A 15-Year Climatology of Warm Conveyor Belts

Abstract

This study presents the first climatology of so-called warm conveyor belts (WCBs), strongly ascending moist airstreams in extratropical cyclones that, on the time scale of 2 days, rise from the boundary layer to the upper troposphere. The climatology was constructed by using 15 yr (1979-93) of reanalysis data and calculating 355 million trajectories starting daily from a 1Њ ϫ 1Њ global grid at 500 m above ground level (AGL). WCBs were defined as those trajectories that, during a period of 2 days, traveled northeastward and ascended by at least 60% of the zonally and climatologically averaged tropopause height. The mean specific humidity at WCB starting points in different regions varies from 7 to 12 g kg Ϫ1. This moisture is almost entirely precipitated out, leading to an increase of potential temperature of 15-22 K along a WCB trajectory. Over the course of 3 days, a WCB trajectory produces, on average, about four (six) times as much precipitation as a global (extratropical) average trajectory starting from 500 m AGL. WCB starting points are most frequently located between approximately 25Њ and 45ЊN and between about 20Њ and 45ЊS. In the Northern Hemisphere (NH), there are two distinct frequency maxima east of North America and east of Asia, whereas there is much less zonal variability in the Southern Hemisphere (SH). In the NH, WCBs are almost an order of magnitude more frequent in January than in July, whereas in the SH the seasonal variation is much weaker. In order to study the relationship between WCBs and cyclones, an independent cyclone climatology was used. Most of the WCBs were found in the vicinity of a cyclone center, whereas the reverse comparison revealed that cyclones are normally accompanied by a strong WCB only in the NH winter. In the SH, this is not the case throughout the year. Particularly around Antarctica, where cyclones are globally most frequent, practically no strong WCBs are found. These cyclones are less influenced by diabatic processes and, thus, they are associated with fewer high clouds and less precipitation than cyclones in other regions. In winter, there is a highly significant correlation between the North Atlantic Oscillation (NAO) and the WCB distribution in the North Atlantic: In months with a high NAO index, WCBs are about 12% more frequent and their outflow occurs about 10Њ latitude farther north and 20Њ longitude farther east than in months with a low NAO index. The differences in the WCB inflow regions are relatively small between the two NAO phases. During high phases of the Southern Oscillation, WCBs occur more (less) frequent around Australia (in the South Atlantic).

A climatology of mid-tropospheric mesoscale strong wind events as observed by the MST radar, Aberystwyth

“…a 15 year climatology is given by Eckhardt et al, 2004), cold conveyor belts (re-examined by Schultz, 2001) and dry intrusions (e.g. Carlson, 1980).…”

Airborne lidar observations in the inflow region of a warm conveyor belt

“…The poleward transport of sensible and latent heat by WCBs is a key component of the overall atmospheric meridional energy transport in midlatitudes, where the temperature contrast between Pole and Equator is reduced mainly by cyclones and anticyclones due to baroclinic instability. The WCB climatology by Eckhardt et al (2004) reveals the large contribution of WCBs to mid-latitude precipitation. The intense latent heat release associated with cloud formation and precipitation influences the life cycle of cyclones and thus directly affects atmospheric dynamics.…”

Vorticity, deformation and divergence signals associated with stratosphere–troposphere exchange

“…This strengthens the claim that TST is not directly linked to the dynamics of upper-level jet streaks. We expect the upward motion to be due to either warm conveyor belts, as suggested by Eckhardt et al (2004), or convection (Sprenger et al, 2009). A further possibility is that these upward motions are related to regions of hydrostatic instability, which was found to have a local maximum in the tropopause region in midlatitudes and can be a source of CAT (Jaeger and Sprenger, 2007).…”

“…The maximum probability is found at the time of the crossing for STT, but about 20 h earlier for TST. The TST cyclone peak in Figure 9(e) most likely represents the rapid transport by the warm conveyor belts of extratropical cyclones (Eckhardt et al, 2004). A clear asymmetry between STT and TST exists for anticyclones.…”