The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratropical weather systems. PV is modified by nonconservative processes, such as cloud latent heating, radiative transfer, and turbulence. Using a Lagrangian method, material PV modification near the tropopause is attributed to specific parameterized processes in the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF). In a case study, several flow features identified in a vertical section across an extratropical cyclone experienced strong PV modification. In particular clear-air turbulence at the jet stream is found to be a relevant process (i) for the PV structure of an upper-level front–jet system, corroborating previous observation-based findings of turbulent PV generation; (ii) for the purely turbulent decay of a tropopause fold, identified as an effective process of stratosphere–troposphere exchange; and (iii) in the ridge, where the Lagrangian accumulated turbulent PV modification exhibits a distinct vertical pattern, potentially impacting the strength of the tropopause inversion layer. In contrast, cloud processes affect the near-tropopause PV structure above a warm conveyor belt outflow in the ridge and above cold-sector convection. In agreement with previous studies, radiative PV production dominates in regions with an anomalously low tropopause, where both radiation and convection act to increase the vertical PV gradient across the tropopause. The particular strengths of the Lagrangian diagnostic are that it connects prominent tropopause structures with nonconservative PV modification along the flow and that it quantifies the relative importance of turbulence, radiation, and cloud processes for these modifications.

Processes that do not conserve potential vorticity (PV) have a profound impact on the intensity, evolution, and mesoscale details of extratropical weather systems. This study aims at quantifying and improving the understanding of how and when physical processes modify PV in cyclones. To this end, a 6-day forecast of a North Pacific cyclone is performed using a recent operational version of the ECMWF global numerical weather prediction model. Hourly instantaneous temperature and momentum tendencies of each parametrized process are used to compute the corresponding PV tendencies. By integrating these diabatic PV rates along backward trajectories, the relative contribution of individual processes for the PV budget can be assessed. The cold front is characterized by an elongated filament of increased PV, generated by latent heating due to condensation at the front as well as long-wave radiative cooling at the surface. Turbulent mixing at the interface of the boundary layer decreases PV behind the cold front during the early stage of the cyclone, while sublimation of snow produces negative PV in the mature phase. A broad region of enhanced PV is found along the warm front, generated by condensation and turbulence at the front as well as long-wave radiative cooling at the surface. The region of decreased PV north of the warm front is mainly modified by snow melting and sublimation. Finally, high values of PV along the bent-back front and the cyclone centre are generated by condensation, convection, snow melting and sublimation. In general, turbulent mixing offsets intense PV modification induced by the other processes. This study highlights the relevance of condensation, melting and sublimation of snow, long-wave radiative cooling, turbulence, and convection for the production of low-level PV anomalies and underlines the importance of correctly representing these processes in weather prediction models. K E Y W O R D Sconvection, diabatic processes, extratropical cyclone, IFS, microphysics, potential vorticity, radiation, turbulence Q J R Meteorol Soc. 2019;145:2454-2476.wileyonlinelibrary.com/journal/qj

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 d model simulations using the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts, this study systematically quantifies the diabatic modification of positive and negative low-level PV anomalies along the cold front, warm front, and in the center of 288 rapidly intensifying extratropical cyclones. Diabatic PV modification is assessed by accumulating PV tendencies associated with each parametrized process along 15 h backward trajectories. The primary processes that modify PV typically remain temporally consistent during cyclone intensification. However, a pronounced case-to-case variability is found when comparing the most important processes across individual cyclones. Along the cold front, PV is primarily generated by condensation in half of the investigated cyclones in the cold season (October to March). For most of the remaining cyclones, convection or long-wave radiative cooling is the most important process. Similar results are found in the warm season (April to September); however, the fraction of cyclones with PV generation by convection as the most important process is reduced. Negative PV west of the cold front is primarily produced by turbulent mixing of momentum, long-wave radiative heating, or turbulent mixing of temperature. The positive PV anomaly at the warm front is most often primarily generated by condensation in the cold season and by turbulent mixing of momentum in the warm season. Convection is the most important process only in a few cyclones. Negative PV along the warm front is primarily produced by long-wave radiative heating, turbulent mixing of temperature, or melting of snow in the cold season. Turbulent mixing of temperature becomes the primary process in the warm season, followed by melting of snow and turbulent mixing of momentum. The positive PV anomaly in the cyclone center is primarily produced by condensation in most cyclones, with only few cases primarily associated with turbulent mixing or convection. A composite analysis further reveals that cyclones primarily associated with PV generation by convection exhibit a negative air–surface temperature difference in the warm sector, which promotes a heat flux directed into the atmosphere. These cyclones generally 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–surface 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–surface temperature difference in the cold sector produces negative PV along the cold and warm front, in particular in the cold season.

Abstract. Ice clouds, so-called cirrus clouds, occur very frequently in the tropopause region. A special class are subvisible cirrus clouds with an optical depth lower than 0.03, associated with very low ice crystal number concentrations. The dominant pathway for the formation of these clouds is not known well. It is often assumed that heterogeneous nucleation on solid aerosol particles is the preferred mechanism although homogeneous freezing of aqueous solution droplets might be possible, since these clouds occur in the low-temperature regime T < 235 K. For investigating subvisible cirrus clouds as formed by homogeneous freezing we develop a reduced cloud model from first principles, which is close enough to complex models but is also simple enough for mathematical analysis. The model consists of a three-dimensional set of ordinary differential equations, and includes the relevant processes as ice nucleation, diffusional growth and sedimentation. We study the formation and evolution of subvisible cirrus clouds in the low-temperature regime as driven by slow vertical updraughts (0 < w ≤ 0. 05 m s−1). The model is integrated numerically and also investigated by means of theory of dynamical systems. We found two qualitatively different states for the long-term behaviour of subvisible cirrus clouds. The first state is a stable focus; i.e. the solution of the differential equations performs damped oscillations and asymptotically reaches a constant value as an equilibrium state. The second state is a limit cycle in phase space; i.e. the solution asymptotically approaches a one-dimensional attractor with purely oscillatory behaviour. The transition between the states is characterised by a Hopf bifurcation and is determined by two parameters – vertical updraught velocity and temperature. In both cases, the properties of the simulated clouds agree reasonably well with simulations from a more detailed model, with former analytical studies, and with observations of subvisible cirrus, respectively. The reduced model can also provide qualitative interpretations of simulations with a complex and more detailed model at states close to bifurcation qualitatively. The results indicate that homogeneous nucleation is a possible formation pathway for subvisible cirrus clouds. The results motivate a minimal model for subvisible cirrus clouds (SVCs), which might be used in future work for the development of parameterisations for coarse large-scale models, representing structures of clouds.