Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part I: Climatology and Potential Vorticity Evolution

Abstract

A global climatology of warm conveyor belts (WCBs) is presented for the years 1979-2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 258 and 508 latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically .20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity unit (PVU) at 700 hPa, and a decrease to less than 0.5 PVU at 300 hPa. These low PV values in the upper troposphere constitute significant negative anomalies with amplitudes of 1-3 PVU, which can strongly influence the downstream flow. Considering the low-level diabatic PV production, (i) WCBs starting at low latitudes (,408) are unlikely to attain high PV (due to weak planetary vorticity) although they exhibit the strongest latent heating, and (ii) for those ascending at higher latitudes, a strong vertical heating gradient and high absolute vorticity are both important. This study therefore provides climatological insight into the cloud diabatic formation of significant positive and negative PV anomalies in the extratropical lower and upper troposphere, respectively.

Atmospheric processes triggering the central European floods in June 2013

“…An important fraction of large-scale precipitation in extratropical cyclones occurs in the warm sector ahead of the cold front where air ascends poleward along the sloped isentropes within so-called warm conveyor belts (WCBs, Browning et al, 1973;Browning, 1990;Madonna et al, 2014). For central Europe, Pfahl et al (2014) quantified that 20-40 % of all extreme precipitation events during 1979-2010 were linked to WCBs and even 40-60 % either to a WCB or a cyclone.…”

“…If moisture decreases by more than 1 g kg −1 within 6 h along a WCB trajectory, all surrounding grid points are marked, and surface precipitation at these locations is associated with the WCB. We also use the ERA-Interim based WCB climatology of Madonna et al (2014) for climatological considerations.…”

“…An equatorward ascending WCB is identified from the general ERAInterim (1979-2012) WCB climatology (Madonna et al, 2014) by the criterion that the latitude at the start of the ascent is further poleward compared to the latitude at the end of the ascent. The equatorward ascending WCB frequency is determined as the fraction of 6-hourly ERA-Interim time steps that exhibit at least one equatorward ascending WCB within the vertical column at the respective grid point.…”

Climatology and long-term evolution of ozone and carbon monoxide in the UTLS at northern mid-latitudes, as seen by IAGOS from 1995 to 2013

“…During winter, the weaker convection and the longer lifetime of CO allow the accumulation of this trace gas in the lower troposphere despite the stronger wintertime frequency of warm conveyor belts (e.g. Madonna et al, 2014), thus leading to a climatological maximum spread from late winter until early spring (Petetin et al, 20 2016). As the convection activity increases, the CO confined in the lower troposphere starts being uplifted to the UT.…”

“…It is likely the resulting combination of stronger emissions from both anthropogenic and biomass burning over the Asian continent (e.g. Jiang et al, 2017), and an efficient vertical transport over there (Madonna et al, 2014;Huang et al, 2016). and a maximum during Summer (79 ppb), characterizing the seasonal peak of photochemical activity.…”