Precipitation Biases in the ECMWF Integrated Forecasting System

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  • 1 European Centre for Medium-Range Weather Forecasts (ECMWF), Shinfield Park, Reading, RG2 9AX, U.K.
  • 2 School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, U.K.
  • 3 Centre for Ecology & Hydrology, Wallingford, OX10 8BB, UK
  • 4 Geography Department, Loughborough University, Loughborough, LE11 3TU, UK
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Abstract

Precipitation is a key component of the global water cycle and plays a crucial role in flooding, droughts, and water supply. One way to manage its socioeconomic effects is based on precipitation forecasts from numerical weather prediction (NWP) models, and an important step to improve precipitation forecasts is by diagnosing NWP biases. In this study, we investigate the biases in precipitation forecasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (IFS). Using the IFS control forecast from 12 June 2019 to 11 June 2020 at 5,219 stations globally, we show that in each of the boreal winter and summer half years, the IFS (1) has an average global wet bias and (2) displays similar bias patterns for forecasts starting at 0000 UTC and 1200 UTC and across forecast days 1 to 5. These biases are dependent on observed (climatological) precipitation; stations with low observed precipitation have an IFS wet bias, while stations with high observed precipitation have an IFS dry bias. Southeast Asia has a wet bias of 1.61 mm day-1 (in boreal summer) and over the study period the precipitation is overestimated by 31.0% on forecast day 3. This is the hydrological signature of several hypothesized processes including issues specifying the IFS snowpack over the Tibetan Plateau which may affect the Meiyu front. These biases have implications for IFS land-atmosphere feedbacks, river discharge, and for ocean circulation in the southeast Asia region. Reducing these biases could lead to more accurate forecasts of the global water cycle.

Corresponding author: David Lavers (david.lavers@ecmwf.int)

Abstract

Precipitation is a key component of the global water cycle and plays a crucial role in flooding, droughts, and water supply. One way to manage its socioeconomic effects is based on precipitation forecasts from numerical weather prediction (NWP) models, and an important step to improve precipitation forecasts is by diagnosing NWP biases. In this study, we investigate the biases in precipitation forecasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (IFS). Using the IFS control forecast from 12 June 2019 to 11 June 2020 at 5,219 stations globally, we show that in each of the boreal winter and summer half years, the IFS (1) has an average global wet bias and (2) displays similar bias patterns for forecasts starting at 0000 UTC and 1200 UTC and across forecast days 1 to 5. These biases are dependent on observed (climatological) precipitation; stations with low observed precipitation have an IFS wet bias, while stations with high observed precipitation have an IFS dry bias. Southeast Asia has a wet bias of 1.61 mm day-1 (in boreal summer) and over the study period the precipitation is overestimated by 31.0% on forecast day 3. This is the hydrological signature of several hypothesized processes including issues specifying the IFS snowpack over the Tibetan Plateau which may affect the Meiyu front. These biases have implications for IFS land-atmosphere feedbacks, river discharge, and for ocean circulation in the southeast Asia region. Reducing these biases could lead to more accurate forecasts of the global water cycle.

Corresponding author: David Lavers (david.lavers@ecmwf.int)
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