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Parameterizing the effects of sub-grid land heterogeneity on the atmospheric boundary layer and convection: Implications for surface climate, variability and extremes

The complexity of spatial patterns over land across a range of scales plays a key role in
convection, mesoscale circulations, hydrologic extremes, biodiversity, and ecosystem resilience.
For this reason, a recurring emphasis to represent its role in climate and Earth system models
has led to significant advances in the representation of sub-grid land heterogeneity over the
last few decades. However, these advances have not been met with complementary advances
in modeling the communication of land heterogeneous states to the atmospheric boundary
layer (ABL). This leads to model deficiencies including: 1) a limited influence of land models
within contemporary sub-grid convection schemes, 2) deficient sub-grid redistribution of key
land surface states, 3) challenges when evaluating modeled surface fluxes using observations,
and 4) unrealistic model feedbacks between the ABL and the heterogeneous land.

To address these deficiencies, a climate process team (CPT) is proposed to implement
sub-grid spatially heterogeneous coupled land and ABL in NCAR’s Community Earth System
Model version 2 (CESM2), GFDL’s Climate Model version 4 (CM4), GMAO’s Goddard Earth
Observing System Model (GEOS), and DOE’s Energy Exascale Earth System Model (E3SM). This
will be accomplished by coupling each climate model’s existing sub-grid tiling scheme over land
to its climate model’s sub-grid parameterization of the ABL (e.g., the Cloud Layers Unified By
Binormals (CLUBB) scheme for clouds and turbulence). The newly proposed parameterization
will be evaluated and constrained using large eddy simulations (LES) and observations of
surface fluxes and other relevant variables. The overarching scientific objective of this climate
process team is to parameterize heterogeneous exchanges between the land and atmosphere
and to characterize its implications for surface climate, variability, and extremes.

This proposal is relevant to this competition as it will leverage advances in the
representation of sub-grid heterogeneity in climate models to parameterize heterogeneous
exchange between the land and ABL in CESM2, CM4, GEOS, and E3SM; this will lead to
improved modeling of the land/atmosphere interface and thus the Earth system. As such, it
aligns with NOAA’s long term research goal to advance our understanding of the Earth’s climate
system and to use this knowledge to improve the resilience of our nation and its partners. The
CPT will combine experimentalists, process modelers and diagnosticians, theoreticians, and
model developers from four modeling centers, two academic centers, and the GEWEX community.
The proposed coupled process improvement is: 1) relevant, because the land-to-atmosphere
exchange of sub-grid states and fluxes is neglected in current Earth system/climate models;
2) ready for implementation due to existing sub-grid parameterizations over land and
in the atmosphere; 3) focused, because the project will revolve around the improvement and
impact of heterogeneous exchange in Earth system/climate modeling; 4) model independent,
as it is of interest to developers from four Earth system/climate modeling centers.

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