Hans von Storch1, Leone Cavicchia2 and Delei Li (李德磊)3 State and perspectives of the concept of large-scale conditioning of coastal weather extremes for deriving forecasts and statistics of such phenomena   Hans von Storch1, Leone Cavicchia2 and Delei Li (李德磊)3 1 Institute of Coastal Research, Helmholtz Center Geesthacht, Germany 2 School of Earth Sciences, University of Melbourne, Australia 3 Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

Constraining by spectral nudging Similarity of zonal wind at 850 hPa between simulations and (driving) NCEP re-analyses in a constrained (black blue) and an unconstrained (red, gren) simulations. Top: Large scales Bottom: small scales von Storch et al., 2000 [Waldron et al., 1996]: the full hydrodynamic field is de-composed into a series of scale-sorted pattens – e.g., spherical harmonics or EOFs; after Earlier methods: Nudging spatial means (Kida et al. (1991), Sasaki et al. (1995), and McGregor et al. (1998) The basic idea that the state of small scale phenomena are conditioned by the regional hydrodynamic state of the atmosphere (or ocean) - at least outside of the tropical belt Dynamical downscaling method, which enforces the model to follow closely prescribed large scales (as given by re-analyses) A state-space formulation: merging empirical (data) and dynamical (model) knowledge von Storch, H., H. Langenberg and F. Feser, 2000: A spectral nudging technique for dynamical downscaling purposes. Mon. Wea. Rev. 128: 3664-3673 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting In regional models Number of medicanes per year. Year-numbers refer to medicane seasons, from August to July next year. Various regions: Europe, Southern Atlantic, East Asia, Mediterranean Sea, Yellow Sea, North Pacific For deriving climatologies and assessments of decadal change: - Polar Lows in the North Atlantic and North Pacific (Zahn, Chen) - Medicanes in the Mediterranean Sea (Cavicchia) - Low Level Jets and other phenomena in the Yellow Sea and Bo Hai (Li) - Typhoons in the West Pacific and South China Sea (Feser, Barcikowska) - Wind and storms in German Bight (Schaaf et al., 2018) When region is small (say: 500 km), then con- straining has little or no effect (Schaaf et al., 2017) Cavicchia, L., H. von Storch, and S. Gualdi, 2014: A long-term climatology of medicanes, Clim. Dyn. 43, Page 1183-1195 DOI: 10.1007/s00382-013-1893-7 (Cavicchia et al., 2014) Medicane track density on a 1° grid 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

in global models Brier Skill Scores over the period of Dec. 1999 – Nov. 2009 for marine surface wind speeds of (left panel) 3 – 25 m/s and (right panel) 10.8 – 25 m/s relative to observational reference data 929 (“truth”) QuikSCAT. Top: Comparison of the regional (55 km) and the global downscaling product; Bottom: Comparison of the regional (7 km) and the global downscaling product. Large scale constraining was introduced for regional models, but the approach may be implemented also in global models (Yoshimura and Kanamitsu, 2008) ECHAM 6, T256 grid resolution (Schubert-Frisius et a., 2017) – grid resolution about 60 km Skill of describing small scale dynamics and features comparable to that achieved in regional models(von Storch et al., 2017) von Storch, H., F. Feser, B. Geyer, K. Klehmet, 李德磊 (Li D.), B. Rockel, M. Schubert-Frisius, N. Tim, and E. Zorita, 2017: Regional re-analysis without local data - exploiting the downscaling paradigm. J. Geophys. Res. - Atmospheres 122, 8631–8649, DOI:10.1002/2016JD026332 NCEP-1 Constrained ECHAM6 Vorticity fields for Typhoon Tokage at maximum strength at 1800 UTC 19 Oct 2004 Schubert-Frisius et al., 2017 6 March 2018

Polar Lows described in constrained global simulation? vorticity No systematic analysis so far, but random samples have shown that some polar lows are realistically generated, but some are missing and some are additionally generated (as was in case of the regional modelling). Done by Zahn and by Woth Woth, pers. comm. Example of a Polar Low forming in the global simulation at the right time and area. 10-m wind speed 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

issue: divergence in phase space Simulated sea level pressure (hPa; isobars) and near surface wind speed fields (ms-1; shaded) of typhoon Winnie in August 1997. Grid resoution: 50-km Top: with constraining Bottom: two simulations without constraining issue: divergence in phase space When run without constraining, regional (and global) models generate intermittently different developments („divergence in phase space“) – which is not indicating „falseness“ of a model but is rightly reflecting the stochastic character of „weather formation“. In constrained models, this effect is largely suppressed. Zahn, M., H. von Storch, and S. Bakan, 2008: Climate mode simulation of North Atlantic Polar Lows in a limited area model, Tellus A, DOI: 10.1111/j.1600-0870.2008.00330.xFeser, F., and H. von Storch, 2008: A dynamical downscaling case study for typhoons in SE Asia using a regional climate model. Mon. Wea. Rev. 136, 1806-1815 10m wind speed (> 13:9 ms-1) and air pressure (at mean sea level) at 0600 UTC 15 October 1993 – Ensembles of four regional simulations Top: with constraining; Bottom: without constraining. 6 March 2018

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining features 6 March 2018 14th European Polar Low Working Group meeting

Purposes (incomplete references) 7-day forecast – case from forecast experiment series by Zhao et al., 2016; forced with GFS global forecast; regional model with (bottom) and without spectral nudging (middle) For constructing climatologies: Europe – Feser et al., 2001 … etc Mediterranean Sea Region – Cavicchia et al., 2014 East Asia / West Pacific – Feser and von Storch, 2008 North Pacific – Chen and von Storch, 2013 Yellow Sea, Bo Hai – Li et al. 2016 Southern Atlantic – Tim et al., Central Sibiria – Klehmet et al., Sea of Okhotsk and the Sakhalin Island - Platonov et al., 2017 For specifying regional detail in forecasting Zhao et al., 2016 For studying process paramterizations in models Kolstad et al., 2016 Zhao Y., Wang D., Liang Z and Xu J., 2016: Improving numerical experiments on persistent severe rainfall events in southern China using spectral nudging and filtering schemes. Q. J. R. Meteorol. Soc. DOI:10.1002/qj.2892 6 March 2018

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

challenge: determining spatial features Definition of spatial properties of Coastal Low Level Jets : 2 examples A B A: starting from weak and small LLJ, then growing and becoming strong LLJ, then decay and disappear B: a well developed LLJ at the beginning, then disintegrates into LLJ pieces, decaying and disappearing Pattern recognition methods need to be implemented to define pattern, size and properties of dynamic features like Low Level jets but also Polar Lows etc. challenge: determining spatial features Cross section1: (lat: 33.62 40.18 N) (lon: 121.37 128.18E) Li and von Storch, in prep.

14th European Polar Low Working Group meeting Simulating small synoptic features conditioned by the large-scale state constraining by spectral nudging in regional models in global models issue: divergence in phase space purposes challenge: determining spatial features 6 March 2018 14th European Polar Low Working Group meeting

14th European Polar Low Working Group meeting 6 March 2018 14th European Polar Low Working Group meeting