Quantifying the role of individual diabatic processes for the formation of PV anomalies in a North Pacific cyclone

Abstract

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

The sensitivity of atmospheric blocking to changes in upstream latent heating – numerical experiments

“…Our sensitivity experiments corroborate earlier studies that the interaction between mobile synoptic-scale eddies and planetary-scale flow anomalies plays an important role for blocking formation and maintenance (Nakamura et al, 1997;Luo et al, 2014;Nakamura and Huang, 2018), and show that diabatic processes can provide the required flow amplification in addition to dry-dynamical forcing. In order to properly represent blocking dynamics, numerical weather prediction and climate models thus have to correctly account for this coupling between dry and moist processes, including the details of microphyical processes that shape the spatial distribution of latent heating in clouds (e.g., Joos and Wernli, 2012;Dearden et al, 2016;Joos and Forbes, 2016;Crezee et al, 2017;Attinger et al, 2019).…”

“…LH is artificially turned off by multiplying the instantaneous temperature tendencies due to parameterized cloud and convection processes with a factor α = 0.0, but still allowing for moisture changes due to cloud and precipitation formation. Other non-conservative processes, such as radiative heating and turbulent mixing, which can also modify PV (Spreitzer et al, 2019;Attinger et al, 2019), are not altered. In contrast to previous studies, here LH is only modified in the region that is identified to be directly relevant for the blocking system, which is typically the WCB ascent region associated with upstream extratropical cyclones (Steinfeld and Pfahl, 2019).…”

Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics

“…The WCB case study was simulated with the limited-area nonhydrostatic model COSMO (Consortium for Small-scale Modeling; Baldauf et al, 2011;Doms and Baldauf, 2018) at 0.02 • (≈ 2.2 km) horizontal grid spacing with 60 vertical levels. The setup of the COSMO simulation and the online trajectories was the same as in Oertel et al (2019).…”

“…Initial and lateral boundary conditions are taken from the ECMWF analyses with a horizontal resolution of 0.1 • every 6 h. The domain is centered in the eastern North Atlantic and extends from about 50 • W to 20 • E and 30 to 70 • N. We apply the standard COSMO setup of the Swiss National Weather Service, which employs a one-moment six-category cloud microphysics scheme including prognostic water vapor (q v ), liquid (LWC) and ice (IWC) cloud water content, rain (LWC), snow (SWC), and graupel (GWC). The graupel category is important for the explicit simulation of deep convection (Baldauf et al, 2011). Deep convection is resolved at 2.2 km (e.g., Ban et al, 2014), while for shallow convection the reduced Tiedtke scheme was applied (Tiedtke, 1989;Baldauf et al, 2011).…”

“…The graupel category is important for the explicit simulation of deep convection (Baldauf et al, 2011). Deep convection is resolved at 2.2 km (e.g., Ban et al, 2014), while for shallow convection the reduced Tiedtke scheme was applied (Tiedtke, 1989;Baldauf et al, 2011). The 3-D COSMO fields are output every 15 min, which allows capturing the large temporal and spatial variability of embedded convection.…”