Characterizing soil moisture regimes and linear and nonlinear soil
moisture-latent heat flux dependency
The control of latent heat flux (LE) by soil moisture (SM) variations is
a key process affecting the moisture and energy balance at the
land-atmosphere interface. SM-LE coupling is conventionally examined by
identifying SM-LE relationships with metrics involving correlation.
However, such a traditional approach, which fits a straight line across
the full SM-LE space to evaluate the dependency, leaves out certain
critical information: nonlinear SM-LE relationships and the
long-recognized thresholds that lead to dramatically different behavior
in different soil moisture regimes. This study examines three aspects of
the SM-LE relationship to diagnose coupling globally: linear
dependencies, nonlinear dependencies, and SM-LE threshold behavior.
Using data from climate models, reanalyses, and
observational-constrained datasets, global patterns of SM-LE regimes are
determined by segmented regression. Mutual information analysis is
applied only for days when SM is in the transitional regime between
critical points defining high sensitivity in the SM-LE dependency.
Sensitivity is further decomposed into linear and nonlinear components.
Our results show discrepancies in the global pattern of existing SM
regimes, but general consistencies among the linear and nonlinear
components of SM-LE coupling. This implies that although models simulate
different surface hydroclimates, the inherent behavior of how LE
interacts with SM is well-described. The pattern of strong SM-LE
coupling in the transition regime resembles the conventional
distribution of “hot spots” of land-atmosphere interactions. This
indicates that only the transitional SM range is necessary to determine
the strength of coupling. This framework can be applied to investigate
extremes and the shifting surface hydroclimatology in a warming climate.