The Role of Warm Conveyor Belts for the Intensification of Extratropical Cyclones in Northern Hemisphere Winter

Abstract

The role of warm conveyor belts (WCBs) and their associated positive low-level potential vorticity (PV) anomalies are investigated for extratropical cyclones in Northern Hemisphere winter, using ERA-Interim and composite techniques. The Spearman correlation coefficient of 0.68 implies a moderate to strong correlation between cyclone intensification and WCB strength. Hereby, cyclone intensification is quantified by the normalized maximum 24-h central sea level pressure deepening and WCB strength by the WCB air mass associated with the cyclone's 24-h period of strongest deepening. Explosively intensifying cyclones typically have strong WCBs and pronounced WCB-related PV production in the cyclone center; they are associated with a WCB of type W2, which ascends close to the cyclone center. Cyclones with similar WCB strength but weak intensification are either diabatic Rossby waves, which do not interact with an upper-level disturbance, or cyclones where much of the WCB-related PV production occurs far from the cyclone center and thereby does not contribute strongly to cyclone deepening (WCB of type W1, which ascends mainly along the cold front). The category of explosively intensifying cyclones with weak WCBs is inhomogeneous but often characterized by a very low tropopause or latent heating independent of WCBs. These findings reveal that (i) diabatic PV production in WCBs is essential for the intensification of many explosive cyclones, (ii) the importance of WCBs for cyclone development strongly depends on the location of the PV production relative to the cyclone center, and (iii) a minority of explosive cyclones is not associated with WCBs.

Significant Impact of Heat Supply From the Gulf Stream on a “Superbomb” Cyclone in January 2018

“…Although we focused specifically on the role of the CCB in this research, recent studies have emphasized that a warm conveyor belt (WCB) plays vital roles in the enhancement of LH within extratropical cyclones and in cyclone intensification (e.g., Binder et al, ; Booth et al, ; Schemm & Wernli, ). We confirmed the poleward moisture transport associated with the WCB during the rapid development stage of the cyclone in the CNTL run (see Figure S6).…”

“…This deepening rate satisfies the criteria defining a “bomb” cyclone (Sanders & Gyakum, ). According to the latest statistics by Binder et al () based on ERA‐Interim (see their Figure ), there were only two cases with maximum deepening rates exceeding 3.3 Bergeron in the Northern Hemisphere between 1979 and 2014. Thus, this bomb cyclone was one of the strongest in recent years.…”

The North Atlantic Waveguide and Downstream Impact Experiment

“…The latent heating in WCBs is strong both in the early phase of the ascent when condensation dominates and later when mixed-phase clouds are formed and vapor deposition on ice crystals and snow becomes important (Joos and Wernli 2012). The effect of the heating on the PV structure is to produce a positive PV anomaly in the lower troposphere (Wernli and Davies 1997), which inf luences the structure and evolution of midlatitude surface cyclones (e.g., Kuo et al 1991;Davis et al 1993;Binder et al 2016). Above the level of maximum latent heating, PV is reduced by cloud diabatic processes, leading to negative PV anomalies in the upper-tropospheric WCB outf low region (Wernli 1997;Pomroy and Thorpe 2000;Madonna et al 2014;Methven 2015).…”

Influential role of inter‐decadal explosive cyclone activity on the increased frequency of winter storm events in Hokkaido, the northernmost island of Japan

“…Rapid development of the cyclones requires not only baroclinic instability but also vertical coupling between an upper‐level disturbance and a surface cyclone (e.g., Hoskins et al, ; Takayabu, ; Shapiro et al, ) and diabatic heating around the cyclone centre (e.g., Kuo et al, ; ; Reed et al, ; Kuwano‐Yoshida and Asuma, ). Latent heat release within the cyclone system through the warm conveyor belt (WCB) contributes to the cyclone intensity because the WCB facilitates water vapour import into the cyclone from low latitudes (e.g., Carlson, ; Booth et al, ; Madonna et al, ; Binder et al, ), whereas a positive feedback process between the cold conveyor belt (CCB) and latent heating over the bent‐back front also contributes to cyclone development through heat and moisture supply from mid‐latitude warm currents (Hirata et al, ; ; ).…”

“…(e.g., Kuo et al, 1991a;1991b;Reed et al, 1993;Kuwano-Yoshida and Asuma, 2008). Latent heat release within the cyclone system through the warm conveyor belt (WCB) contributes to the cyclone intensity because the WCB facilitates water vapour import into the cyclone from low latitudes (e.g., Carlson, 1980;Booth et al, 2012;Madonna et al, 2014;Binder et al, 2016), whereas a positive feedback process between the cold conveyor belt (CCB) and latent heating over the bent-back front also contributes to cyclone development through heat and moisture supply from mid-latitude warm currents (Hirata et al, 2015;2016;.…”