The Impact of Analysis Correction-based Additive Inflation on subseasonal tropical prediction in the Navy Earth System Prediction Capability

Abstract

Accurately simulating the Madden-Julian Oscillation (MJO), which dominates intraseasonal (30-90 day) variability in the tropics, is critical to predicting tropical cyclones (TCs) and other phenomena at extended-range (2-3 week) timescales. MJO biases in intensity and propagation speed are a common problem in global coupled models. For example, the MJO in the Navy Earth System Prediction Capability (ESPC), a global coupled model, has been shown to be too strong and too fast, which has implications for the MJO-TC relationship in that model. The biases and extended-range prediction skill in the operational version of the Navy ESPC are compared to experiments applying different versions of Analysis Correction-based Additive Inflation (ACAI) to reduce model biases. ACAI is a method in which time-mean and stochastic perturbations based on analysis increments are added to the model tendencies with the goals of reducing systematic error and accounting for model uncertainty. Over the extended boreal summer (May-November), ACAI reduces the root mean squared error (RMSE) and improves the spread-skill relationship of the total tropical and MJO-filtered OLR and low-level zonal winds. While ACAI improves skill in the environmental fields of low-level absolute vorticity, potential intensity, and vertical wind shear, it degrades the skill in the relative humidity, which increases the positive bias in the Genesis Potential Index (GPI) in the operational Navy ESPC. Northern Hemisphere integrated TC genesis biases are reduced (increased number of TCs) in the ACAI experiments, which is consistent with the positive GPI bias in the ACAI simulations.