Thomas Spengler

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Extratropical cyclones (ETCs) are strongly influenced by moist processes, rendering the impact of diabatic heating critical for forecast performance. We systematically evaluate short-range (12-hour) forecast biases of wintertime maritime ETCs over the North Atlantic, North Pacific, and Southern Ocean for the period 2023–2024. Employing a cyclone-centred composite framework, cyclones are categorised into strong and weak diabatic heating groups. We compare forecasts from the ECMWF high resolution operational 9-km Integrated Forecasting System (IFS) and the data-driven Artificial Intelligence Forecasting System (AIFS). Both the higher resolution of IFS and the data-driven AIFS significantly reduce the ETC propagation biases previously identified in ERA5. However, both models exhibit more pronounced errors in cyclones with strong diabatic heating. In the physics-based IFS, while the previous severe dry and cold biases are largely improved, the model still underestimates cyclone intensity and warm sector wind speeds. Furthermore, IFS displays a distinct spiral-shaped positive bias in the 850–500 hPa temperature difference, suggesting a misrepresentation of the vertical distribution and depth of diabatic heating. AIFS generally yields similar but smaller biases in most fields compared to IFS. Despite these improvements, AIFS features a notable physical inconsistency as it demonstrates a weaker mean sea level pressure (MSLP) bias but a stronger 10 m wind bias compared to IFS. This is related to an underestimation of the near-surface ageostrophic wind speed.

Extratropical cyclones (ETCs) are strongly influenced by moist processes, rendering the impact of diabatic heating critical for forecast performance. We systematically evaluate short-range (12-hour) forecast biases of wintertime maritime ETCs over the North Atlantic, North Pacific, and Southern Ocean for the period 2023–2024. Employing a cyclone-centred composite framework, cyclones are categorised into strong and weak diabatic heating groups. We compare forecasts from the ECMWF high resolution operational 9-km Integrated Forecasting System (IFS) and the data-driven Artificial Intelligence Forecasting System (AIFS). Both the higher resolution of IFS and the data-driven AIFS significantly reduce the ETC propagation biases previously identified in ERA5. However, both models exhibit more pronounced errors in cyclones with strong diabatic heating. In the physics-based IFS, while the previous severe dry and cold biases are largely improved, the model still underestimates cyclone intensity and warm sector wind speeds. Furthermore, IFS displays a distinct spiral-shaped positive bias in the 850–500 hPa temperature difference, suggesting a misrepresentation of the vertical distribution and depth of diabatic heating. AIFS generally yields similar but smaller biases in most fields compared to IFS. Despite these improvements, AIFS features a notable physical inconsistency as it demonstrates a weaker mean sea level pressure (MSLP) bias but a stronger 10 m wind bias compared to IFS. This is related to an underestimation of the near-surface ageostrophic wind speed.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Extratropical cyclones (ETCs) are strongly influenced by moist processes, rendering the impact of diabatic heating critical for forecast performance. We systematically evaluate short-range (12-hour) forecast biases of wintertime maritime ETCs over the North Atlantic, North Pacific, and Southern Ocean for the period 2023–2024. Employing a cyclone-centred composite framework, cyclones are categorised into strong and weak diabatic heating groups. We compare forecasts from the ECMWF high resolution operational 9-km Integrated Forecasting System (IFS) and the data-driven Artificial Intelligence Forecasting System (AIFS). Both the higher resolution of IFS and the data-driven AIFS significantly reduce the ETC propagation biases previously identified in ERA5. However, both models exhibit more pronounced errors in cyclones with strong diabatic heating. In the physics-based IFS, while the previous severe dry and cold biases are largely improved, the model still underestimates cyclone intensity and warm sector wind speeds. Furthermore, IFS displays a distinct spiral-shaped positive bias in the 850–500 hPa temperature difference, suggesting a misrepresentation of the vertical distribution and depth of diabatic heating. AIFS generally yields similar but smaller biases in most fields compared to IFS. Despite these improvements, AIFS features a notable physical inconsistency as it demonstrates a weaker mean sea level pressure (MSLP) bias but a stronger 10 m wind bias compared to IFS. This is related to an underestimation of the near-surface ageostrophic wind speed.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Extratropical cyclones (ETCs) are strongly influenced by moist processes, rendering the impact of diabatic heating critical for forecast performance. We systematically evaluate short-range (12-hour) forecast biases of wintertime maritime ETCs over the North Atlantic, North Pacific, and Southern Ocean for the period 2023–2024. Employing a cyclone-centred composite framework, cyclones are categorised into strong and weak diabatic heating groups. We compare forecasts from the ECMWF high resolution operational 9-km Integrated Forecasting System (IFS) and the data-driven Artificial Intelligence Forecasting System (AIFS). Both the higher resolution of IFS and the data-driven AIFS significantly reduce the ETC propagation biases previously identified in ERA5. However, both models exhibit more pronounced errors in cyclones with strong diabatic heating. In the physics-based IFS, while the previous severe dry and cold biases are largely improved, the model still underestimates cyclone intensity and warm sector wind speeds. Furthermore, IFS displays a distinct spiral-shaped positive bias in the 850–500 hPa temperature difference, suggesting a misrepresentation of the vertical distribution and depth of diabatic heating. AIFS generally yields similar but smaller biases in most fields compared to IFS. Despite these improvements, AIFS features a notable physical inconsistency as it demonstrates a weaker mean sea level pressure (MSLP) bias but a stronger 10 m wind bias compared to IFS. This is related to an underestimation of the near-surface ageostrophic wind speed.

The development of extratropical cyclones (ETCs) is often significantly altered by diabatic processes, yet the representation of these processes in numerical weather prediction models has been shown to lead to significant forecast biases. To provide a systematic quantification of 12-h ETC forecast errors, this study uses a cyclone-centred composite framework for North Atlantic wintertime (DJF) ETCs using the ERA5 reanalysis for the period 1979 to 2022. Cyclones are categorised into strong and weak diabatic heating at the time of their maximum intensification based on the domain-averaged 70th and 30th percentiles of vertically integrated diabatic heating. While both groups exhibit a systematic underestimation of cyclone intensity, the error structures are markedly distinct. The weak heating group is characterised by an intensity underestimation near the cyclone core, whereas the strong heating group features a pronounced southwestward displacement bias together with a domain-wide intensity underestimation. After removing the displacement bias, the strong heating group exhibits distinct structural errors. In the warm sector, a clear underestimation of moisture transport and temperature, combined with an underdeveloped upper-level ridge, indicates a mis-representation of the intense moisture transport pathways and associated warm-sector moist processes. Conversely, in the cold sector, low-level winds are overestimated within the cold conveyor belt (CCB), sting jet (SJ), and dry intrusion (DI) regions. The wind field biases are accompanied by a pronounced overestimation of 850 hPa kinematic frontogenesis near the centre. The strong frontogenesis is associated with an enhanced secondary circulation and vertical velocity, yielding the overestimation of total column liquid water observed along the bent-back warm front. In contrast, cyclones in the weak heating group exhibit an underestimation of wind speed and moisture near the centre, consistent with the near-centre intensity underestimation. Overall, our findings demonstrate the critical impact of diabatic heating on structural forecast biases, highlighting that the representation of moist processes and the interaction with atmospheric dynamics through diabatic processes is a key area for future model developments.