EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Quantifying Convective Aggregation using the Moist Tropical Margin’s Length

Julia Windmiller1, David Leutwyler1, and Tom Beucler2,3
Julia Windmiller et al.
  • 1Max-Planck-Institut für Meteorologie, Hamburg, Germany
  • 2Department of Earth System Science, University of California, Irvine, CA, USA
  • 3Department of Earth and Environmental Engineering, Columbia University, New York, NY, USA
A peculiar feature of the tropical atmosphere is its tendency to be either rather moist or very dry. It has
been shown that the two states are separated by a sharp margin, well identified by the anti-mode of
the bimodal tropical column water vapor distribution (CWV). Despite recent progress in understanding
physical processes governing the spatio-temporal variability of CWV near the Equator, the behavior of
this meander remains elusive, and we lack a simple framework to understand the bimodality of tropical
CWV in observations. A tendency of the atmosphere to be either moist or dry can also be found in
idealized simulations of radiative-convective equilibrium (RCE). A number of studies assessing self-
aggregation of convection in RCE simulations have indicated that positive feedbacks between
radiation and moisture lead to spatially concentrated moisture in a small area of the computational
domain. As a result, RCE simulations also exhibit a bimodal distribution of CVW and a distinct moist
Motivated by theory originating from statistical physics, we argue that, in steady-state RCE, the length
of the margin separating moist and dry regions should minimize. Based on these ideas, we design a
diagnostic relating the moist margin length to the circumference of a defined equilibrium shape. Using
that Moist Margin Length Index (MMLi), we assess the evolution of self-aggregation in idealized
convection-resolving RCE simulations and contrast it to the oscillations of the Atlantic Inter-Tropical
Convergence Zone in the ERA5 reanalysis product.
We find that MMLi successfully describes aspects of convective organization not captured by more
traditional metrics of convective organization (e.g., the CVW's interquartile range) while offering a new
way to characterize the seasonal cycle of convective organization in the Atlantic ITCZ. Overall, our
new framework uses the moist meander margin to connect the observationally-motivated, object-
oriented view of convective organization with the processes driving the spatio-temporal evolution of
the CWV field. It can be broadly deployed across models and observations to lead further insight into
the bimodal nature of CWV while offering an efficient and visual way to quantify convective

How to cite: Windmiller, J., Leutwyler, D., and Beucler, T.: Quantifying Convective Aggregation using the Moist Tropical Margin’s Length, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18988,, 2020.