Solar Eclipses in the IFS

Title
Solar Eclipses in the IFS
Technical memorandum
Date Published
01/2025
Secondary Title
Technical Memoranda
Number
920
Author
Philippe Lopez
Publisher
ECMWF
Abstract Until now, ECMWF’s Integrated Forecasting System (IFS) was not able to represent the meteorological consequences of the strong reduction in incoming solar radiation associated with occasional solar eclipses. Therefore, developments have been made in the IFS to include the effects of solar eclipses in both operational weather forecasts and data assimilation (from IFS cycle 50R1 onwards), as well as in ECMWF’s future reanalysis ERA6 (to be based on IFS cycle 49R2). As illustrated for the recent total solar eclipse of 8 April 2024 over North America, the inclusion of such event in IFS forecasts has a significant and widespread impact on low-level tropospheric temperature, humidity and wind over land, as well as on temperature in the stratospheric ozone layer. Consistent with previous studies based on observations and numerical simulations, the strongest impacts are found over land regions with a stable planetary boundary layer and clear-sky conditions. The drop in predicted 2-m temperature reaches 7◦C inside the band of totality, while a persistent cooling in excess of 1◦C affects most of North America, even several hours after the eclipse. These simulated impacts are successfully validated using high temporal frequency ground-based observations provided by the Purdue University in Indiana (USA). It is also demonstrated that including solar eclipses in ECMWF’s 4D-Var data assimilation system helps to reduce background departures over the whole region affected by the eclipse, especially for temperature in the low troposphere and in the stratosphere. This, in turn, can increase the number of effectively assimilated observations by several percent. Furthermore, even bigger benefits are expected when satellite solar reflectances start to be assimilated in the IFS. Even though these developments are not likely to noticeably improve the overall performance of the IFS, they should significantly improve both analyses and forecasts during the two to five eclipse days that occur every year.
URL https://www.ecmwf.int/en/elibrary/81633-solar-eclipses-ifs
DOI 10.21957/720a4af0c5