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 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.

Highlights

  • Subgrid-scale physical processes such as cloud microphysics, radiation, and turbulence can substantially influence the life cycle of extratropical cyclones (e.g., Kuo et al, 1991)

  • The present study aims at quantifying the relative importance of individual diabatic processes for the generation of positive and negative lowlevel potential vorticity (PV) anomalies during the intensification period of extratropical cyclones

  • The total number of extratropical cyclones identified in the Northern Hemisphere and the number of explosively deepening cyclones in each month are provided in the upper part of Table 3

Read more

Summary

Introduction

Subgrid-scale physical processes such as cloud microphysics, radiation, and turbulence can substantially influence the life cycle of extratropical cyclones (e.g., Kuo et al, 1991). Diabatic cooling of the warm sector by latent and sensible heat fluxes directed into the ocean was found to increase the stratification of the marine boundary layer, thereby enhancing low-level PV along the cold front in a North Atlantic (Neiman et al, 1990) and North Pacific (Attinger et al, 2019) cyclone. Low-level PV is generally reduced in the cold sector of extratropical cyclones, where the comparatively warm ocean destabilizes the boundary layer through surface sensible heat fluxes directed into the atmosphere (Chagnon et al, 2013; Vannière et al, 2016). This enables the assessment of the relative contribution of different diabatic processes to the modification of PV in a large set of extratropical cyclones and the investigation of the variability of relevant processes across different cases

Data and methods
Model simulations
Linking diabatic processes to PV modification
Identification of cyclones and fronts
Selection of cyclones
Investigated PV anomalies
Systematic assessment of diabatic PV modification in extratropical cyclones
The influence of the environment on the generation of positive PV anomalies
The cold front
The warm front
The cyclone center
Conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call