Enhanced multi-model ensembles with the ESRL FIM-HYCOM coupled model at both NWP and subseasonal forecast time scales

There is a strong desire from NOAA's users for weather products at lead times from one week to two months (hereafter, called “sub-seasonal.”), which are beyond the time scale of traditional numerical weather prediction. In this weather-climate gap, two strategies possible are to extend NWP model duration and to shorten the time-scale at which seasonal forecast models are applied. In both areas, ensemble forecasting is essential and multi-model ensembles have been shown to be beneficial.

The coupled atmospheric and ocean model developed at NOAA/ESRL, consisting of the Flow-following Icosahedral Model (FIM, Bleck et al, MWR 2015, accepted for publication) atmospheric and the icosahedral Hybrid Coordinate Ocean Model (iHYCOM) appears to be capable of application of bridging the full NWP-through-seasonal application toward improved multi-model ensemble forecasts. FIM is now participating in experimental NWP global forecasting in atmosphere-only mode as part of the NOAA HIWPP (http://hiwpp.noaa.gov), where it has shown state-of-the-art skill for NWP measures including winds, precipitation, and height anomaly correlation. FIM's unique icosahedral horizontal grid, minimization of vertical diffusion with its adaptive vertical coordinate, and efficient scaling have provided a unique numerical approach for these global applications.

NOAA has facilitated the construction of ensemble coupled prediction systems with the goal of making sub-seasonal to seasonal forecasts that can provide information on anticipated high-impact events such as monsoons, MJO, and extreme weather. The North American Multi-Model Ensemble (NMME) project organized by NOAA/CPC aims at improving sub-seasonal to seasonal forecast skill by employing a multi-model ensemble strategy. Studies (e.g., Kirtman, et. al. 2014) have shown that multi-model ensembles improved the skill over each individual model. There is a need to better understand the variability and ensemble forecast spread in coupled models applied to this sub-seasonal forecast problem. FIM-iHYCOM is now in initial retrospective (hindcast) testing toward an experimental NMME model contributor in fulfilling NMME entry protocol. ESRL/GSD will test validation techniques for assessing coupled model forecast skill across the time scales mentioned. ESRL/GSD will test validation techniques for assessing coupled model forecast skill across the time scales mentioned.