Systematic assessment of the diabatic processes that modify low-level potential vorticity in extratropical cyclones

Abstract

Abstract. Diabatic processes significantly affect the development and structure of extratropical cyclones. Previous studies quantified the dynamical relevance of selected diabatic processes by studying their influence on potential vorticity (PV) in individual cyclones. However, a more general assessment of the relevance of all PV-modifying processes in a larger ensemble of cyclones is currently missing. Based on a series of twelve 35-day model simulations using the Integrated Forecasting System (IFS) of the European Centre for Medium-range Weather Forecasts (ECMWF), this study systematically quantifies the relevance of individual diabatic processes for the dynamics of 288 rapidly intensifying extratropical cyclones. To this end, PV tendencies associated with each parametrized process in the model are accumulated along 15 h backward trajectories. The investigation focuses on regions of high PV (≥ 1 PVU) along the cold front, warm front, and in the cyclone center, as well as of negative PV (≤ −0.1 PVU) along the cold and warm front in the lower troposphere. On average, the primary processes that modify PV during the 24 h period of most rapid cyclone intensification remain temporally consistent for all anomalies considered. However, a pronounced case-to-case variability is found when comparing the dominant processes across individual cyclones. Along the cold front, PV is primarily generated by condensation in half of the investigated cyclones. For the remaining cyclones, convection or long-wave radiative cooling become the dominant process depending on environmental conditions. Results are similar for both seasons, with a reduced role of convection for the generation of PV along the cold front in the warm season. Negative PV west of the cold front is produced by turbulent exchange of momentum and temperature as well as long-wave radiative heating. The relevance of long-wave radiative heating is reduced during summer. The positive PV anomaly at the warm front is predominantly generated by condensation in the cold season, whereas turbulent mixing becomes the prevalent process during the warm season. Convection only plays a minor role for the generation of PV at the warm front. Negative PV along the warm front is produced by long-wave radiative heating, turbulent temperature tendencies, or melting of snow in the cold season. Turbulent temperature tendencies become the dominant process decreasing PV at the warm front in the warm season, together with melting of snow and turbulent exchange of momentum. The positive PV anomaly in the cyclone center is primarily produced by condensation, with only few cyclones where PV production is mainly associated with turbulent mixing or convection. A composite analysis further reveals that PV anomalies generated by convection require a negative air-sea temperature difference in the warm sector of the cyclone, which promotes a heat flux directed into the atmosphere and destabilizes the boundary layer. These cyclones primarily occur over warm ocean currents in the cold season. On the other hand, cyclones that occur in a significantly colder environment are often associated with a positive air-sea temperature difference in the warm sector, leading to PV generation by long-wave radiative cooling. Finally, long-wave radiative heating due to a negative air-sea temperature difference in the cold sector can produce negative PV along the cold and warm front. The general agreement between accumulated PV tendencies and the net PV change along trajectories is good. Therefore, the approach used in this study yields valuable insight regarding the specific physical processes that modify low-level PV in rapidly deepening extratropical cyclones.

Origin of low-tropospheric potential vorticity in Mediterranean cyclones

“…Nevertheless, the boxes and whiskers suggest that there is a large case-by-case variability. These distributions are comparable in sign and amplitude to the PV modifications in extratropical cyclones in the main storm track region recently analyzed by Attinger et al (2021). Furthermore, the overall strong positive contribution by latent heat release by both convection and microphysics in the lower troposphere and the overall positive contribution by turbulent momentum forcing with some individual negative cases are in accordance with the findings of Flaounas et al (2021).…”

“…However, the contribution of each process shows a large case-by-case variability. Comparing with North Atlantic cyclones (Attinger et al, 2021), Mediterranean cyclones show a similar contribution of diabatic processes to their lowertropospheric PV anomaly. From the perspective of atmospheric processes, this suggests that Mediterranean cyclones do not form a unique category of extratropical cyclones except for cases forming close to mountains.…”

“…From ERA5, we use wind and PV fields (PV calculated on model levels), regridded to a 0.5 • × 0.5 • grid on 98 vertical model levels. We also use output from 12 monthly IFS simulations, which were obtained using cycle 43R1 of the hydrostatic IFS model (Roberts et al, 2018), as described in Attinger et al (2021). Each simulation covers 35 d and is initialized at the beginning of each month from December 2017 to November 2018.…”

“…The special IFS version provides detailed output of temperature and momentum tendencies from all parameterizations, from which we can calculate the corresponding PV tendencies, enabling a process-based investigation of the formation of PV anomalies along trajectories. Here we follow the approach by Joos and Wernli (2012), which was later used, for example, by Attinger et al (2019Attinger et al ( , 2021 and Spreitzer et al (2019). Table 1 summarizes the processes and corresponding PV tendencies considered in our study.…”

“…However, a systematic assessment of the processes that produce PV in Mediterranean cyclones, also by other processes, such as turbulence, is currently missing. The recent study by Attinger et al (2021) showed that, in North Atlantic cyclones, turbulent and radiative processes can also contribute to the PV production in the center and along the fronts of the cyclone. In fact, they presented a case study of a cyclone in which the lowertropospheric PV anomaly within the cyclone center could be almost entirely attributed to frictional forces.…”