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The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity

Research Review. The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity. 25 June 2009 Song- Lak Kang. The ABL … . typically 1-2 km high. the lowest 10-20 % of the troposphere. directly influenced by the earth’s surface. Convective Boundary Layer.

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The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity

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  1. Research Review The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity 25 June 2009 Song-Lak Kang

  2. The ABL … • typically 1-2 km high. • the lowest 10-20 % of the troposphere. • directly influenced by the earth’s surface.

  3. Convective Boundary Layer 1700 MST ON 17 JUNE 2009 The sun’s ray heats the earth’s surface, which then heats and moistens the air above it.

  4. Horizontally Homogeneous CBL From Wyngaard (1990) • Bottom-up (Surface heat and moisture fluxes) and • Top-down (entrainment from the free atmosphere) approaches. • The characteristic length scale of energy containing eddies is about height of the ABL

  5. Mesoscale fluctuations in the CBL Mesoscale and turbulent fluctuations coexist in the CBL Aircraft Observation at 33 m AGL From Mahrt et al. (1994) Aircraft Observation at various levels From LeMone et al. (2002)

  6. Does the spectral gap exists? Assumption of mesoscale modeling ln E Mesoscale fluctuations Turbulent fluctuations lnκ

  7. Horizontal flows generated by mesoscale surface heat flux variation U Warm Cool

  8. Horizontal flows generated by mesoscale surface heat flux variation High-amplitude surface heat flux variation Non-stationary flows Low-amplitude surface heat flux variation Quasi-stationary flows

  9. Why do the horizontal flows oscillate?Divergence of vertical heat flux .vs. Temperature advection Temperature gradient increased by divergence of vertical heat flux Temperature gradient reduced by temperature advection

  10. Ramp event in wind power forecasting The Ramp Event One of the most difficult issues that wind power forecasting system has encountered is a so-called ramp event. An unforeseen ramp event may be costly to balance the supply and demand of power and affect power system security (Cutler et al. 2007) . The red solid lines represent observed wind speed and the blue solid lines wind power produced. From Cutler et al. (2007)

  11. Realistic surface heat flux variations The diurnal cycle of surface heat flux variation Multi-scale surface heat flux variation

  12. Comparison of LES results with WRF results

  13. Summary • In the CBL, mesoscale horizontal flows generated by differential heating may temporally fluctuate. • The temporal fluctuations are suggested as one of the reasonsto cause the ramp event in horizontal wind speed. • With more realistic conditions, LES experiments are being performed. • The LES results are being compared with WRF results in order to improve the performance of wind power forecasting.

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