Predictability Associated with Northern Hemisphere Retrograde Disturbances as Inferred from Reforecast
Abstract
Retrograde long waves in the higher latitudes of Northern Hemisphere are known to episodically sustain coherent phase propagation over multiple weeks. The influence of such waves on extended-range weather forecast has been speculated in previous studies but not systematically quantified. This study uses the NOAA GEFS Reforecast II data to compare the week-2 predictability, measured by the anomaly correlation of 500 mb geopotential height over 40oN - 70oN, between active and inactive periods for retrograde waves. The analysis is guided by a newly created catalog of prominent retrograde-wave events identified from reanalysis. For each winter season with a significant presence of retrograde waves, days are divided into active and inactive sub-periods for the evaluation of anomaly correlation for the 11-member ensemble mean and up to 16 days of forecast lead time. Screening through the reforecast data from 1985-2017, it is found that the anomaly correlation for the long-wave (zonal wavenumber 1 and 2) component of 500 mb geopotential height is consistently higher within the active periods. In many cases, the elevated anomaly correlation sustains into the end of week 2. In some strong retrograde-wave events, the presence of the retrograde disturbances also leads to an increase in the anomaly correlation for the short waves (zonal wavenumber 5-9) well into week 2. To understand the local behavior of predictability, anomaly correlations are further evaluated for selected longitudinal sectors within the 40oN - 70oN band. It is found that the enhancement of predictability associated with retrograde waves is usually more significant over the Pacific-East Asia sector. The signal outside this sector is more mixed and occasionally cancels that in the Pacific-East Asia sector. A further analysis is underway to identify the structure of atmospheric circulation associated with the state of high predictability.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA226.0013R
- Keywords:
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- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3362 Stratosphere/troposphere interactions;
- ATMOSPHERIC PROCESSES;
- 3373 Tropical dynamics;
- ATMOSPHERIC PROCESSES