Systematic daytime increases in atmospheric biases linked to dry soils in irrigated areas in Indian operational forecasts

The representation of land-atmosphere coupling in forecast models can significantly impact weather prediction. A previous case study in Northern India incorporating both model and observational data identified atmospheric biases in a high-resolution forecast linked to soil moisture that impacted the representation of the monsoon trough, an important driver of regional rainfall. The aim of the current work is to understand whether this behavior is present in operational forecasts run by the India National Centre for Medium Range Weather Forecasting (NCMRWF). We utilize satellite observations and reanalysis to evaluate model fields in June, July, August, and September forecasts from 2020. Our analysis reveals systematic rapid growth in warm boundary layer biases during the daytime over North West India, which weaken overnight, consistent with excessive daytime surface sensible heat flux. The cumulative effect of these biases produces temperatures more than 4K warmer in 60-h forecasts. These effects are enhanced by dry surface conditions. The biases impact circulation in the forecasts, which have implications for regional rainfall. The spatial distribution of warm biases in the Indo-Gangetic Plain is remarkably consistent with the location of areas equipped for irrigation, a process that is not explicitly represented in the model. Our results provide compelling evidence that the development of an irrigation scheme within the model is needed to address the substantial forecast biases that we document.

Automated summary

The representation of land-atmosphere coupling in forecast models has a significant impact on weather prediction. A previous case study in Northern India identified atmospheric biases in a high-resolution forecast related to soil moisture, affecting the monsoon trough representation. This study aims to investigate if the biases exist in operational forecasts by the India NCMRWF, revealing warm biases in the boundary layer over North West India during the daytime, which weaken overnight.