Diabatic processes modulating the vertical structure of the jet stream above the cold front of an extratropical cyclone: sensitivity to deep convection schemes

The effect of deep convection parameterization on the jet stream above the cold front of an explosive extratropical cyclone is investigated in the global numerical weather prediction model ARPEGE, operational at Météo-France. Two hindcast simulations differing only in the deep convection scheme used are systematically compared with each other, with (re)-analysis datasets and with NAWDEX airborne observations. The deep convection representation has an important effect on the vertical structure of the jet stream above the cold front at one-day lead time. The simulation with the less active scheme shows a deeper jet stream, associated with a stronger potential vorticity (PV) gradient in the jet core in middle troposphere. This is due to a larger deepening of the dynamical tropopause on the cold-air side of the jet and a higher PV destruction on the warm-air side, near 600 hPa. To better understand the origin of this stronger PV gradient, Lagrangian backward trajectories are computed. On the cold-air side of the jet, numerous trajectories undergo a rapid ascent from the boundary layer to the mid levels in the simulation with the less active deep convection scheme, whereas they stay at mid levels in the other simulation. This ascent explains the higher PV noted on that side of the jet in the simulation with the less active deep convection scheme. These ascending air masses form mid-level ice clouds that are not observed in the microphysical retrievals from airborne radar-lidar measurements. On the warm-air side of the jet, in the warm conveyor belt (WCB) ascending region, the Lagrangian trajectories with the less active deep convection scheme undergo a higher PV destruction due to a stronger heating occurring in the lower and middle troposphere. In contrast, in the simulation with the most active deep convection scheme, both the heating and PV destruction extend further up in the upper troposphere.

On the relationship between CYGNSS surface heat fluxes and the lifecycle of low-latitude ocean extratropical cyclones

Surface latent and sensible heat fluxes are important for extratropical cyclone evolution and intensification. Because extratropical cyclone genesis often occurs at low-latitude, CYGNSS surface latent and sensible heat flux retrievals are composited to provide a mean picture of their spatial distribution in low-latitude oceanic extratropical cyclones. CYGNSS heat fluxes are not affected by heavy precipitation and offer observations of storms with frequent revisit times. Consistent with prior results obtained for cyclones in the Gulf Stream region, the fluxes are strongest in the wake of the cold fronts, and weakest to negative in the warm sector in advance of the cold fronts. As cyclone strength increases, or mean precipitable water decreases, the maximum in surface heat fluxes increases while the minimum decreases. This impacts the changes in fluxes during cyclone intensification: the post-cold frontal surface heat flux maximum increases due to the increase in near surface winds. During cyclone dissipation, the fluxes in this sector decrease, due to the decrease in winds and in temperature and humidity contrast. The warm sector minimum decreases throughout the entire cyclone lifetime and is mostly driven by sea-air temperature and humidity contrast changes. However, during cyclone dissipation, the surface heat fluxes increase along the cold front in a narrow band to the east, independently from changes in the cyclone characteristics. This suggests that, during cyclone dissipation, energy transfers from the ocean to the atmosphere are linked to frontal in addition to synoptic-scale processes.

Impact of Extratropical Cyclone Intensity and Speed on the Extreme Wave Trends in the Atlantic Ocean

This work analyzes the extratropical cyclone-related extreme waves in the ocean surface and their trends in the North and South Atlantic Oceans. Atmospheric and ocean wave products are obtained from ERA5, from 1979 to 2020 with 1-hourly outputs, covering 42 years with the present climate changes evaluated by the difference between the two 21-years time slices. The cyclones are tracked through the relative vorticity at 850 hPa and then associated with extreme wave events using an automated scheme that searches for an extreme wave region 1500 km from the centre of the cyclone, following criteria that exclude possible swell dominated events. The hot-spot regions of cyclone-related waves occurrence found by the method are in agreement with previous studies and are related to the cyclogenesis region and storm track orientation. Most cyclones associated with extreme wave events are generated in the western boundary of the domains. The east-poleward side of the ocean basins presents the highest density of occurrences related to the higher density of cyclone track and the dominance of more mature stage cyclones while in the west side prevail systems on developing stages, with notable propagating fronts and consequently, lower wind persistence. Changes in occurrence cannot be explained just by the storm track variation during the period due to the lack of statistical confidence. However, the wave occurrence responds to changes in the cyclone intensity, modulated by cyclone displacement speed. Regions with an increase of extreme waves are related to the effect of more intense cyclones or cyclones with slower propagation, being the last associated with a longer interaction of winds with the ocean surface.

Heavy winter precipitation events with extratropical cyclone diagnosed by GPM products and trajectory analysis

<p>In Japan, Extratropical cyclone sometimes causes sporadic heavy snow in the coastal cites or heavy rains on snow covers in mountainous areas. Ando and Ueno (2015) identified that heavy precipitation events tend to occur with occluding cyclones. However, three-dimensional structure of precipitation system embedded in the cyclone system are difficult to capture by surface observation network over Japanese archipelago that are composed of complex coastal lines and mountains. This study identified heavy precipitation events during the cold seasons of 2014-2019 by two-day accumulated precipitation data at 137 stations of the Japan Meteorological Agency. The mechanisms for producing heavy precipitation in relation to the structure of an occluding extratropical cyclone were analyzed with the aid of the products of the Dual-frequency Precipitation Radar onboard the Global Precipitation Measurement (GPM) core satellite and trajectory analysis on European Centre for Medium-range Weather Forecasts atmospheric reanalysis data. Upper-ranked events with heavy precipitation were mostly due to extratropical cyclones, and many of them were in mature stages. In the top 50 ranked events, three south-coast cyclones were nominated, and relationships between the development of the mesoscale precipitation system and airstreams were intensively diagnosed. Hourly precipitation changes at stations that recorded heavy precipitation were primary affected by a combination of the warm conveyor belt (WCB), the cold conveyor belt (CCB) and the dry intrusion (DI). Wide-ranging stratiform precipitation in the east of cyclone center was composed of low-level WCB over the CCB and the upper WCB, and convective clouds around the cyclone center was associated with the upper DI over the WCB that provided an extreme precipitation rate at the surface, including formation of a band-shaped precipitation system. The convective cloud activities also contributed to moist air advection over the stationary stratiform precipitation areas recognized as the upper WCB. DPR products also identified deep stratiform precipitation in the cloud-head area behind the cyclone center with mid-level (near-surface) latent heat release (absorption) with increased potential vorticity along the CCB that was made feed-back intensification of the cyclone possible. (This study will be published in GPM special issue of JMSJ)　</p>