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Land atmosphere interaction – metrics and examples

Land atmosphere interaction – metrics and examples. Bart van den Hurk (KNMI/IMAU). Preparing land surface assignment. Prepare an experimental set-up using the conceptual model Write down, hand over to me (will be commented), add student nr Criteria

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Land atmosphere interaction – metrics and examples

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  1. Land atmosphere interaction – metrics and examples Bart van den Hurk (KNMI/IMAU) Land atmosphere interaction

  2. Preparing land surface assignment • Prepare an experimental set-up using the conceptual model • Write down, hand over to me (will be commented), add student nr • Criteria • Should be inspired by a “true” physical question • Should include at least 2 experiments • Should describe the analysis method and conclusions expected • Example: • Strong/weak coupling affects the gradient of precipitation more than the gradient of soil moisture Land atmosphere interaction

  3. Interaction between land and boundary layer Land atmosphere interaction Van Heerwaarden et al, 2009

  4. Coupled Penman-Monteith equation • Consider the following system: • And the Penman-Monteith equation: A = (1-)Sin + Lin-Lout D = qsat(T) – q  = dqsat/dT Van Heerwaarden et al, 2010 Land atmosphere interaction

  5. Derivative dLE/dt Forcing Feedback Heating/moistening through advection Change of radiation in time Changes in aerodynamic coupling due to stability effects Heating/moistening through boundary layer growth Soil moisture depletion Longwave cooling Soil heating A = (1-)Sin + Lin-Lout D = qsat(T) – q  = dqsat/dT Van Heerwaarden et al, 2010 Land atmosphere interaction

  6. Results for Cabauw and Niamey Van Heerwaarden et al, 2010 Land atmosphere interaction

  7. Rising air and condensation Rising air cools with 1 K per 100m ……….. Until it becomes so cold that it starts to condensate… rising air rising air water vapour condensates to droplets water vapour molecules And a cloud is born!!! Land atmosphere interaction sheets from Pier Siebesma, KNMI

  8. Conditions for precipitation Necessary (but not sufficient) condition: Low Level Moisture Convergence Land atmosphere interaction

  9. Examples of Moist Convergence Mid-latitudes : Low pressure Systems Tropics: Intertropical Convergence Zone (ITCZ) Land atmosphere interaction

  10. Causes of rising air:1. Orography Lenticularis above Mount Etna seen from Taormina, Sicily Italy. Land atmosphere interaction

  11. 2. Convection • The sun heats the soil so that…….. • Thermals are formed…. • that rise because of buoyancy…. • And a cloud forms as a wig on top of an invisible man 24-07-2006 12:30 Amsterdam: cumulus humilis or “fair weather” cumulus www.sky-catcher.nl Land atmosphere interaction

  12. Condensational Heating allows cumulus to grow • Wolken top (~3 km) • Humidity condensates into cloud water….. • And produces latent heat • Which serves as onboard fuel that allows the cloud to rise further….. • With ~5 m/s…. • Until the cloud is stopped by a temperature inversion. • Wolken basis (~1km) 24-07-2006 Amsterdam: cumulus mediocris. 15:30 Land atmosphere interaction

  13. Poor man’s cloud model: adiabatic ascent Mean profile “Level of zero kinetic energy” Inversion Level of neutral buoyancy (LNB) non-well mixed layer height Level of free convection (LFC) Lifting condensation level (LCL) well mixed layer Land atmosphere interaction

  14. A true profile CAPE = Convective Available Potential Energy. CIN = Convection Inhibition LCL = Lifting Condensation Level LFC = Level of Free convection CAPE CIN parcel eq. temperature true potential temperature Land atmosphere interaction

  15. 3. Large Scale Lifting through fronts } Occuring at mid-latitudes } Land atmosphere interaction

  16. Convection and land-atmosphere coupling • Findell and Eltahir (2003) did a systematic analysis of soil moisture – precipitation feedback • Start with atm. sounding of 6:00 am • Use simple Land-PBL model driven by obs. soundings • Diagnose convective triggering or shallow cumulus formation for 2 runs (dry and wet soil) • Classes of cases: • soil has no impact (atm. controlled) • convection favoured over wet soils • convection favoured over dry soils Land atmosphere interaction

  17. Soil-PBL feedback • Dry soils favoring convection (negative feedback) • Wet soils favoring convection (positive feedback) dry wet PBL growth reaches LCL LCL LCL Build-up of MSE gives convective potential LCL LCL Land atmosphere interaction

  18. Findell’s diagnostics • Convective Triggering Potential (CTP) • Dewpoint depression in low levels (HIlow): • too wet: rain always likely • too dry: no moisture available Land atmosphere interaction

  19. Findell’s map of feedback Land atmosphere interaction

  20. Large scale coupling Koster et al, 2004, Science Land atmosphere interaction

  21. The coupling coefficient  • What is the contribution of the interactive land-atmosphere coupling on the hydrological cycle? • How to answer? • Simple. Simulate the hydrological cycle without interactive land-atmosphere coupling and compare. • How? • Simple. Replace interactive landsurface flux/state by something that is prescribed and not interactive. • How to distinguish between interactive and prescribed landsurface? • Ensemble simulations Land atmosphere interaction

  22. The coupling coefficient  • Two ensembles of 16 model simulations, with varying atmosphere for each member • One ensemble (W) ‘normal’ (full coupling between land and atmosphere) • soil moisture from member one is written every time step • One ensemble (S) with varying atmosphere prescribed soil moisture • soil moisture from each member is read from W1 • Evaluate ensemble variance of precipitation Koster et al, 2004, Science Land atmosphere interaction

  23. Effect on precipitation All simulations in ensemble respond to the land surface boundary condition in the same way strong coupling Simulations in ensemble have no coherent response to the land surface boundary condition weak coupling Koster et al, 2004, Science Land atmosphere interaction

  24. Definition of  (simplified version) • Take variance of precipitation across ensemble, P2 • Compare P2 from ensemble W with ensemble S • If P2(W)  P2(S)    0, low coupling • If P2(S) disappears    1, strong coupling Koster et al, 2004, Science Land atmosphere interaction

  25. Similar exp, not an ensemble but multiyear simulation • Compare two multi-year simulations: • One normal (‘coupled’) simulation • One ‘uncoupled’ simulation with fixed land cond’s • See what is the effect on variability of T, P soil water precipitation Land atmosphere interaction

  26. Land-atmosphere coupling in Europe: climate change scenario Precipitation • Change in interannual variability • T,p2(future) - T,p2(control) Temperature Coupled Uncoupled Seneviratne et al, 2006, Nature Land atmosphere interaction

  27. Temperature extremes in Europe • Various runs with an RCM covering 1959-2006 • a cooling period 1959-1980 • a warming between 1981-2006 • Cut land-atmosphere interaction by prescribing time-filtered soil moisture using different time scales • Check the trends in these periods for different filtering time scales IAV = interannual variability removed Jaeger et al (2010) Land atmosphere interaction

  28. Results in trends Thus: cutting land-atmosphere interaction affects trends in Tmax Also feedbacks with cloud cover play a role Land atmosphere interaction

  29. Material for the land-assignment • Hydrological balance • Land parameterization • Land use • Land-atmosphere interaction Land atmosphere interaction

  30. Material for the land-assignment • Hydrological balance • Land parameterization • Land use • Land-atmosphere interaction Land atmosphere interaction

  31. Schedule • Next week: start of Atmospheric Dynamics • 1 May: presentations land assignment and deadline land assignment report • 15 May: start Cloud/aerosol section • 12 and 26 June: presentations Dynamics/Clouds Land atmosphere interaction

  32. More information • Bart van den Hurk • hurkvd@knmi.nl • www.knmi.nl/~hurkvd Land atmosphere interaction

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