Aerosols in the convective boundary layer: effects on the radiation field and the land-atmosphere system Eduardo Barbaro* with contributions of: Jordi Vila, Maarten Krol, Huug Ouwersloot, Henk Baltink, Fred Bosveld, Dave Donovan, Wouter Knap, Ping Wang CESAR DAY KNMI *Meteorology and air-quality group Wageningen University email@example.com
Research question How do the CBL dynamics and the land-atmosphere system react to the SW radiation absorbed by the aerosols during the day ? Combining observations and numerical modeling Strategy to answer:
Comprehensive observation dataset Radiation budget: (LW ↕and SW ↕) Aerosol properties: AOD, ω, g CBL height θ q SEB: QNET, SH,LE,G0
Numerical modeling • LES:3-D high-resolution model able to reproduce detailed CBL dynamics. • MXL: Simplified bulk model able to reproduce the most important CBL dynamics. • Penman-Monteith: Land-surface model able to calculate the SEB. • Delta-Eddington: Broadband radiative transfer code able to calculate SW radiation profiles accounting for the aerosol information. Broader quantification! The MXL model is used to perform 256 systematic runs (sensitivity analysis) varying the initial aerosol properties (AOD and ω).
CBL prototypes h h 25th September 2003 8th May 2008 Aerosol layer Aerosol layer t t
Aerosol temporal evolution and vertical structure 8th May 2008 • We constrain the aerosol data in our LES and MXL models (red dashes). h Similar to Wang et al 2009 t
Initial conditions: θ and q 8th May 2008 Residual layer Aerosol layer
Radiation budget LES CESAR 8th May 2008 R2 = 0.99 RMSE = 8.4 Wm-2 R2 = 0.93 RMSE = 9.7 Wm-2
SEB and CBL height LES CESAR 8th May 2008
Thermodynamic variables LES CESAR 8th May 2008 Entrainment of drier air
Sensitivity Analysis: AOD Beijing Barcelona Oslo Quebec Cabauw τCONTROL = 0.2 τAERO = 0.6 τCLEAR = 0.0 CONTROL CLEAR AERO+
SW and SEB modifications 25th September 2003 - Aerosols directly reduce downward irradiance - Relatively constant reduction on LE (10-20%) - SH is influenced more strongly - Aerosols increase EF (up to 20%) τ = 0.6 τ = 0.2 τ = 0.0
Vertical heat budget and θ 25th September 2003 • Aerosols: • Morning (dotted lines): • reduce the surface fluxes • warm the residual layer • Afternoon (solid lines): • Heat the CBL τ = 0.6 τ = 0.2 τ = 0.0
CBL height evolution • Aerosols shallow the CBLbecause of less entrainment • Aerosols delay/anticipate the CBL onset/collapse τ = 0.6 τ = 0.2 τ = 0.0
A C A C MXL: sensitivity analysis (τ x ω) 25th September 2003 AOD and SSA CBL height Irradiance Evaporative fraction A C A C
Take home message: Aerosols will (in a nutshell): Disrupt the land-atmosphere diurnal cycle • Reduce irradiance, SH and LE • Shallow and warm the CBL h When also located above the CBL (I): • Strongly shallow the CBL • Delay the CBL onset (I) (II) When located within the CBL (II): • Shallow the CBL (also reduce Δθ) • Anticipate the CBL afternoon collapse t
Aerosols on the land-atmosphere system Δθ θ zi ω ω τ ω ω τ τ ω HR τ - - - - - - - + + + + + + + SH τ QNET LE
(τ x ω) 25th September 2003
Two-stream approximation Two stream approach is an approximation of the RTE in which radiation is propagating in only two discrete directions. N=1 The multiple scattering contribution is represented by up(down)ward intensities weighted by the appropriated asymmetry factor Diffuse radiance production by simple scattering of direct solar radiation Diffuse radiance production by scattering of diffuse radiation available in dτ.
Two-stream approximation Two stream approach is an approximation of the RTE in which radiation is propagating in only two discrete directions. Diff (2) and filling (1) in we have: Boundary conditions: TOP -> I↓ = 0 SURF -> I↑ = albedo*I ↓
Two-stream + Eddington’s approximation Eddinton’s approximation is an improvement on two-stream approach. It can be used to obtain the radiance in a plane-parallel medium with ISOTROPIC SCATTERING. The scattering is also assumed frequency-independent (representative λ) ->not true for aerosols!. Example: Stellar atmospheres (Eddington, 1916). OPS! I μ Boundary conditions: TOP -> I↓ = 0 SURF -> I↑ = albedo*I ↓
The DELTA-Eddington principle The Eddinton-two stream approach produces very good results for thick layers but is inaccurate for thin layers and when significant absorption is involved. f, fraction of scattered energy residing in the forward peak We remove f=g2 (f≈0.5) from τ, ω, and g in order to better define the phase function.
A complex system: (Interconnection between radiation – land surface – CBL dynamics - Aerosols) CABAUWTOA TOA (100km) Almost no mass here! (39 km) θCBL↑ Rayleigh scattering Direct Diffuse Tsurf ↓ 870 Wm-2 (<2km) Mie scattering + absorption -> attenuates shortwave radiation! Big particles (both absorption + scattering) BL height LE 800 Wm-2 25% H SW Reflection