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Advancing 3D Surrogate Cloud Fields Using Measured Structures and LES Modelling Techniques

This study presents the development of 3D surrogate cloud fields based on measured structures and Large Eddy Simulation (LES) modeling, conducted by researchers at the University of Bonn, Germany. By leveraging measurements from BBC campaigns and satellite data, we explore the relationship between cloud structure and radiation transfer. We assess the efficacy of Independent Pixel Approximation (IPA) and demonstrate that our statistical approaches yield accurate estimates of radiative properties. These findings advance our understanding of boundary layer clouds and their impact on climate.

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Advancing 3D Surrogate Cloud Fields Using Measured Structures and LES Modelling Techniques

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  1. 3-Dimensional surrogate cloud fields with measured structures Victor Venema Clemens Simmer University of Bonn, Germany Cloud measurements: Susanne Crewell, Ulrich Löhnert Radiative transfer: Sebastián Gimeno García, Anke Kniffka, Steffen Meyer LES Modelling: Andreas Chlond, Frederick Chosson, Siegfried Raasch, Michael Schroeter

  2. BBC campaigns • Keywords: boundary layer water clouds, structure & radiation, climate • Airplanes • Tethered balloons • Satellites • Regional network with lidar, ir-radiometers • Ground based remote sensing

  3. BBC campaigns • BBC1: August, September 2001 • BBC2: May 2003 • Chaotic skies, typically Dutch cloudscapes

  4. BBC campaigns • BBC1: August, September 2001 • BBC2: May 2003 • Chaotic skies, typically Dutch cloudscapes • High quality measurements for testing • Open database • ftp://bbc.knmi.nl • Beautiful measurements, but modellers need 3D fields • Dimensionally challenged

  5. Motivation • Can not measure a full 3D cloud field • Can measure many (statistical) cloud properties • Generate cloud field based on measurements • Emphasis on good structure for radiative transfer

  6. Structure & radiation • Distribution • Amplitude (LWP, LWC, ) alone is already good • Success of Independent Pixel Approximation (IPA) at large scales

  7. Structure & radiation • Power spectrum • Spatial linear correlations • Full spectrum

  8. Measured power spectrum • Scale breaks • Waves • Land sea mask • … Satellite pictures: Eumetsat

  9. Validation surrogate clouds • Is this statistical description good enough? • 3D LWC fields from LES modelling • Make surrogates from their statistics • Calculate radiative properties • Radiances (remote sensing; Steffen Meyer) • Irradiances (radiative budget; Sebastián Gimeno García) • Actinic fluxes(chemistry; Anke Kniffka) • Compare them

  10. LES Surrogates - LWC LES originals Surrogates LES: Andreas Chlond

  11. Validation results • Irradiances • RMSE mean: < 0.03 % of the radiative budget • Monte Carlo error, statistically not significant • Radiances • RMSE mean: < 0.3 % • Monte Carlo error, statistically not significant • Actinic flux • RMSE mean: < 1.4 % • No error estimate (SHDOM)

  12. Validation results • Fields with only one statistic (i.e. PDF or Fourier) • clearly worse • statistically significant • The statistical description is probably good enough • LES stratocumulus and within 1 % • Limitation is likely the amount of data

  13. Validation cumulus • Developed a more accurate Stochastic IAAFT algorithm • Surrogates are copies of templates • In practise the bias is likely still there as you cannot measure the power spectrum that accurately

  14. Validation cumulus LES original Surrogate

  15. Validation broken clouds LES originals Surrogates LES: Frederick Chosson

  16. Iterative algorithm (Schreiber and Schmitz, 1996) Time series Flow diagram Time series Distribution

  17. Iterative algorithm • Original problem • Generate a cloud similar to measurements • New problem • Based on limited data estimate: • distribution • power spectrum

  18. Power Frequency (Hz) Estimate spectrum – method 1 • Add a dimension • Assume isotropy • Rotate and scale power spectrum LWP Measurement 1D power spectrum 2D power spectrum LWP Power Frequency (Hz) Time (s) Frequency (Hz)

  19. Estimate spectrum – method 2 • Scanning measurement • Estimate 2D-autocorrelation function • 2D anisotropic power spectrum

  20. Estimate distribution - zenith • Option: PDF field is measured PDF • However! • Inhomogeneous (non-stationary) field • Underestimate the width distribution

  21. Estimate distribution - zenith • Relative reduction in variance • Zenith measurement • Simulated on LES clouds

  22. Estimate distribution - scan • Correcting • Frank Evans • More data • Spread • Scanning

  23. Estimate distribution - scan

  24. Estimate distribution - height • Distribution as a function of height

  25. Added: Local values • Input from scanning measurement • Amplitude distribution • 2D power spectrum • The measured values on the spiral

  26. Added: Local constraint & mask Surrogate with cloud mask

  27. Added: Coarse means Original Surrogate Coarse means Coarse fraction

  28. Added: Coarse means & mask Original Surrogate Coarse means Coarse fraction

  29. Applications • Closure studies • Bring micro-physics and radiation together • Poster: 3D surrogates from in situ measurements (Sebastian Schmidt, Ronald Scheirer, Francesca Di Giuseppe) • Structure studies • Fractal generator • Geophysics

  30. Applications • Closure studies • Structure studies • How good is the fractal approximation? • How accurate do you need to know the PDF? • Fractal generator • Geophysics

  31. Structure studies • 2D tdMAP clouds • IAAFT surrogates • Full structure • Only correlations at small scales • Compare radiative properties

  32. 2D & 3D reflectance RT: Sebastián Gimeno García

  33. Applications • Closure studies • Structure studies • Fractal generator • Sensitivity studies • Retrievals and parameterisations • Spectrum and PDF varied independently • Broad-band radiometer EarthCARE • Jaime F. Gimeno (University of Valencia) & Howard Barker • Geophysics

  34. Applications • Closure studies • Structure studies • Fractal generator • Soil temperature fields LES • Rain fields • Downscaling low resolution atmospheric models to high resolution hydrological models • Surrogate run-off • Wind stress fields • Soil properties • …

  35. 3,2: 3D, 2D; S: Structure; P: PDF; L: Local values; M: Mask IAAFT method Cumulus fields (Evans; structure of a binary mask) CLABAUTAIR (Scheirer and Schmidt) Shift cloud (Schmidt; Los and Duynkerke) 2D-2D Ice cloud (Liou et al.) tdMAP (A. Benassi, F. Szczap, et al.) Multi-fractal clouds Bounded Cascade and other fractal clouds Fourier method SITCOM (F. di Giuseppe; 2D structure) Ice clouds (R. Hogan, S. Kew; 2.5D structure) 3SPLM 3SP__ 3SPL_ 2__L_ 2__L_ 2SP__ 2SP__ 2S___ 2S___ Comparison cloud generators

  36. Conclusions • Developed/extended an algorithm • Full 3D structure • LWC height profile • Local measured constraints (fine or coarse) • Cloud mask • Advantages • Flexible • Dimensions • Instruments • Vary the statistics easily and independently

  37. More information • Homepage • Papers, Matlab-programs, examples • http://www.meteo.uni-bonn.de/ venema/themes/surrogates/ • Google: surrogate clouds • BBC campaign surrogates • ftp://bbc.knmi.nl/bbc1/model/ • Victor.Venema@uni-bonn.de

  38. Constrained surrogates • Arbitrary constraints • Evolutionary search algorithm • Better convergence • Try new statistics • Fractal geometry for cloud boundaries

  39. Evolutionary search algorithm

  40. Constrained surrogates • height profiles • cloud base • cloud top • cloud cover • average LWC • Histograms • LWP • LWC • number of layers • Power spectra & length • LWP • Highest cloud top • Lowest cloud base

  41. Coarse mean surrogates • Input • Local coarse mean LWP, τ • Local coarse mean cloud fraction • Power spectrum, extrapolated to small scales • Combine satellite and ground based orin situ measurements • Measured small scale spectrum or PDF • Downscaling models • A priory small scale spectrum

  42. Instruments • 3 microwave radiometers, • 3 cloud radars, • 4 Micro Rain Radars (MRRs), • 2 wind profiler-RASS systems, to measure wind and temperature profiles, • 4 lidar ceilometers, • 2 lidars • numerous radiation, precipitation, turbulence and meteorological instruments.

  43. LES Surrogates - Radiation Optical depth Transmittance Actinic flux Radiance Original Fourier &distribution Fourier Distribution

  44. LES Surrogates - Radiation

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