Using microwave radiometer data in the AROME model: case study in an Alpine valley
The town of Passy in the Arve Valley, Haute-Savoie, France, is regularly affected by peaks of intense pollution. These occur as a result of anticyclonic winter conditions, during which weather forecasts are often uncertain.
In order to improve understanding and forecasting of these events, Météo-France launched the Passy-2015 measurements campaign in 2015 and installed a microwave radiometer on the ground. It takes continuous measurements of temperature and humidity profiles as well as content integrated into water from the atmosphere. Its unrivalled temporal resolution (of around several minutes) enables evolution of the boundary layer’s life cycle to be monitored; in other words, the lower part of the atmosphere located just above the Earth's surface, in order to incorporate this data into the numerical prediction model, AROME. The initial state of the atmosphere taken into account by the model is therefore closer to reality, which ultimately improves the forecasts it provides.
Work conducted during the Passy-2015 campaign highlighted the limitations of the AROME model which largely underestimated cooling that occurs near the surface, with forecasting errors reaching 12°C during peaks of pollution. By combining microwave radiometer observations with the model’s 1-hour forecasts, the error observed in the model’s initial state could be greatly reduced, both on the surface (going from 8°C to 0.5°C) and at the base of stratus clouds. Beyond the initial state, the temporal evolution of this boundary layer is also far better represented in the model thanks to its consideration of these high-frequency observations.
This work led to studies on operational assimilation of the data in the AROME model. It should bring about improvement in the forecasting of a number of events that pose significant economic and social issues: pollution, but also fog and intense rainfall events.