The Gulf Stream influence on wintertime North Atlantic jet variability


CH O'Reilly, S Minobe, A Kuwano-Yoshida, T Woollings

THE GLOBAL AEROSOL SYNTHESIS AND SCIENCE PROJECT (GASSP): Measurements and Modeling to Reduce Uncertainty


CL Reddington, KS Carslaw, P Stier, N Schutgens, H Coe, D Liu, J Allan, J Browse, KJ Pringle, LA Lee, M Yoshioka, JS Johnson, LA Regayre, DV Spracklen, GW Mann, A Clarke, M Hermann, S Henning, H Wex, TB Kristensen, WR Leaitch, U Poeschl, D Rose, MO Andreae, J Schmale, Y Kondo, N Oshima, JP Schwarz, A Nenes, B Andersrson, GC Roberts, JR Snider, C Leck, PK Quinn, X Chi, A Ding, JL Jimenez, Q Zhang

The impact of stochastic physics on tropical rainfall variability in global climate models on daily to weekly time scales


PAG Watson, J Berner, S Corti, P Davini, J von Hardenberg, C Sanchez, A Weisheimer, TN Palmer

Climate SPHINX: evaluating the impact of resolution and stochastic physics parameterisations in the EC-Earth global climate model


P Davini, J von Hardenberg, S Corti, HM Christensen, S Juricke, A Subramanian, PAG Watson, A Weisheimer, TN Palmer

Introducing independent patterns into the Stochastically Perturbed Parametrization Tendencies (SPPT) scheme

Quarterly Journal of the Royal Meteorological Society 143 (2017) 2168-2181

HM Christensen, SJ Lock, IM Moroz, TN Palmer

© 2017 Royal Meteorological Society The Stochastically Perturbed Parametrization Tendencies (SPPT) scheme is used at weather and climate forecasting centres worldwide to represent model uncertainty that arises from simplifications involved in the parametrization process. It uses spatio-temporally correlated multiplicative noise to perturb the sum of the parametrized tendencies. However, SPPT does not distinguish between different parametrization schemes, which do not necessarily have the same error characteristics. A generalization to SPPT is proposed, whereby the tendency from each parametrization scheme can be perturbed using an independent stochastic pattern. This acknowledges that the forecast errors arising from different parametrizations are not perfectly correlated. Two variations of this ‘independent SPPT’ (iSPPT) approach are tested in the Integrated Forecasting System (IFS). The first perturbs all parametrized tendencies independently, while the second groups tendencies before perturbation. The iSPPT schemes lead to statistically significant improvements in forecast reliability in the Tropics in medium-range weather forecasts. This improvement can be attributed to a large, beneficial increase in ensemble spread in regions with significant convective activity. The iSPPT schemes also lead to improved forecast skill in the extratropics for a set of cases in which the synoptic initial conditions were more likely to result in European ‘forecast busts’. Longer 13 month simulations are also considered to indicate the effect of iSPPT on the mean climate of the IFS.



MS Lozier, S Bacon, AS Bower, SA Cunningham, MF de Jong, L de Steur, B deYoung, J Fischer, SF Gary, BJW Greenan, P Heimbach, NP Holliday, L Houpert, ME Inall, WE Johns, HL Johnson, J Karstensen, F Li, X Lin, N Mackay, DP Marshall, H Mercier, PG Myers, RS Pickart, HR Pillar, F Straneo, V Thierry, RA Weller, RG Williams, C Wilson, J Yang, J Zhao, JD Zika

Variability in seasonal forecast skill of Northern Hemisphere winters over the 20th century

Geophysical Research Letters American Geophysical Union (AGU) (2017)

C O'Reilly, MacLeod, A Weisheimer, T Palmer, T Woollings

A family of super congruences involving multiple harmonic sums

International Journal of Number Theory 13 (2017) 109-128

M McCoy, K Thielen, L Wang, J Zhao

The impact of Southern Ocean residual upwelling on atmospheric CO2 on centennial and millennial timescales

CLIMATE DYNAMICS 48 (2017) 1611-1631

JM Lauderdale, RG Williams, DR Munday, DP Marshall

A deformation-based parametrization of ocean mesoscale eddy reynolds stresses

Ocean Modelling 112 (2017) 99-111

JA Anstey, L Zanna

© 2017 The Authors Ocean mesoscale eddies strongly affect the strength and variability of large-scale ocean jets such as the Gulf Stream and Kuroshio Extension. Their spatial scales are too small to be fully resolved in many current climate models and hence their effects on the large-scale circulation need to be parametrized. Here we propose a parametrization of mesoscale eddy momentum fluxes based on large-scale flow deformation. The parametrization is argued to be suitable for use in eddy-permitting ocean general circulation models, and is motivated by an analogy between turbulence in Newtonian fluids (such as water) and laminar flow in non-Newtonian fluids. A primitive-equations model in an idealised double-gyre configuration at eddy-resolving horizontal resolution is used to diagnose the relationship between the proposed closure and the eddy fluxes resolved by the model. Favourable correlations suggest the closure could provide an appropriate deterministic parametrization of mesoscale eddies. The relationship between the closure and different representations of the Reynolds stress tensor is also described. The parametrized forcing possesses the key quasi-geostrophic turbulence properties of energy conservation and enstrophy dissipation, and allows for upgradient fluxes leading to the sharpening of vorticity gradients. The implementation of the closure for eddy-permitting ocean models requires only velocity derivatives and a single parameter that scales with model resolution.

On the spatio-temporal representativeness of observations

Atmospheric Chemistry and Physics European Geosciences Union (EGU) (2017)

NJ Schutgens, S Tsyro, E Gryspeerdt, D Goto, N Weigum, M Schulz, P Stier

Variability in seasonal forecast skill of Northern Hemisphere winters over the twentieth century


CH O'Reilly, J Heatley, D MacLeod, A Weisheimer, TN Palmer, N Schaller, T Woollings

The Dynamical Influence of the Atlantic Multidecadal Oscillation on Continental Climate

JOURNAL OF CLIMATE 30 (2017) 7213-7230

CH O'Reilly, T Woollings, L Zanna

Constraining the instantaneous aerosol influence on cloud albedo.

Proceedings of the National Academy of Sciences of the United States of America 114 (2017) 4899-4904

E Gryspeerdt, J Quaas, S Ferrachat, A Gettelman, S Ghan, U Lohmann, H Morrison, D Neubauer, DG Partridge, P Stier, T Takemura, H Wang, M Wang, K Zhang

Much of the uncertainty in estimates of the anthropogenic forcing of climate change comes from uncertainties in the instantaneous effect of aerosols on cloud albedo, known as the Twomey effect or the radiative forcing from aerosol-cloud interactions (RFaci), a component of the total or effective radiative forcing. Because aerosols serving as cloud condensation nuclei can have a strong influence on the cloud droplet number concentration (Nd ), previous studies have used the sensitivity of the Nd to aerosol properties as a constraint on the strength of the RFaci. However, recent studies have suggested that relationships between aerosol and cloud properties in the present-day climate may not be suitable for determining the sensitivity of the Nd to anthropogenic aerosol perturbations. Using an ensemble of global aerosol-climate models, this study demonstrates how joint histograms between Nd and aerosol properties can account for many of the issues raised by previous studies. It shows that if the anthropogenic contribution to the aerosol is known, the RFaci can be diagnosed to within 20% of its actual value. The accuracy of different aerosol proxies for diagnosing the RFaci is investigated, confirming that using the aerosol optical depth significantly underestimates the strength of the aerosol-cloud interactions in satellite data.

A note on ‘Toward a stochastic parameterization of ocean mesoscale eddies’

Ocean Modelling 113 (2017) 30-33

I Grooms, L Zanna

© 2017 Porta Mana and Zanna (2014) recently proposed a subgrid-scale parameterization for eddy-permitting quasigeostrophic models. In this model the large-scale fluid is represented as a non-Newtonian viscoelastic medium, with a subgrid-stress closure that involves the Lagrangian derivative of large-scale quantities. This note derives this parameterization, including the nondimensional proportionality coefficient, using only two statistical assumptions: that the subgrid-scale term is locally homogeneous and decorrelates rapidly in space. The parameterization is then verified by comparing against eddy-resolving quasigeostrophic simulations, independently reproducing the results of Porta Mana and Zanna in a simpler model.

Strong constraints on aerosol-cloud interactions from volcanic eruptions.

Nature 546 (2017) 485-491

FF Malavelle, JM Haywood, A Jones, A Gettelman, L Clarisse, S Bauduin, RP Allan, IHH Karset, JE Kristjánsson, L Oreopoulos, N Cho, D Lee, N Bellouin, O Boucher, DP Grosvenor, KS Carslaw, S Dhomse, GW Mann, A Schmidt, H Coe, ME Hartley, M Dalvi, AA Hill, BT Johnson, CE Johnson, JR Knight, FM O'Connor, DG Partridge, P Stier, G Myhre, S Platnick, GL Stephens, H Takahashi, T Thordarson

Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol-cloud interactions. Here we show that the massive 2014-2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets-consistent with expectations-but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around -0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

Seasonal Climate Prediction: A New Source of Information for the Management of Wind Energy Resources


V Torralba, FJ Doblas-Reyes, D MacLeod, I Christel, M Davis

Stochastic Subgrid-Scale Ocean Mixing: Impacts on Low-Frequency Variability

JOURNAL OF CLIMATE 30 (2017) 4997-5019

S Juricke, TN Palmer, L Zanna

Observational evidence against strongly stabilizing tropical cloud feedbacks


IN Williams, RT Pierrehumbert

Impact of stochastic physics on tropical precipitation in the coupled ECMWF model


A Subramanian, A Weisheimer, T Palmer, F Vitart, P Bechtold