962 Precipitation Forecasts and Their Relationship with Large-Scale Tropical Dynamics in the GFS and IFS

Wednesday, 10 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Maria Gehne, CIRES/Univ. of Colorado and NOAA, Boulder, CO; and J. Dias and G. N. Kiladis

Quantitative precipitation forecast (QPF) skill in the tropics is assessed and compared to the extra-tropics in two operational forecast models, the NCEP Global Forecast System (GFS) and the ECMWF Integrated Forecast System (IFS) model. Guidance from both models was used during NOAA's 2016 El Niño Rapid Response field campaign and it was observed that both models perform with much higher skill for mid-latitude precipitation forecasts than for the tropics. Here we first examine the forecast skill from April 2015 through March 2016 and because of the predictability potential from convectively coupled equatorial waves (CCEWs), we focus mainly on these large scale features.
Results reveal that differences in model analyses are smaller than differences to global reanalyses products and both models show relatively good QPF scores for the 6-12 hour forecasts, indicating that in general initial conditions are reasonably well estimated in both forecast systems. However, overall, the tropical QPF in both systems was not considered useful by typical metrics much beyond 6 days. By forecast day 10 the GFS converges to a double ITCZ in the central Pacific, which is consistent with the 10 day pattern of low-level divergence at the equator and off-equatorial convergence in both hemispheres. Coherence spectra of observed precipitation and model divergence show signals along CCEW dispersion curves, indicating that CCEWs are initialized in the model analyses. Convectively coupled equatorial Kelvin waves coherently propagate for only about 3 days in the GFS and 7 days in the IFS. This difference in forecast skill is consistent with the differences in the low-level flow that each model converges to and it is in sharp contrast to both models' ability to accurately forecast extra-tropical storm track activity. The divergence along the equator that develops in the GFS is not conducive to convectively coupled equatorial Kelvin waves, whereas the IFS divergence field is. This indicates that initialization of CCEWs is not the limiting factor in forecasting this type of variability, but rather the relationship between large scale dynamics and parametrization of moist convective processes is. This issue raises the question of the respective role of CCEWs versus the basic model state as well as their interplay in deteriorating tropical forecast skill.
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