Forest response to cloud and canopy effects on incident radiation

In the 17 years since I first met Andy Black, eddy flux observations have gone from being an exotic rarity to a commodified ‘gap-filled' data product, a trade perhaps like replacing venison for processed canned meat product. There are, of course, real needs to have easily accessed homogeneous data sets with which to assess model outcomes. However, does this mean that the field micrometeorologist is readily (or ready to be) supplanted by technicians?

Correctly describing the light environment impinging upon the forest canopy and penetrating to the forest floor still remains a challenge. Moreover, few efforts have been made to date to examine the consequences that evolving and thriving in a fluctuating light environment has on canopy carbon dioxide (Fco2), heat (H) and water vapor (LE) fluxes. (See Doughty et al., 2006). The eddy covariance method as conventionally performed is not suitable to address this issue. In this presentation, I will report what kinds of transient light regimes are produced by clouds and canopy. In addition, I will discuss briefly how one may develop data analysis approaches that invoke alternate ways to interpret the ensemble mean at the heart of the EC method, following earlier work by Czikowsky and Fitzjarrald (2009). This is an event-based compositing approach to estimate H, LE and Fco2.

To describe the radiation environment of the upper and lower canopy, during the field phases of LBA-ECO our team operated the laser ceilometer at the km67 site near Santarém Pará Brazil and an array of 16 subcanopy radiation sensors. One sensor was a PAR sensor; the remaining were pyranometers, multiplexed every 10 seconds. In conjunction with G. G. Parker (Smithsonian Environmental Research Center, MD), measurements of the spectral character of subcanopy light were made during July 2004, to account for spectral response differences between the global solar and PAR sensors. Recording these data on the same time base as the above-canopy radiation and fast-response flux sensors, facilitate comparison of the influence of clouds on radiation at both levels.

References

Czikowsky, M. J., and D. R. Fitzjarrald (2009), Detecting rainfall interception in an Amazonian rain forest with eddy flux measurements, Journal of Hydrology, 377(1-2), 92-105, doi:10.1016/j.jhydrol.2009.08.002.

Doughty, C. E., M. L. Goulden, S. D. Miller, and H. R. da Rocha (2006), Circadian rhythms constrain leaf and canopy gas exchange in an Amazonian forest, Geophys. Res. Lett, 33, L15404.