Brier Score Skill

India Meteorological Department (IMD) maintains a continuous watch over the north Indian Ocean (NIO) including the Bay of Bengal (BoB) and the Arabian Sea(AS) and issues extended range probabilistic cyclogenesis (formation of depression)forecast over the BoB & AS for the next two weeks, every Thursday.

IMD introduced extended range forecast on 22nd April, 2018. The probability of cyclogenesis inextended range forecast (ERF) is issued in four categories viz. nil (0%), low (1-33%),moderate (34-67%) and high (68-100%) based on diagnosis of various large scale features leading to cyclogenesis and the guidance from various global deterministic & ensemble numerical weather prediction (NWP) models & extended range forecasting models.

IMD evaluates the performance of operational ERF of cyclogenesis based on the forecasts issued for week 1 and week 2 for AS, BoB and NIO during April 2018 to 2023. For this purpose, Brier Score Operational (BSo) is calculated and analysed. To evaluate the skill,the climatological probability of cyclogenesis for each week is derived basedon the data during 1891-2020 and the same is used to determine reference BS ref. Subsequently, comparing the operational BSo with reference BS ref, the Brier Skill Score (BSS) has been calculated and analysed.

The Brier Score Skill (BSS) indicates the degree of improvement of the BS of a forecast (BSv) compared to the BS of a climatological forecast (BSc) as reference, based on an assumed equal occurrence probability of 33% for each event. The forecast is perfect when the BSS is equal to one.

Similarly, in the medium range, IMD commenced the cyclogenesis forecast for next 3 days in 2014, next 5 days in 2018 and next 7 days in 2023 as nil (0% probability), low(1-33% probability), moderate (34-67% probability) and high (68-100% probability). To evaluate the skill of genesis forecast, in a similar manner the BSS is calculated from day1 to day5 for BoB, AS and NIO regions. Every forecast is valid upto 0300 UTC of next day.

Fig.1: Annual BSS in the extended range scale for next 2 weeks

Fig.2: Annual BSS for medium range forecast for day 1 to day 5:

Tabular Representation of BSS Data for verification of medium range forecast over (a) BOB, (b) AS & (c) NIO and (d) extended range forecast for week 1 & 2 for BOB, AS & NIO during 2019-2023 for

Colour Legend for BSS (Red shades indicate BSS <50 and Green shades indicate BSS>50)

Track forecast issued upto 72 hours since 2009 and 120 hours since 2013.