Mattias Mohr, Johan Arnqvist, Hans BergströmUppsala University (Sweden)

Implementation of forest canopy in the MIUU mesoscale model

Project Goals

• Project: Wind power over forests (Vindforsk III)

• Better estimation of energy yield (wind resource)

• Better estimation of turbine loads (wind shear, turbulence, forest clearings)

Models should be developed for these purposes

Ryningsnäs test site

140m high mast

18m high mast

T1, T2 = wind turbines

Measurement setupU, T, , Global radiation

U, T

U, T, , q

U, T,

U, T, q

U, T, , Net radiation

TU ,

Sonic ane-mometers, LiCor

MIUU mesoscale model

• Used for wind mapping of Sweden (Uppsala University, Weathertech)

• Higher order closure, prognostic TKE, no terrain smoothing, 1km resolution (mapping), 100m resolution (forest modelling)

• Very high resolution in boundary layer (canopy modelling: 1, 3, 6, 10, 16, 24, 35, 52, … m)

Wind profile over forests

• Bulk versuscanopy modelling

• Does it make any difference at allin mesoscalemodels?

Bulk versus canopy modelling

• Resource assessment benefit? Not sure

• Micro-scale siting benefit? Definitely

• In MIUU model wind-mapping setup: 5 vertical levels within forest anyway, so why not include canopy?

How to include this in the model?

• Production/dissipation term in TKE equation

LAD | horizontal | 3 - | horizontal | q2

where q2 = turbulent kinetic energy (TKE)

βp = 1.0 (canopy production coefficient)βd = 4.0 (canopy dissipation coefficient)

Seems to make little difference above forest. (Main part of TKE produced by strong wind shear above forest.)

Drag term for horizontal wind components (u, v)

LAD | horizontal | (same for v-component)

, = wind component

Halldin, S. (1985): Leaf and bark area distribution in a pine forest. In The forest atmosphere interaction, edited by B. A. Hutchison and B. B. Hicks (Dordrecht: Reidel Publishing Company), p. 39–58.

Lalic, B. and D. T. Mihailovic (2004): An Empirical Relation Describing Leaf-Area Density inside the Forest for Environmental Modeling. Journal of Applied Meteorology, Notes and Correspondence, Vol. 43, p. 641-645.

”Elevated” Monin Obukhov (MO) theory in model

• Replace elevation above ground with elevation above zero displacement

• Replace MO-similarity theory terms in forest with something else (what?)

• Lower boundary conditions have to be modified (energy balance, u*, … )

Master length scale• Length scale within forest

• Simple model of Inoue (1963):

l = 0.47 · (h – d) ≈ 2m

• Within canopy: Length scale constant with height

Seems to have very little influence on results.

Energy balance

• Has to be solved at each model level within canopy

• Direct shortwave radiation follows Beer’s law S↓ = S↓0 · exp(-0.5 · )

• Longwave radiation (Zhao and Qualls, 2006)

Start with idealised 1D simulations

• Run several days (diurnal cycle)

• Parameters used: 10m/s geostr. wind, average temperature profile, z0 = 1 m, h = 20 m, LAI = 5, pine forest, spring

• Compare results with bulk version

Idealised 1D results – diurnal variation

0 0 0

1 122

3 3

4

4

4

4

4

5

5

555

5

5

5

5

55

5

6

6

6

6

66

6

6

6

6

6

6

6

7

7

7

7

77

7

7

7

7

7

7

7

7

7

7

7

8

8

8

8

8

8

9

9

9

Model-prediced wind speed (fair-weather test case)

Local standard time

Hei

ght a

bove

gro

und

(m)

12 0 12 0 12 0 12 00

20

40

60

80

100

120

140

160

180

0

1

2

3

4

5

6

7

8

9

10

11

m/s

No wind in forest

Idealised 1D results – mean profiles

0 1 2 3 4 5 6 7 80

20

40

60

80

100

120

140

160

180

200

Mean Wind Speed (m/s)

Heig

ht a

bove

gro

und

(m)

Comparison of Bulk and Canopy Wind Profiles (4 day 1D test run)

Bulk forest (z0 = 1 m)

Forest canopy (additional drag terms)Logarithmic wind profile

4 day 1D simulation – Input data

• Temperature profiles from radio soundings atRyningsnäs

• Global radiation from measurements

• Geostrophic winds from Reanalysis

• Forest: hc, LAI, LAD(z), zm best guess

4 day 1D simulation - Ryningsnäs

0 2 4 6 8 10 12

20

40

60

80

100

120

140

Comparison of Bulk and Canopy Wind Profiles (05/04 - 08/04/2011)

Mean Wind Speed (m/s)

Hei

ght a

bove

gro

und

(m)

Forest canopy (additional drag terms)MeasurementsBulk forest (z0 = 1 m)

4 day 1D simulation - shear

• Comparison of shear exponents (4 days):

Shear exponentMeasurements 0.374

Forest canopy model 0.365

Bulk model 0.39

For comparison (annual values)42 Swedish forest site: 0.25 - 0.40 (median value 0.33)

(Source: ”Wind power in forests”, final report, Elforsk, published March 2013)

Summary & Conclusions

• Preliminary 1D results promising

• Still lot of work to do (lower boundary conditions, canopy energy balance, length scale…)

• Vertical resolution of 1D runs too time-consuming for 3D runs?

• Is vertical resolution of 3D runs enough for canopy model?

Future plans

• Refine forest canopy module in MIUU model

• Implement and run in 3D

• Study effects on resource assessment

• Implement forest canopy in WRF