The Atmospheric Boundary Layer (ABL) over Mesoscale Surface Heterogeneity

25 June 2009Song-Lak Kang

Research Review

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

Aircraft Observation at 33 m AGLFrom Mahrt et al. (1994)

Aircraft Observation at various levelsFrom LeMone et al. (2002)

Mesoscale and turbulent fluctuations coexist in the CBL

Does the spectral gap exists?

ln κ

ln E Mesoscale fluctuations

Turbulentfluctuations

Assumption of mesoscale modeling

Horizontal flows generated by mesoscale surface heat flux variation

U

7

Warm Cool

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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

w

zxu

tTemperature gradient increased

by divergence of vertical heat flux

w

zxu

tTemperature gradient reduced

by temperature advection

Why do the horizontal flows oscillate?Divergence of vertical heat flux .vs.

Temperature advection

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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

Realistic surface heat flux variations

The diurnal cycle of surface heat flux variation

Multi-scale surface heat flux variation

Comparison of LES results with WRF results

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.