COSMO/CLM Training Course

Transcription

1 COSMO/CLM Training Course Langen, February 2012 Physical Parameterisations I: Radiation Bodo Ritter, DWD GB FE 14 with contributions by Matteo Buzzi, Meteo Suisse Kathrin Wapler, DWD FEZE Ulrike Wißmeier, LMU and Robert Pincus, CIRES FE 14

2 Outline of the presentation Some basic facts and principles concerning radiative processes as energy source&sink in the earth-atmosphere system Radiative transfer as key component of a complex NWP system Concepts for the parameterisation of RT in NWP models Validation issues Work in progress / Open issues Summary and conclusions

3 Basic facts & principles Sources of radiative energy and interaction with atmospheric constituents i) radiative energy sources: Planck s Law states that so-called black bodies emit electromagnetic energy depending on wavelength and the temperature of the body, i.e. I 2 2 hc 1 W ( λ, T ) = 5 hc 2 λ m e λ kt 1 implications for the earth-atmosphere system receives radiative energy emitted by sun (T Sun ~5800 K) emits and thereby loses radiative energy to space (T earth ~255 K) since the wave length for maximum emission depends on inverse of temperature (Wien s displacement law) a strong shift occurs between the spectral composition of solar and terrestrial radiation sr 1 m integration of Planck s Law over solid angle and wave length leads to the Stefan-Boltzmann Law, describing the total radiated energy per unit area as E ( T ) = σ T 4 W m 2 Note: values for the sun apply at the surface of the sun; at the top of the earth s atmosphere the solar flux is smaller by a factor of

4 Basic facts & principles ii) interaction between radiation and atmospheric constituents: within the atmosphere and at the earth s surface radiation can be absorbed, leading to an increase of temperature scattered, leading to change in the direction of propagation emitted, leading to a decrease of temperature The interaction efficiency, i.e. the so-called optical depth of atmospheric constituents is most pronounced for cloud water and cloud ice minor trace gases (e.g. H 2 O, CO 2, O 3, O 2, CH 4, ) aerosols (e.g. dust, sea salt, soot, ) Depending on surface type and actual conditions (e.g. snow cover) the earth s surface scatters a substantial proportion of impeding radiation at solar wavelengths back into the atmosphere For longer (terrestrial) wavelengths the earth s surface behaves almost as a black body, i.e. emissivity and absorptivity are very close to unity

5 Basic facts & principles iii) relevance of radiative transfer for numerical weather prediction The divergence of radiative fluxes contributes to the local temperature tendency within the atmosphere, i.e.: Absorption of solar radiation and emission (and absorption) of terrestrial radiation are important components of the energy budget at the earths surface, i.e. where J s, J q S T t sol rad ~ are surface fluxes of sensible and latent heat, respectively. It will be demonstrated later that sensible and latent heat fluxes are to a certain extent slaves of the solar radiative forcing! ter F rad E = F + F + J + s J q

6 Radiative transfer as key component schematic view of interaction between various processes

7 Radiative transfer as key component schematic view of interaction between various processes Thermal&solar radiation interact with all atmospheric processes, in particular those related to clouds There is a large variety of interactions between all processes. Modifications in any of these processes will have effects on all other processes. soil processes, in particular in the presence of snow

8 Radiative transfer as key component Some more facts radiation is the ultimate source and sink of energy for the whole earth-atmosphere system (solar) radiation is primary cause for phenomena like diurnal and seasonal cycle the spatial and temporal mean of the radiation balance at the earth s surface is positive, since it is dominated by a gain of solar radiation this gain is compensated by turbulent fluxes of sensible heat and moisture and longwave radiative fluxes from the earth s surface to the atmosphere the energy gained by the atmosphere through turbulent fluxes and longwave radiation from the ground and absortion of solar radiation compensates the loss through longwave radiation at the top of the atmosphere global annual mean gain of solar radiation at top of atmosphere: ~240W/m 2 global annual mean loss of thermal radiation at top of atmosphere: ~240 W/m 2 Stefan-Boltzmann-Law: F (T) = σ T 4 T earth ~255 K

9 An example of process interaction radiative forcing total cloud cover low cloud cover net solar flux cloud effect sensible heat flux radiative forcing latent heat flux surface energy balance surface temperature clear sky thermal radiative cooling evolution of fluxes and near surface parameters at a single grid point over two days low cloud effect net thermal flux 2m temperature

10 Global energy budget annual mean energy budget of the earth-atmosheresystem

11 Model simulation of energy budget components on a regional scale Energy budget components on a regional scale Europe July 2007 TOP OF ATMOSPHERE Energy budget at top 72h of the forecasts european atmosphere with GME July 2007 & COSMO-EU 400 GME COSMO-EU W/m** ASOB_T ATHB_T Net summer balance > Note: -300 The naming convention used here follows the COSMO model notation, e.g. ASOB_T= time integrated net solar flux at top of atmosphere

12 Model simulation of energy budget components on a regional scale Energy budget components on a regional scale Europe July 2007 SURFACE 72h forecasts with GME & COSMO-EU Energy budget at earth's surface of Europe July 2007 GME COSMO-EU W/m** ASOB_S ATHB_S ALHFL_S ASHFL_S Net summer balance > 0-100

13 Model simulation of energy budget components on a regional scale Energy budget components on a regional scale Europe July 2007 ATMOSPHERE 72h forecasts with GME & COSMO-EU Energy budget of the european atmosphere July GME COSMO-EU Note: A net gain of 100 W/m 2 corresponds to a warming of the whole atmosphere by ~1K/day 0 W/m** ASOB_A ATHB_A ALHFL_A ASHFL_A Net Not included: transport through lateral boundaries by advection!

14 Basic concepts of RT parameterization in NWP models Radiative transfer in a plane parallel, horizontally homogeneous atmosphere can be described by the monochromatic radiative transfer equation (RTE): L( τ, µ, ϕ) µ τ scattering of direct beam incoming radiance emission P(cosΘ = L( τ, µ, ϕ) (1 ~ ω) B( τ ) ~ ω 4π 2π + 1 P Θ ~ (cos ) ω L( τ, µ, ϕ ) dµ dϕ 4π 0 1 scattering of diffuse radiance components ) S 0 e τ µ 0 0 with Lλ monochromatic directional radiance τ λ optical thickness µ,µ 0 cosine of zenith angle for diffuse resp. direct radiation ϕ azimuth angle single scattering albedo λ Bλ Planck function Pλ Phase function for scattering θ,θ 0 scattering angle for diffuse resp. direct solar radiation solar constant ω ~ S 0 λ ASSUMPTIONS

15 The electromagnetic spectrum For the simulation of the atmospheric evolution in NWP & Climate models, we are only interested in a very small portion of the electromagnetic spectrum, but for other purposes, e.g. remote sensing, other wavelengths may be of interest too!

16 The electromagnetic spectrum relevant wavelengths for energetics of earth & atmosphere For the simulation of the atmospheric evolution in NWP & Climate models, we are only interested in a very small portion of the electromagnetic spectrum, but for other purposes, e.g. remote sensing, other wavelengths may be of interest too!

17 Spectral dependance of gaseous absorption scattering cross section for air molecules (top panel) and absorption cross section of major absorbing gases at different wavelengths important note: optically active are mainly those gases, which contribute little to the total atmospheric mass absorption coefficients of gases vary by several orders of magnitude within a few micrometers of wavelength The non-linearity of the RTE in conjunction with the extreme wavelength dependency of gaseous absorption properties creates severe problems for the computational efficiency of radiative transfer algorithms

18 Uncertainties related to gaseous absorption longwave cooling rates in a cloud-free atmosphere computed by different line-by-line models even for simple atmospheric situations computationally expensive radiation codes show a certain level of uncertainty

19 Simplifications for NWP applications Justification: Exact (LBL) solutions of the RTE carry the burden of spectral integration over several thousand spectral intervals and solid angle of monochromatic, directional radiances and are unfeasible with regard to the severe computational constraints of the real-time application NWP Relevant input (e.g. cloud distribution and optical properties) is not known with sufficient accuracy A massive reduction of the number of spectral intervals in conjunction with a simplified description of directional aspects yields acceptable results for radiative fluxes and heating rates δ-two-stream approximation for wide spectral bands

20 Simplifications, continued A major simplification (and computational saving) is achieved by replacing the integral over all directions of irradiances through up- and downward directed flux densities ( two stream ). For solar radiation an additional term results from the description of the direct solar beam, i.e.: with F 1,2 F1 ( τ ) τ = α F 1 1 F2 ( τ ) α 2F2 α3j = α 2F1 τ S( τ ) S = (1 ω) τ µ 0 α F diffuse upward and downward flux density (W/m2) S parallel solar flux density (W/m 2 ) τ optical thickness α J 4 J =S(τ)/µ 0 for the solar part of the spectrum =πb(τ) for the thermal part of the spectrum α i coefficients describing layer optical properties (cf. Ritter&Geleyn, 1992)

21 Simplifications, continued A major simplification (and computational saving) is achieved by replacing the integral over all directions of irradiances through up- and downward directed flux densities ( two stream ). For solar radiation an additional term results from the description of the direct solar beam, i.e.: But! F1 ( τ ) F2 ( τ ) Even after all = simplifications, α1f1 α 2F2 α3j radiative = α 2F1transfer α1f2 + α4schemes J are τ τ computationally so expensive that the so-called full S( τ ) S computation is performed = at (1 a reduced ω) frequency compared with τ µ 0 to the NWP model timestep! F 1,2 diffuse upward and downward flux density (W/m2) (model S namelist parallel variables: solar flux hincrad_s, density hincrad_t (W/m 2 ) respectively nincrad_s, nincrad_t) τ optical thickness This has implications for the simulation of the intercation between radiative J processes =S(τ)/µ 0 for and the other solar processes part of the simulated spectrumby the NWP model! =πb(τ) for the thermal part of the spectrum α i coefficients describing layer optical properties (cf. Ritter&Geleyn, 1992)

22 Simplifications, continued Separation of the solar and thermal radiative transfer problem The use of separated equations for solar and thermal radiative transfer is possible due to Wien s displacement law, which states that the wavelength of maximum intensity of radiation emitted by a so-called black body is inversely proportional to its temperature T, i.e.: λ max = b T As a consequence the spectrum of solar radiation, which is emitted at ~ 6000 K and that emitted by the earth and the atmosphere show almost no overlap!

23 Simplifications, continued Spectral intervals employed in the radiative transfer scheme of COSMO interval limits spectral region major optically active constituents solar O 3,O 2,H 2 o, clouds &aerosols solar H 2 O, CO 2, O 2, clouds & aerosols solar H 2 O, CO 2, CH 4, N 2 O, clouds & aerosols thermal H 2 O, CO 2, CH 4, N 2 O, clouds & aerosols & thermal H 2 O, CO 2, N 2 O, clouds & aerosols thermal H 2 O, O 3, CO 2, N 2 O, clouds & aerosols thermal H 2 O, CO 2, N 2 O, clouds & aerosols thermal H 2 O, clouds & aerosols

24 Validation: Comparison to other schemes Solar radiative heating rates in cloudy atmosphere during COPS: Comparison between RRTM and COSMO RT scheme (By courtesy of S.Crewell, Uni Köln)

25 Validation: Comparison to other schemes (&observations) Dependency of solar radiative transfer calculation on cloud microphysical properties, here: effective droplet radius (By courtesy of S.Crewell, Uni Köln) Smaller droplets are (much) more efficient with regard to scattering than larger ones! This implies a strong dependence of radiative fluxes and heating rates on poorly known atmospheric properties, even within one RT scheme! observed flux

26 Validation: Comparison to observations Comparison of observed and simulated OLR Observed OLR (NOAA 12/ /2009) Simulated OLR (GME 48h forecast annual mean) Interpolated OLR data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at

27 Open issues & Work in progress limitations of one-dimensional radiative transfer schemes optical properties of key atmospheric constituents atmospheric distribution of important constituents

28 Open issue: 3d-problem Solar radiative transfer in slow motion 1D-simulated position of solar flux minimum true position of solar flux minimum model grid column boundaries

29 Open issue: 3d-problem the standard approach of column-by-column processing of individual physical processes implies the neglection of any direct interaction between adjacent columns pure geometrical considerations show that for solar radiation this neglection is not justified for NWP models of high horizontal resolution 1D-approach neglects also effects related to deviation of earth s surface from horizontal orientation, i.e. slope effects fully 3D-radiative transfer schemes require enourmous computational ressources and are not feasible for operational NWP

30 comparison of 3D- and 1D-RT calculations at very high horizontal resolution 3D-Simulation 1D-Simulation Courtesy of U.Wißmeier, LMU Munich

31 Work in progress: 3d-problem Optical depth for the attenuation of the slanted direct solar beam in a checker board cloud situation (cf. Wapler, 2008) TICA optical depth ICA

32 Work in progress: 3d-problem TICA = tilted independent column approximation (cf. Wapler, 2008) TICA will be implemented and tested in COSMO in the framework of a collaboration between DWD & Uni Munich 1st order approximation for cloud shadow effects on solar surface flux in adjacent grid columns z x optical depth for direct solar beam derived from true path of ray and associated with target grid column realistic attenuation of direct beam improved location & intensity of cloud shadows interaction of clouds with diffuse radiation needs additional modifications

33 Work in progress: Topographic effects Sloping topography & shadowing effects (Meteo Swiss, M.Buzzi)

34 Sloping topography & shadowing effects Shortwave downward radiation Longwave radiation Diffuse radiation

35 Sloping topography & shadowing effects Approximative solution of the topography problem Simple approach: correction factors for surface radiation components computed by the 1D-code a modified Müller and Scherrer (2005) scheme scheme implemented & operational at MeteoSwiss

36 Sloping topography & shadowing effects Grid scale (implemented, but topo parameters + additional reading routine are needed): COSMO Topo slope angle,slope aspect skyview, horizon COSMO run correction factors Subgrid scale (partially implemented but tools and routines available at MeteoSwiss): DEM topo slope angle,slope aspect skyview, horizon correction factors skyview aggregation COSMO run

37 Sloping topography & shadowing effects Direct solar radiation correction SW dir SW * dir = cosα sinθ S cosα = cosθ N sinθ S + sinθ N cosθ S cos(φ S φ N ) Kondratiev (1977) SW * dir = fcor SW dir fcor sun azimuth sin θ N = mask cos + cos( ) shadow θ N φ S φ N tan θ S θ S φ φ S N slope aspect θ N slope angle N θ N : slopeangle θ :sun elevation angle φ S N : slope aspect φ :sun azimuth angle S SW SW * dir dir :radiation on a sloping surface : radiation on a horizontal surface

38 Sloping topography & shadowing effects Correction factor: shortwave radiation UTC, gridscale option (2.2 km)

39 Sloping topography & shadowing effects Validation: Global radiation in winter Observational data based MSG (CMSAF B. Dürr)

40 Sloping topography & shadowing effects 2m temperature winter Comparison to SYNOP data Comparison to ANETZ/ENET data

41 Work in progress: Ice clouds Revision of optical properties of ice clouds current parameterisation of ice optical properties in COSMO ice spheres optical properties function of IWC only (Rockel et al, 1991) revised optical properties of ice clouds will be based on parametrisation of FU, 1996 and FU et al.,1998 includes dependence on IWC and a generalized effective radius allows, in principle, a separate contribution of snow particles to optical properties

42 Work in progress: Ice clouds Revision of optical properties of ice clouds current parameterisation of ice optical properties in COSMO ice spheres optical properties function of IWC only (Rockel et al, 1991) revised optical properties of ice clouds will be based on parametrisation of FU, 1996 and FU et al.,1998 includes dependence on IWC and a generalized effective radius allows, in principle, a separate contribution of snow particles to optical properties

43 Open issues: critical input cloud properties: here vertically integrated cloud liquid water content Monthly mean August 2008 GME

44 Open issues: critical input cloud properties: here vertically integrated cloud liquid water content Monthly mean August 2008 COSMO-EU

45 Open issues: critical input cloud properties: here vertically integrated cloud liquid water content There are obviously large differences in simulated model properties which are important input quantities for radiative transfer calculations between various models! This applies in particular to cloud related variables, is evident even in spatial & temporal averages and depends strongly on rather uncertain aspects of the cloud microphysics. The truth is largely unknown! In addition to cloud water and cloud ice the COSMO model carries also rain water and snow as prognostic variables. Neither rain nor snow are (yet) considered as contributions to the optical opacity of the model atmosphere. However, in particular the snow component may be quite large compared to cloud ice and cloud water! Monthly mean August 2008 COSMO-EU

46 Work in progress Other areas of research relevant to the COSMO radiative transfer scheme low temporal frequency, full spectral integration high temporal frequency, random spectral sampling The climatological fields for the distribution of various types of aerosols are being revised. This work has been completed in the framework of DWDs global model GME, the adaptation to COSMO is underway (Climatological aerosol fields are not to be confused with the so-called COSMO-ART modifications/extensions (see presentation by B.Vogel) The speed versus accuracy problem for RT simulations is being adressed by means of two independent approaches: a so-called adaptive RT scheme (Venema et al., 2007) in collaboration with the Uni Bonn as part of the so-called extra-mural research of DWD an alternative to the classical method used for spectral integration via a so-called Monte Carlo Spectral integration (Pincus&Stevens, 2008)

47 Summary and conclusion Parameterisation of radiative transfer is both a key component and also a complex part of NWP models in general and the COSMO model in particular Radiative transfer depends strongly on input provided by other components of the NWP system (e.g. cloud scheme, snow model) The radiative transfer problem for NWP models is far from being solved and requires special attention in conjunction with high resolution models!

48 Deutscher Wetterdienst for your attention