Smoothing and spatial verification of global fields
Pith reviewed 2026-05-25 08:24 UTC · model grok-4.3
The pith
Two new smoothing methods make spatial verification metrics feasible for global weather forecast grids.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper claims that two newly presented smoothing methodologies overcome the computational complexity of global spherical geometry and non-equidistant grids sufficiently to enable practical smoothing of high-resolution global fields while respecting variable grid-point areas and handling missing data, thereby making smoothing-based verification metrics such as the Fraction Skill Score usable in the global domain.
What carries the argument
Two new global-domain smoothing methodologies that incorporate variable grid areas and missing-data handling.
If this is right
- Smoothing-based metrics such as the Fraction Skill Score can be calculated for global forecast fields.
- Operational high-resolution global precipitation forecasts can be evaluated with spatial verification scores.
- Machine-learning global models can be assessed using the same spatial metrics.
- Verification can now address limitations of traditional non-spatial scores in the global setting.
Where Pith is reading between the lines
- The methods may apply to other global fields such as temperature or wind beyond the precipitation example shown.
- Similar smoothing logic could support additional spatial verification techniques not based on the Fraction Skill Score.
- Faster global smoothing opens the possibility of routine use in model development cycles for next-generation global forecasts.
Load-bearing premise
The new smoothing methods produce results close enough to exact smoothing that the derived verification metrics remain reliable.
What would settle it
Running the new methods and a brute-force exact smoother on the same moderate-resolution global field and checking whether the resulting smoothed values differ by more than a small tolerance.
Figures
read the original abstract
Forecast verification plays a crucial role in the development cycle of operational numerical weather prediction models. At the same time, verification remains a challenge as the traditionally used non-spatial forecast quality metrics exhibit certain drawbacks, with new spatial metrics being developed to address these problems. Some of these new metrics are based on smoothing, with one example being the widely used Fraction Skill Score (FSS) and its many derivatives. However, while the FSS has been used by many researchers in limited area domains, there are no examples of it being used in a global domain yet. The issue is due to the increased computational complexity of smoothing in a global domain, with its inherent spherical geometry and non-equidistant and/or irregular grids. At the same time, there clearly exists a need for spatial metrics that could be used in the global domain as the operational global models continue to be developed and improved, along with the new machine-learning-based models. Here, we present two new methodologies for smoothing in a global domain that are potentially fast enough to make the smoothing of high-resolution global fields feasible. Both approaches also consider the variability of grid point area sizes and can handle missing data appropriately. This, in turn, makes the calculation of smoothing-based metrics, such as FSS and its derivatives, in a global domain possible, which we demonstrate by evaluating the performance of operational high-resolution global precipitation forecasts provided by the European Centre for Medium-Range Weather Forecasts.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces two new smoothing methodologies for global spherical domains that incorporate variable grid-cell areas via explicit integration and handle missing data through masking. These methods are positioned as computationally feasible for high-resolution fields, thereby enabling practical application of the Fraction Skill Score (FSS) and its derivatives on global grids; the claim is supported by timing benchmarks and an example evaluation of ECMWF high-resolution precipitation forecasts.
Significance. If the efficiency and reliability claims hold, the work removes a longstanding barrier to spatial verification on global domains, which is relevant for ongoing development of operational global NWP models and emerging machine-learning forecasts. The explicit area weighting and missing-data handling, together with reported scaling to ~0.1° resolution, constitute a concrete, usable advance over prior limited-area FSS implementations.
minor comments (3)
- [§3] §3 (Method 2 description): the integration over variable cell areas is described in prose but would be clearer with an explicit summation formula or pseudocode block showing how the neighborhood integral is discretized on the irregular grid.
- [Figure 5] Figure 5 (timing benchmarks): report the number of repeated runs and any variability measure; single-run timings leave open whether the reported scaling is representative.
- [§4.2] §4.2 (ECMWF example): the neighborhood radii used for the FSS curves are stated in degrees but the conversion to grid-point counts on the native grid is not tabulated; adding this would aid reproducibility.
Simulated Author's Rebuttal
We thank the referee for the positive assessment of the manuscript, the recognition of its significance for global NWP verification, and the recommendation for minor revision. No major comments were listed in the report.
Circularity Check
No significant circularity; methodological extension is self-contained
full rationale
The manuscript introduces two new smoothing algorithms for global spherical grids that incorporate explicit area weighting and missing-data masks, then demonstrates their use for FSS on ECMWF precipitation fields. No equations are presented that equate a derived quantity to a fitted parameter or to a self-referential definition. No load-bearing uniqueness theorem or ansatz is imported via self-citation. The central claim (practical feasibility of global FSS) rests on explicit algorithmic descriptions and timing benchmarks rather than on any reduction to the paper's own inputs. This is the normal case of an independent methodological contribution.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Smoothing operations on global fields must account for spherical geometry, non-equidistant grids and variable grid point areas.
Reference graph
Works this paper leans on
-
[1]
ISSN 0001-0782. doi: 10.1145/361002.361007. J. Bentley. Multidimensional binary search trees in database applications. IEEE Transactions on Software Engineering, SE-5:333–340, 7
-
[2]
ISSN 0098-5589. doi: 10.1109/TSE.1979.234200. K. Bi, L. Xie, H. Zhang, X. Chen, X. Gu, and Q. Tian. Accurate medium-range global weather forecasting with 3d neural networks. Nature, 619,
-
[3]
doi: 10.1038/s41586-023-06185-3
ISSN 14764687. doi: 10.1038/s41586-023-06185-3. Z. B. Bouallègue, S. E. Theis, and C. Gebhardt. Enhancing cosmo-de ensemble forecasts by inexpensive techniques. Meteorologische Zeitschrift, 22:49–59, 2
-
[4]
doi: 10.1127/0941-2948/2013/0374
ISSN 0941-2948. doi: 10.1127/0941-2948/2013/0374. B. G. Brown, E. Gilleland, and E. E. Ebert. Forecasts of Spatial Fields. In Forecast Verification, pages 95–117. John Wiley and Sons, Ltd, Chichester, UK, feb
-
[5]
doi: 10.1002/9781119960003.ch6. URLhttps://onlinelibrary. wiley.com/doi/10.1002/9781119960003.ch6. R. A. Brown. Building a Balanced k-d Tree in O(kn log n) Time. Journal of Computer Graphics Techniques (JCGT), 4(1):50–68, mar
-
[6]
ISSN 1477-870X. doi: 10.1002/qj.3964. URL http://dx.doi.org/10.1002/qj.3964. B. Casati. New developments of the intensity-scale technique within the spatial verification methods intercomparison project. Weather and Forecasting, 25:113–143, 2
-
[8]
doi: 10.1017/S1350482704001239
ISSN 1350-4827. doi: 10.1017/S1350482704001239. B. Casati, C. Lussana, and A. Crespi. Scale-separation diagnostics and the symmetric bounded efficiency for the inter-comparison of precipitation reanalyses. International Journal of Climatology, 43:2287–2304, 4
-
[9]
ISSN 0899-8418. doi: 10.1002/joc.7975. C. Davis, B. Brown, and R. Bullock. Object-based verification of precipitation forecasts. part i: Methodology and application to mesoscale rain areas. Monthly Weather Review, 134:1772–1784, 7 2006a. ISSN 1520-0493. doi: 10.1175/MWR3145.1. C. Davis, B. Brown, and R. Bullock. Object-based verification of precipitation ...
-
[11]
ISSN 0027-0644. doi: 10.1175/MWR-D-14-00172.1. S. R. A. Dey, R. S. Plant, N. M. Roberts, and S. Migliorini. Assessing spatial precipitation uncertainties in a convective- scale ensemble. Quarterly Journal of the Royal Meteorological Society, 142:2935–2948, 10
-
[12]
ISSN 0035-9009. doi: 10.1002/qj.2893. M. Dorninger, E. Gilleland, B. Casati, M. P. Mittermaier, E. E. Ebert, B. G. Brown, and L. J. Wilson. The setup of the MesoVICT project. Bulletin of the American Meteorological Society, 99(9):1887–1906,
-
[13]
ISSN 00030007. doi: 10.1175/BAMS-D-17-0164.1. L. Duc, K. Saito, and H. Seko. Spatial-temporal fractions verification for high-resolution ensemble forecasts. Tellus A: Dynamic Meteorology and Oceanography, 65:18171, 12
-
[14]
doi: 10.3402/tellusa.v65i0.18171
ISSN 1600-0870. doi: 10.3402/tellusa.v65i0.18171. E. Ebert and J. McBride. Verification of precipitation in weather systems: determination of systematic errors. Journal of Hydrology, 239:179–202, 12
-
[15]
doi: 10.1016/S0022-1694(00)00343-7
ISSN 00221694. doi: 10.1016/S0022-1694(00)00343-7. ECMWF. Ifs documentation cy48r1 - part iii: Dynamics and numerical procedures. 2023a. doi: 10.21957/26F0AD3473. URL https://www.ecmwf.int/en/elibrary/ 81369-ifs-documentation-cy48r1-part-iii-dynamics-and-numerical-procedures . ECMWF. Ifs documentation cy48r1 - part iv: Physical processes. 2023b. doi: 10.2...
-
[16]
doi: 10.54302/mausam.v66i3.555
ISSN 0252-9416. doi: 10.54302/mausam.v66i3.555. J. H. Friedman, J. L. Bentley, and R. A. Finkel. An algorithm for finding best matches in logarithmic expected time. ACM Transactions on Mathematical Software, 3:209–226, 9
-
[17]
ISSN 0098-3500. doi: 10.1145/355744.355745. A. Gainford, S. L. Gray, T. H. A. Frame, A. N. Porson, and M. Milan. Improvements in the spread–skill relationship of precipitation in a convective-scale ensemble through blending. Quarterly Journal of the Royal Meteorological Society, 150:3146–3166, 7
-
[18]
ISSN 0035-9009. doi: 10.1002/qj.4754. E. Gilleland. A new characterization within the spatial verification framework for false alarms, misses, and overall patterns. Weather and Forecasting, 32(1):187–198,
-
[19]
ISSN 15200434. doi: 10.1175/W AF-D-16-0134.1. E. Gilleland, D. Ahijevych, B. G. Brown, B. Casati, and E. E. Ebert. Intercomparison of spatial forecast verification methods. Weather and Forecasting, 24:1416–1430, 10
-
[21]
ISSN 1520-0493. doi: 10.1175/MWR3457.1. C. Keil and G. C. Craig. A displacement and amplitude score employing an optical flow technique. Weather and Forecasting, 24:1297–1308, 10
-
[23]
ISSN 2299-3835. doi: 10.26491/mhwm/171699. R. Lam, A. Sanchez-Gonzalez, M. Willson, P. Wirnsberger, M. Fortunato, F. Alet, S. Ravuri, T. Ewalds, Z. Eaton-Rosen, W. Hu, A. Merose, S. Hoyer, G. Holland, O. Vinyals, J. Stott, A. Pritzel, S. Mohamed, and P. Battaglia. Learning skillful medium-range global weather forecasting. Science, 382:1416–1421, 12
-
[24]
ISSN 0036-8075. doi: 10.1126/science.adi2336. S. Lang, M. Alexe, M. Chantry, J. Dramsch, F. Pinault, B. Raoult, M. C. A. Clare, C. Lessig, M. Maier-Gerber, L. Magnusson, Z. B. Bouallègue, A. P. Nemesio, P. D. Dueben, A. Brown, F. Pappenberger, and F. Rabier. Aifs – ecmwf’s data-driven forecasting system,
- [25]
-
[26]
ISSN 2073-4433. doi: 10.3390/atmos9020043. S. Malardel, N. Wedi, W. Deconinck, M. Diamantakis, C. Kuehnlein, G. Mozdzynski, M. Hamrud, and P. Smolarkiewicz. A new grid for the ifs. ECMWF Newsletter, (146):23–28,
-
[27]
URL https: //www.ecmwf.int/node/17262
doi: 10.21957/ZWDU9U5I. URL https: //www.ecmwf.int/node/17262. M. Markou and P. Kassomenos. Cluster analysis of five years of back trajectories arriving in athens, greece.Atmospheric Research, 98:438–457, 11
-
[28]
doi: 10.1016/j.atmosres.2010.08.006
ISSN 01698095. doi: 10.1016/j.atmosres.2010.08.006. 18 Skok and Kosovelj, 2025 Smoothing and spatial verification of global fields A PREPRINT C. Marzban, S. Sandgathe, H. Lyons, and N. Lederer. Three spatial verification techniques: Cluster analy- sis, variogram, and optical flow. Weather and Forecasting, 24:1457–1471, 12
-
[30]
ISSN 1520-0493. doi: 10.1175/mwr-d-15-0167.1. URL http://dx.doi.org/10.1175/MWR-D-15-0167.1 . M. P. Mittermaier. Using an intensity-scale technique to assess the added benefit of high-resolution model precipitation forecasts. Atmospheric Science Letters, 7:36–42, 4
-
[31]
ISSN 1530-261X. doi: 10.1002/asl.127. M. P. Mittermaier. Is there any skill in daily global precipitation forecasts over the maritime continent? Quarterly Journal of the Royal Meteorological Society, 1
-
[32]
ISSN 0035-9009. doi: 10.1002/qj.4877. T. Necker, L. Wolfgruber, L. Kugler, M. Weissmann, M. Dorninger, and S. Serafin. The fractions skill score for ensemble forecast verification. Quarterly Journal of the Royal Meteorological Society, 150:4457–4477, 10
-
[33]
ISSN 0035-9009. doi: 10.1002/qj.4824. N. Roberts. Assessing the spatial and temporal variation in the skill of precipitation forecasts from an NWP model. Meteorological Applications, 15(1):163–169, mar
-
[34]
ISSN 13504827. doi: 10.1002/met.57. URL https: //onlinelibrary.wiley.com/doi/10.1002/met.57. N. M. Roberts and H. W. Lean. Scale-Selective Verification of Rainfall Accumulations from High-Resolution Forecasts of Convective Events. Monthly Weather Review , 136(1):78–97, jan
-
[35]
ISSN 1520-0493. doi: 10.1175/ 2007MWR2123.1. URL http://journals.ametsoc.org/doi/10.1175/2007MWR2123.1. C. S. Schwartz, J. S. Kain, S. J. Weiss, M. Xue, D. R. Bright, F. Kong, K. W. Thomas, J. J. Levit, M. C. Coniglio, and M. S. Wandishin. Toward improved convection-allowing ensembles: Model physics sensitivities and optimizing probabilistic guidance with...
-
[37]
doi: 10.1016/j.atmosres.2015.04.012
ISSN 01698095. doi: 10.1016/j.atmosres.2015.04.012. G. Skok. A New Spatial Distance Metric for Verification of Precipitation. Applied Sciences, 12(8):4048, apr
-
[38]
ISSN 2076-3417. doi: 10.3390/app12084048. URL https://www.mdpi.com/2076-3417/12/8/4048. G. Skok. Precipitation attribution distance. Atmospheric Research, 295,
-
[39]
ISSN 01698095. doi: 10.1016/j.atmosres. 2023.106998. G. Skok. Snapshot of the smoothing on sphere package, Mar
-
[40]
URL https://doi.org/10.5281/zenodo. 15100264. G. Skok and V . Hladnik. Verification of gridded wind forecasts in complex alpine terrain: A new wind verification methodology based on the neighborhood approach. Monthly Weather Review, 146:63–75, 1
-
[41]
ISSN 0027-0644. doi: 10.1175/MWR-D-16-0471.1. G. Skok and L. Lledó. Spatial verification of global precipitation forecasts. Quarterly Journal of the Royal Meteorolog- ical Society, 5
-
[42]
ISSN 0035-9009. doi: 10.1002/qj.5006. G. Skok and N. Roberts. Analysis of Fractions Skill Score properties for random precipitation fields and ECMWF forecasts. Quarterly Journal of the Royal Meteorological Society, 142(700):2599–2610, oct
-
[43]
ISSN 00359009. doi: 10.1002/qj.2849. URL https://onlinelibrary.wiley.com/doi/10.1002/qj.2849. G. Skok and N. Roberts. Estimating the displacement in precipitation forecasts using the Fractions Skill Score.Quarterly Journal of the Royal Meteorological Society, 144(711):414–425,
-
[44]
ISSN 1477870X. doi: 10.1002/qj.3212. S. W. Smith. The Scientist and Engineer’s Guide to Digital Signal Processing. California Technical Publishing,
-
[45]
ISSN 00270644. doi: 10.1175/2008MWR2415.1. H. Wernli, C. Hofmann, and M. Zimmer. Spatial forecast verification methods intercomparison project: Application of the sal technique. Weather and Forecasting, 24,
-
[46]
doi: 10.1175/2009W AF2222271.1
ISSN 08828156. doi: 10.1175/2009W AF2222271.1. J. A. Weyn, D. R. Durran, and R. Caruana. Improving data-driven global weather prediction using deep convolutional neural networks on a cubed sphere. Journal of Advances in Modeling Earth Systems, 12,
-
[47]
ISSN 19422466. doi: 10.1029/2020MS002109. D. S. Wilks. Statistical Methods in the Atmospheric Sciences. Elsevier,
-
[48]
ISBN 9780128158234. doi: 10.1016/ C2017-0-03921-6. URL https://linkinghub.elsevier.com/retrieve/pii/C20170039216. 19 Skok and Kosovelj, 2025 Smoothing and spatial verification of global fields A PREPRINT B. J. Woodhams, C. E. Birch, J. H. Marsham, C. L. Bain, N. M. Roberts, and D. F. A. Boyd. What is the added value of a convection-permitting model for fo...
work page 2025
-
[49]
ISSN 0027-0644. doi: 10.1175/MWR-D-17-0396.1. P. Zacharov and D. Rezacova. Using the fractions skill score to assess the relationship between an ensemble qpf spread and skill. Atmospheric Research, 94:684–693, 12
-
[50]
doi: 10.1016/j.atmosres.2009.03.004
ISSN 01698095. doi: 10.1016/j.atmosres.2009.03.004. 20
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