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 (LE+Hs) 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 Qair=Qa+Qw (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) Cloudiness=Pext-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 (10:00~12:00 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. Qs Qs EBC 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.