Energy balance closure at four forest sites in Wisconsin

1
Energy balance closure at four forest sites in
Wisconsin

• Nan Lu
• LEES Lab, University of Toledo

10/27/06
2
Energy balance closure

• Rn LE Hs G Qs
• Qs Qsoil Qair Qbiomass

3
Energy balance

• Evaluation method
• 1. Linear regression coefficient (slope
  intercept) between (LEHs) and (Rn-G-Qs) (EBC)
• 2. Ratio (EBR)
• Energy Imbalance!
• 55-99 at 50 site-years (Wilson et al., 2002)

Oliphant et al.,2004. AFM
4
Is Qs important?

• Qs was typically 5 of Rn in a mature mixed
  forest and it could be up to 10 under some
  particular conditions, e.g. overcast days, during
  or immediately following rainfall (McCaughey and
  Saxton, 1985).
• The assessment of the contribution of storage
  heat to the total energy balance is few for both
  forest and agricultural ecosystems (Oliphant,
  2004 Mayer, 2004) .

5
What are the conditions under which energy
balance is not closed?

• The lacks of energy balance closure in the forest
  were usually identified at night with low
  friction velocity (u) (Wilson et al., 2002).
• Clouds could play an important role in regulating
  the energy balance closure by limiting
  radioactive energy input as well as evaporation
  (Eltahir and Humphries, 1998 Petrone et al.,
  2002) .

6
Effects of forest type?

• Physical properties of the land surface such as
  albedo, roughness and root zone depth affect
  different components of the energy balance by Rn
  as well as its partition into Hs and LE (Eltahir
  and Humphries, 1998).

7
Objectives

• 1) Dose heat storage (including Qsoil and Qair)
  significantly contribute to the energy balance?
• 2) Do friction velocity and clouds have effects
  on energy balance closure?
• 3) Is energy balance closure different among
  different forest types?

8
Study site
23m
9m
26m
3m
9
Methods

• QairQaQw

(Oliphant et al.,2004. AFM)
10
Methods Definition of cloudiness
Comparison of a sunny day (Day 186) and a cloudy
day (Day 187) (at IHW, 2003)
CloudinessPext-PAR Relative Cloudiness(Pext-PAR)
/Pext
11
Results

• 1. Measured energy fluxes (Rn, LE, Hs, G) and
  storage heat (Qs)

12
Seasonal variation of LE/Rn, Hs/Rn, G/Rn and Qs/Rn
13
Rn, LE, Hs and G of growing season
Comparison of daily variation of Rn, G, Hs, LE
among sites, error bar SE
14
Comparison of maximum of Rn, LE, Hs and G of
growing season among sites

• Repeated ANOVA

Multiple comparisons of maximum (10001200 AM)
Rn, LE, Hs and G among sites in the growing season
15
Qs in the growing season

• No difference on the daily scale!

Comparison of daily variation of storage heat
fluxes (Qs, Qsoil, Qair) among sites, error bar
SE
16
Comparison of Qs in different time periods of a
day among sites
17
Results

• 2. Energy balance closure

18
Contribution of Qs to the energy balance closure
Two particular time periods
All day
19
Energy balance closure under different conditions
20
Energy balance closure under different conditions

• All data available

21
Discussion
22
Discussion
23
Linear regression between EBC and the canopy
height (account for Qs and not account for Qs)
24
Conclusions

• 1. Net radiation and its portioning to LE, Hs and
  G were different at half-hourly scale among
  sites largest difference occurred around noon.
  But there was not a difference in Qs among sites.
• 2. Qs was different among sites during the hours
  after dawn and around dusk when Qs was a larger
  proportion of Rn. Storage was greater in the
  taller than shorter canopies.

25
Conclusions

• 3. Storage energy improved the energy balance
  closure by 1-6 at of our study sites during the
  particular time periods of dawn and dusk, Qs
  could increased energy balance closure by 9-11
  for tall canopies.
• 4. Energy balance closure was higher when
  friction velocity was greater and the sky was
  clearer.