Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 88

0.71 to 0.78, for the better models, to R 2 values ranging from 0.16 to 0.18 for
models developed in mature forests (Lu et al. 2005a ).
AGB modeling of SS features using remotely sensed data is sufficiently
accurate for providing good qualitative estimates of biomass and is becoming
increasingly important in providing quantitative measures. Nevertheless,
additional research is needed in accuracy assessment to better assess the
degree to which AGB estimates can be used in models that focus on carbon
sequestration and potential for CO 2 /C exchanges. More research is needed in
integrating remotely sensed data and ancillary data using advanced models,
such as neural networks, to more accurately estimate mature forest AGB.
Selection of suitable remote sensing variables, reducing uncertainties caused
by AGB field data collection and the position errors between sample plots and
image pixels, are critical in improving AGB accuracy. AGB estimation and
validation of results is difficult; however, progress has been made in biomass
modeling, particularly for SS features, and many of the results have value for
scientists with CO 2 and sequestered carbon interests.
3.5.3 Linking atmospheric research with LULC processes
Progress has been made toward improving understanding of linkages
between LULC and aspects of Amazonia local and regional climate and its
effects on global climate. LBA atmospheric research has raised questions and
has provided some answers about humid tropical climate processes, forcings,
and consequences associated with LULC. This research primarily addresses:
surface fluxes and boundary layer growth; convection, clouds and rainfall;
and climate modeling.
Artaxo et al.( 2002 ) indicate that cloud structure over Amazonia is unique
among terrestrial ecosystems. Anthropogenic emissions have converted
unperturbed clouds into perturbed clouds with smaller cloud droplets and a
much higher number of cloud condensation nuclei (CCN). Concentrations of
aerosols and CCN are largely determined by the nature of Amazonia LULC.
Vegetation directly emits aerosol particles but they are also formed from
oxidation of gaseous volatile organic compounds (VOC). Increased CCN
results in reducing droplet size and forces precipitation to form at higher
elevations through ice processes, which, in turn, affect local precipitation
patterns. This process occurs predominantly during the dry season related to
higher intensity of biomass burning.
Amazonia is a major direct source of organic aerosols to the global atmo-
sphere that, coupled with vegetation and soil ammonia emissions, generate
large-scale atmospheric effects. The tropical troposphere is responsible for
about 70% of the global atmospheric oxidation of long-lived gases including
CH 4 , CO, HCFCs and CH 3 Br. Results from the LBA program indicate that: