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

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

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

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

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

10. Ramp event in wind power forecasting The red solid lines represent observed wind speed and the blue solid lines wind power produced. From Cutler et al. (2007) 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 Ramp Event

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 reasons to 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.