Accounting for Land Model Uncertainty in Numerical Weather Prediction Ensemble Systems: Toward Ensemble-Based Coupled Land-Atmosphere Data Assimilation

The ensembles used in the NOAA National Centers for Environmental Prediction (NCEP) global data assimilation and numerical weather prediction (NWP) system are underdispersed at and near the land surface, preventing their use in ensemble-based land data assimilation. Comparison to offline (land-only) data assimilation ensemble systems suggests that while the relevant atmospheric fields are underdispersed in NCEP's system, this alone cannot explain the underdispersed land component, and an additional scheme is required to explicitly account for land model uncertainty. This study then investigates several schemes for perturbing the soil (moisture and temperature) states in NCEP's system, qualitatively comparing the induced ensemble spread to independent estimates of the forecast error standard deviation in soil moisture, soil temperature, 2-m temperature, and 2-m humidity. Directly adding perturbations to the soil states, as is commonly done in offline systems, generated unrealistic spatial patterns in the soil moisture ensemble spread. Application of a stochastically perturbed physics tendencies scheme to the soil states is inherently limited in the amount of soil moisture spread that it can induce. Perturbing the land model parameters, in this case vegetation fraction, generated a realistic distribution in the ensemble spread, while also inducing perturbations in the land (soil states) and atmosphere (2-m states) that are consistent with errors in the land-atmosphere fluxes. The parameter perturbation method is then recommended for NCEP's ensemble system, and it is currently being refined within the development of an ensemble-based coupled land-atmosphere data assimilation for NCEP's NWP system.