New set of weather patterns to help predict floods
Indian meteorologists can now objectively identify specific weather patterns in active monsoon periods that could trigger high intensity rainfall and set off floods or landslides. How? Scientists from the United Kingdom (UK) and India have generated a new set of 30 daily weather patterns, within existing large weather systems, to determine the variability in rainfall within different phases of the season.
A seven-member team led by the Met Office – the UK’s national weather service – along with the National Centre for Medium Range Weather Forecasting (NCMRWF), under the union ministry of earth sciences, and Department of Geography at King’s College London used a statistical method called cluster analysis to produce the new sets of weather patterns, based on a 37-year analysis (1979 to 2016) of wind speed and direction covering India. In 2016, the Met Office used the same cluster method to develop a set of 30 weather patterns for Europe, based on the analysis of mean sea level pressure observations from 1850 to 2003.
Researchers said generating different weather patterns that represent variability in precipitation (rain, snow, hail) is meteorologically important. India is not only heavily dependent on rainfall for agriculture, but also experiences significant impacts from flooding, such as the Kerala floods in August 2018 and rainfall-induced hazards in the form of landslides that occurred in Uttarakhand in June 2013. “Identifying variations in weather patterns is useful to detect periods that are most susceptible to high‐impact weather within a large‐scale weather regime, such as pinpointing the most flood-prone periods within the active monsoon,” said Robert Neal, principal investigator, Met Office. “Once the characteristics of weather patterns are understood meteorologically, or in terms of their impact on agriculture, energy or transport, it will become easier to interpret the output of the forecast.”
A weather pattern is one of many circulation types over a defined region – in this case over India or a smaller regional area – which differs in its characteristics from other weather patterns over the same region. Circulation types can be cyclonic, anti-cyclonic, northerly, easterly, southerly, westerly and north‐westerly. A weather pattern stays for two to three days on an average, before transitioning to another weather pattern or to a new weather system.
Unlike most previous studies that generated weather patterns for precipitation only during the summer monsoon (June to September) in India, the new set of 30 patterns represent variability for the entire year. The research was published in the International Journal of Climatology, an international scientific peer-reviewed journal of the Royal Meteorological Society last month.
Each of the 30 patterns, chosen from a pool of 192 sets, can be attributed to one of the seven large-scale weather systems – winter dry period, western disturbances, pre or post monsoon, monsoon onset, active monsoon, break monsoon and retreating monsoon. Western disturbances originate over the Mediterranean Sea and Atlantic Ocean, and are responsible for almost one-third of the annual precipitation over north Indian, with the risk of heavy precipitation to north-eastern parts during early spring. “These weather patterns can be used in forecasting tools by meteorologists where multiple forecast scenarios from numerical weather prediction models can be assigned to the closest matching definition of a weather pattern,” said Ashis Mitra, co-author, NCMRWF. “This can be used to predict a probabilistic weather pattern two weeks in advance, and forecast the most likely transition of a weather pattern with associated impacts.”
For instance, Mumbai records some of its highest rainfall totals during the active monsoon, and the four wettest weather patterns fall within the active monsoon types. Weather pattern 19 is the wettest (average 46mm/day) and weather pattern 10 experiences the lowest rainfall (average 23mm/day). Researchers said a high confidence forecast for weather pattern 19 could suggest an increased risk of flooding across the city – information that can be disseminated to government, policy makers and citizens to act accordingly.
The team is currently testing a live forecasting tool with scientists from the Indian Meteorological Department (IMD) and NCMRWF. “Future research will focus on the skill of forecasting applications designed predominantly to predict weather patterns one to two weeks in advance at both regional and national scale. Shorter-range high-resolution forecast models used by IMD will still be used for issuing detailed weather warnings,” said Mitra.
The new set of 30 weather patterns represent variability in precipitation across the whole of India for the entire year.
With the focus on wind because India’s climate is predominantly influenced by changes in the direction of the prevailing wind, historically pattern 1 of the 30 weather patterns occurs most often annually (5.2% of the time) and pattern 30 occurs least often annually (1.5% of the time).
With the exception of the monsoon onset which has only one weather pattern (number 26) that occurs 17.7% in June or 5.6% in July every year, all other weather systems comprise a collection of weather patterns.
Weather patterns also demonstrate seasonality – for instance patterns 2, 3, 7, 8, 9, 16 occur during the winter dry period across India from December to March, and patterns 10, 17, 19, 21 occur during different phases of the active monsoon season from June to September. Some weather patterns do not occur during summer or winter, but only during onset and withdrawal of the main monsoon season.