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Predicted tropical storm frequency
These parameters are calculated over the same weekly periods as the other monthly forecast products and are averaged over an ocean basin. The tropical cyclone identifying algorithms are the same as are used for the medium range tropical cyclone activity products. Evaluation of TC tracks for the sub-seasonal range ensemble is computed at 6hr intervals in the same way as medium range ensemble and the outputs are consistent.
Fig8.2.7-1: To view tropical storm frequency.
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This normalisation makes the orange and green bars directly comparable.
Fig8.2.7-2: An example chart of monthly forecast tropical storm frequency. Not much activity is shown over the ocean basins but there is slightly more activity over tropical waters north of Australia in the Timor Sea and Arafura Sea areas (green bar = 0.4; orange bar = 0.2).
Predicted accumulated cyclone energy (ACE)
These parameters are calculated over the same weekly periods as the other monthly forecast products and are averaged over an ocean basin. The tropical cyclone identifying algorithms are the same as are used for the medium range tropical cyclone activity products.
Fig8.2.7-3: To view accumulated cyclone energy.
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A further normalisation sets the climatological value (orange) equal to 1. The forecast value (green) indicates above average (value greater than 1) or below average (value less than 1) Accumulated Cyclone Energy respectively.
Fig8.2.7-4: An example of the accumulated cyclone energy (ACE) product. This takes into account number, strength, and duration of all the tropical storms forecast by the sub-seasonal ensemble during the weekly period within each arbitrarily defined ocean basin, and totals their energy. This value is compared with a similarly derived ACE from the sub-seasonal range climate (SUBS-M-Climate) for the same week to determine the significance level for the given basin. If significance is greater than 90% the basin is shaded. Green bars represent the forecast and orange bars the model climatology - these can be directly compared.
Tropical storm strike probabilities
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Fig8.2.7-5: To view storm strike probabilities
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Note neither Raw Strike Probability data nor Probability Anomaly data incorporate any bias correction to adjust for differences relative to tropical cyclone observations. Therefore both products may suffer from systematic under- or over-estimations.
Fig8.2.7-6: Weekly mean tropical storm strike probability of the sub-seasonal range forecast. Colours give probability in percentages.
Fig8.2.7-7: Weekly mean tropical storm strike probability deduced from the sub-seasonal range re-forecasts, for the same week as Fig8.2.7-6. Colours give probability in percentages.
Fig8.2.7-8: Weekly mean anomaly of tropical storm strike probability for the same week as Fig8.2.7-6. Colours give indication of more likely (positive probabilities, red to brown/black), or less likely (negative probabilities, cyan to blue/black) than shown in the sub-seasonal range re-forecasts.
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