The GMAO atmospheric GCM (the GEOS-5 AGCM) has a “single column model” (SCM) counterpart, a model that includes all of the vertical physics of the atmospheric model (the radiation physics, the convection scheme, etc.). The GEOS-5 SCM can be used for fast, inexpensive sensitivity experiments. In this study, we use it to determine how details of the imposed atmospheric turbulence and convection parameterizations affect land-atmosphere coupling strength, i.e., the response of simulated precipitation to variations in soil moisture. The ultimate goal is to improve the simulation of coupling strength in the GEOS-5 AGCM modeling and prediction system.

Sixteen parameters associated with the AGCM cloud moisture and turbulence parameterization schemes were identified as candidates for the sensitivity experiments. Observations from the Atmospheric Radiation Measurement/Cloud and Radiation Testbeds (ARM/CART) measurement station in Oklahoma were used for the SCM's lateral boundary conditions. By merging 2 observation episodes, a 63-day long set of lateral forcing for boreal summer was created, and through the repeated random sampling of this dataset, we derived forcing for 100-member ensembles of simulations, each lasting two months. (Ensembles are needed for the coupling strength diagnostic examined.) Idealized cycles of latent and sensible heat fluxes were prescribed at the land surface in each simulation; the cycles were given a two week period, with a tunable amplitude.

The SCM sensitivity analyses identified two parameters with a particularly strong impact on land atmosphere coupling strength: a parameter related to the relative humidity threshhold for cloud water formation, and a parameter associated with the assumed subgrid variability in the atmosphere. The impact of these particular parameters on coupling strength was tested in the full atmospheric GCM.