Group seminar on 6. December, 14:15 CET

Properties of Kelvin Waves forecast errors in the ECMWF deterministic forecasts

Paolo Andreozzi

Kelvin waves (KWs) are eastward-propagating planetary-scale features associated with significant variability of the tropical atmosphere. They are known to be essential ingredients of tropical circulation such as the El-Niño southern oscillation (ENSO), the quasi-biennial oscillation (QBO) and the Madden- Julian oscillation (MJO) and are continuously forced by convection in the troposphere. Convection also constitutes a challenge for numerical weather prediction, ultimately affecting forecast skill in the tropics. The impact of poor convection simulation on tropical predictability is nonetheless still matter of ongoing research. This work investigates KWs forecast errors and is branched in two research questions. One asks to what extent KWs forecast errors reflect errors in simulated convection. A second one asks whether the spectral coefficients allow to investigate forecast errors attributable to phase shifts. We use ECMWF deterministic forecasts covering the period from November, 2021 to September, 2022; 00 UTC analyses are used to validate 10-day long forecasts. KWs spectral coefficients are obtained from the operational MODES analysis of ECMWF deterministic forecasts. OLR is used as a proxy for deep tropical convection and is verified against NOAA daily observations over the 2019-2021 period. Results show a close reflection of systematic errors in OLR and KW fields in several tropical regions. For the KW, these concern progressively deeper layers of the upper tropopause and lower stratosphere as forecast lead time increases. In the lower stratosphere, KW in forecasts is systematically shifted westward than in analysis, indicating a tendency toward too slow predicted propagation.