Reducing transport errors in WRF modeling of greenhouse gas distributions through a combination of grid-nudging and regular restarts

Atmospheric transport models are often used to simulate the distribution of Greenhouse Gases (GHGs) for atmospheric inverse modeling. However, errors in simulated transport are often neglected in the context of inverse flux estimation. We coupled the commonly used Weather Research and Forecasting (WRF) model with the greenhouse gas module (WRF-GHG), to enable passive tracer transport simulation of CO₂ and CH₄. As a mesoscale numerical weather prediction model, WRF’s transport is only constrained by global meteorological fields via initialization and at the lateral boundaries; over time the winds in the center of the domain can deviate considerably from these (re-)analysis fields that are constrained by observations. The aim of this study is to have the WRF-simulated transport represent reality as closely as possible, which in this case means staying consistent with the (ERA5) reanalysis fields used as boundary conditions.

Therefore, two ways of blending ERA5 with WRF-GHG were tested: (a) regularly restarting the model with fresh initial conditions from ERA5, and (b) nudging the atmospheric winds, temperatures, and moisture to those from ERA5 continuously, using the built-in FDDA option (four-dimensional data assimilation). FDDA constantly forces the model towards the physical reference state (ERA5) by adding an additional tendency term at each grid point and time step.

Meteorological variables, as well as the concentrations of CO₂ and CH₄, were analyzed by comparing with observations. We also compared mixed layer heights (PBLH) with radiosonde-derived observation. We found that performance in horizontal winds and PBLH are slightly better in the nudged simulation (NS) compared to the simulation incorporating frequent restarts (RS). The advantage of grid-nudging is notable when comparing CH₄ with aircraft measurements from the CoMet campaign. However, differences in soil moisture increase over time, as soil moisture is not used for nudging. The consequence is a change in the Bowen ratio and thus in vertical mixing, impacting the distribution of GHG tracers in general.

To preserve the benefits of nudging and avoid the divergence of soil moisture, we recommend a hybrid approach, combining nudging with daily re-initializations. This technique will be used in an ensemble-based regional inversion system currently under development to make use of satellite-based measurements of GHGs.