale eddies and narrow boundary currents. We consider lessons learned from forced ocean/sea-ice simulations; from studies concerning the SST resolution required to impact atmospheric simulations; and from coupled predictions. Impacts of the mesoscale ocean in western boundary current regions on the large-scale atmospheric state have been identified. Understanding of air-sea feedback in western boundary currents is modifying our view of the dynamics in these key regions. It remains unclear whether variability associated with open ocean mesoscale eddies is equally important to the large-scale atmospheric state. We include a discussion of what processes can presently be parameterised in coupled models with coarse resolution non-eddying ocean models, and where parameterizations may fall short. We discuss the benefits of resolution and identify gaps in the current literature that leave important questions unanswered.

Relationships between Arctic sea ice drift and strength modelled by NEMO-LIM3.6

D. Docquier, F. Massonnet, A. Barthélemy, N. F. Tandon, O. Lecomte, and T. Fichefet • December 2017The Cryosphere • doi:10.5194/tc-11-2829-2017 • View publication

Abstract. Sea ice cover and thickness have substantially decreased in the Arctic Ocean since the beginning of the satellite era. As a result, sea ice strength has been reduced, allowing more deformation and fracturing and leading to increased sea ice drift speed. We use the version 3.6 of the global ocean–sea ice NEMO-LIM model (Nucleus for European Modelling of the Ocean coupled to the Louvain-la-Neuve sea Ice Model), satellite, buoy and submarine observations, as well as reanalysis data over the period from 1979 to 2013 to study these relationships. Overall, the model agrees well with observations in terms of sea ice extent, concentration and thickness. The seasonal cycle of sea ice drift speed is reasonably well reproduced by the model. NEMO-LIM3.6 is able to capture the relationships between the seasonal cycles of sea ice drift speed, concentration and thickness, with higher drift speed for both lower concentration and lower thickness, in agreement with observations. Model experiments are carried out to test the sensitivity of Arctic sea ice drift speed, thickness and concentration to changes in sea ice strength parameter P*. These show that higher values of P* generally lead to lower sea ice deformation and lower sea ice thickness, and that no single value of P* is the best option for reproducing the observed drift speed and thickness. The methodology proposed in this analysis provides a benchmark for a further model intercomparison related to the relationships between sea ice drift speed and strength, which is especially relevant in the context of the upcoming Coupled Model Intercomparison Project 6 (CMIP6).

The benefits of global high-resolution for climate simulation: process-understanding and the enabling of stakeholder decisions at the regional scale

M. J. Roberts, P. L. Vidale, C. Senior, H. T. Hewitt, C. Bates, S. Berthou, P. Chang, H. M. Christensen, S. Danilov, M.-E. Demory, S. M. Griffies, R. Haarsma, T. Jung, G. Martin, S. Minobe, T. Ringler, M. Satoh, R. Schiemann, E. Scoccimarro, G. Stephens, M. F. Wehner,   • November 2018BAMS • doi:10.1175/BAMS-D-15-00320.1 • https://journals.ametsoc.org/doi/10.1175/BAMS-D-15-00320.1

Abstract. The timescales of the Paris Climate Agreement indicate urgent action is required on climate policies over the next few decades, in order to avoid the worst risks posed by climate change. On these relatively short timescales the combined effect of climate variability and change are both key drivers of extreme events, with decadal timescales also important for infrastructure planning. Hence, in order to assess climate risk on such timescales, we require climate models to be able to represent key aspects of both internally driven climate variability, as well as the response to changing forcings.
In this paper we argue that we now have the modelling capability to address these requirements - specifically with global models having horizontal resolutions considerably enhanced from those typically used in previous IPCC and CMIP exercises. The improved representation of weather and climate processes in such models underpins our enhanced confidence in predictions and projections, as well as providing improved forcing to regional models, which are better able to represent local-scale extremes (such as convective precipitation). We choose the global water cycle as an illustrative example, because it is governed by a chain of processes for which there is growing evidence of the benefits of higher resolution. At the same time it comprises key processes involved in many of the expected future climate extremes (e.g. flooding, drought, tropical and mid-latitude storms).

Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

McCoy, D. T., Field, P. R., Schmidt, A., Grosvenor, D. P., Bender, F. A. M., Shipway, B. J., Hill, A. A.; Wilkinson, J. M., Elsaesser, G. S. • April 2018 • Journal of Atmospheric Chemistry • doi:10.5194/acp-18-5821-2018 • https://www.atmos-chem-phys.net/18/5821/2018/

Abstract: Aerosol-cloud interactions are a major source of uncertainty in inferring the climate sensitivity from the observational record of temperature. The adjustment of clouds to aerosol is a poorly constrained aspect of these aerosol-cloud interactions. Here, we examine the response of midlatitude cyclone cloud properties to a change in cloud droplet number concentration (CDNC). Idealized experiments in high-resolution, convection-permitting global aquaplanet simulations with constant CDNC are compared to 13 years of remote-sensing observations. Observations and idealized aquaplanet simulations agree that increased warm conveyor belt (WCB) moisture flux into cyclones is consistent with higher cyclone liquid water path (CLWP). When CDNC is increased a larger LWP is needed to give the same rain rate. The LWP adjusts to allow the rain rate to be equal to the moisture flux into the cyclone along the WCB. This results in an increased CLWP for higher CDNC at a fixed WCB moisture flux in both observations and simulations. If observed cyclones in the top and bottom tercile of CDNC are contrasted it is found that they have not only higher CLWP but also cloud cover and albedo. The difference in cyclone albedo between the cyclones in the top and bottom third of CDNC is observed by CERES to be between 0.018 and 0.032, which is consistent with a 4.6-8.3gWmg‾² in-cyclone enhancement in upwelling shortwave when scaled by annual-mean insolation. Based on a regression model to observed cyclone properties, roughly 60g% of the observed variability in CLWP can be explained by CDNC and WCB moisture flux.

Predicting decadal trends in cloud droplet number concentration using reanalysis and satellite data

McCoy, D. T., Bender, F. A-M., Grosvenor, D. P., Mohrmann, J. K., Hartmann, D. L., Wood, R., Field, P. R.  • Feb 2018 • Journal of Atmospheric Chemistry and Physics • doi:10.5194/acp-18-2035-2018 • https://www.atmos-chem-phys.net/18/2035/2018/

Abstract: Cloud droplet number concentration (CDNC) is the key state variable that moderates the relationship between aerosol and the radiative forcing arising from aerosol–cloud interactions. Uncertainty related to the effect of anthropogenic aerosol on cloud properties represents the largest uncertainty in total anthropogenic radiative forcing. Here we show that regionally averaged time series of the Moderate-Resolution Imaging Spectroradiometer (MODIS) observed CDNC of low, liquid-topped clouds is well predicted by the MERRA2 reanalysis near-surface sulfate mass concentration over decadal timescales. A multiple linear regression between MERRA2 reanalyses masses of sulfate (SO₄), black carbon (BC), organic carbon (OC), sea salt (SS), and dust (DU) shows that CDNC across many different regimes can be reproduced by a simple power-law fit to near-surface SO₄, with smaller contributions from BC, OC, SS, and DU. This confirms previous work using a less sophisticated retrieval of CDNC on monthly timescales. The analysis is supported by an examination of remotely sensed sulfur dioxide (SO₂) over maritime volcanoes and the east coasts of North America and Asia, revealing that maritime CDNC responds to changes in SO₂ as observed by the ozone monitoring instrument (OMI). This investigation of aerosol reanalysis and top-down remote-sensing observations reveals that emission controls in Asia and North America have decreased CDNC in their maritime outflow on a decadal timescale.

Simulated lightning in a convection permitting global model

P. Field, M. Roberts, J. Wilkinson. Journal of Geophysical Research - Atmospheres. Accepted. View publication

Abstract: High resolution (grid spacing ~8km in midlatitudes) model simulations using explicitly resolved convection in the Met Office Unified Model, as part of the EU Horizon 2020 PRIMAVERA project, are used to provide a global lightning climatology. The results show for the first time that global simulations can capture the strong diurnal flash rate variation as well as the seasonal variation. The lightning parametrization uses information about the graupel and ice water path to estimate a total lightning flash rate. Comparisons are made with the World Lightning Location Network (that mainly detects cloud to ground lightning) and combined LIS(Lightning Imaging Sensor), OTD(Optical transients detector) dataset (that provides an estimate of total flash rate). The model results generally capture the temporal behavior and spatial distribution of the lightning over land. Over the ocean the lightning in the ITCZ appears excessive.

The following are papers written before PRIMAVERA, but with methods or processes that will be applied to PRIMAVERA multi-model simulations.

The resolution sensitivity of Northern Hemisphere blocking in four 25‑km atmospheric global circulation models

R. Schiemann, M.-E. Demory, L. C. Shaffrey, J. Strachan, P. L. Vidale, M. S. Mizielinski, M. J. Roberts, M. Matsueda, M. F. Wehner, T. Jung • December 2016Journal of Climate • doi:10.1175/JCLI-D-16-0100.1 • View publication

Abstract. The aim of this study is to investigate if the representation of Northern Hemisphere blocking is sensitive to resolution in current-generation atmospheric global circulation models (AGCMs). An evaluation is conducted of how well atmospheric blocking is represented in four AGCMs whose horizontal resolution is increased from a grid spacing of more than 100 km to about 25 km. It is shown that Euro-Atlantic blocking is simulated overall more credibly at higher resolution (i.e., in better agreement with a 50-yr reference blocking climatology created from the reanalyses ERA-40 and ERA-Interim). The improvement seen with resolution depends on the season and to some extent on the model considered. Euro-Atlantic blocking is simulated more realistically at higher resolution in winter, spring, and autumn, and robustly so across the model ensemble. The improvement in spring is larger than that in winter and autumn. Summer blocking is found to be better simulated at higher resolution by one model only, with little change seen in the other three models. The representation of Pacific blocking is not found to systematically depend on resolution. Despite the improvements seen with resolution, the 25-km models still exhibit large biases in Euro-Atlantic blocking. For example, three of the four 25-km models underestimate winter northern European blocking frequency by about one-third. The resolution sensitivity and biases in the simulated blocking are shown to be in part associated with the mean-state biases in the models’ midlatitude circulation.