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The Atmospheric Boundary Layer (ABL) over Mesoscale Surface HeterogeneityPowerPoint Presentation

The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity

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### 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

1700 MST ON 17 JUNE 2009

The sun’s ray heats the earth’s surface, which then heats and moistens the air above it.

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

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)

Does the spectral gap exists?

Assumption of mesoscale modeling

ln E

Mesoscale fluctuations

Turbulent

fluctuations

lnκ

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

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

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)

Realistic surface heat flux variations

The diurnal cycle of surface heat flux variation

Multi-scale surface heat flux variation

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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