The dynamics of a midlatitude cyclone with very strong latent‐heat release

Abstract

SUMMARYThe role of latent-heat release in an explosively developing cyclone from the Fronts and Atlantic Storm-Track Experiment is examined within the potential vorticity (PV) framework. Using conventional synoptic analysis, piecewise PV inversion and numerical simulations, the development is described in terms of an upper-level PV (UPV) anomaly, a surface potential-temperature anomaly and diabatically generated PV anomalies in the lower and upper troposphere. The relative contributions of each anomaly to the cyclogenesis are investigated, along with the interactions between anomalies.In contrast to previous case-studies, the results of PV inversion and sensitivity tests reveal that the contribution from the surface thermal anomaly is much weaker than those associated with the upper-level and diabatically generated PV anomalies. A low-level PV (LPV) anomaly is generated rapidly in response to the UPV anomaly, and dominates the intensi cation of the system, but its associated circulation has only a modest feedback effect on the upper levels. However, diabatic reduction of PV in the upper troposphere is found to have important effects on the UPV anomaly. In particular, latent heating tends to destroy the UPV anomaly by inhibiting its downward penetration and reducing its horizontal extent.

Extratropical Cyclones in Idealized Simulations of Changed Climates

“…13). The anomaly in the lower troposphere, with a maximum slightly above 900 hPa, most probably is produced by diabatic processes, mostly by the latent heating during cloud formation, as shown in various case studies (Reed et al 1992;Davis 1992;Davis et al 1993;Stoelinga 1996;Lackmann 2002;Ahmadi-Givi et al 2004). While typically these two anomalies are displaced in the horizontal during cyclone intensification, with the upper anomaly farther to the west (Hoskins et al 1985), they often align at the time of highest intensity to form a so-called PV tower (Reed et al 1992;Rossa et al 2000).…”

“…Plant et al 2003) or for deepening rates rather than measures of intensity. LH has been shown to be important for cyclone intensification in theoretical studies (e.g., Emanuel et al 1987), idealized life cycle experiments (Boutle et al 2011;Booth et al 2013;Schemm et al 2013), and various case studies (e.g., Kuo et al 1991;Stoelinga 1996;Ahmadi-Givi et al 2004), but climatologically the role of moisture for the statistical distribution of cyclone intensities is less clear. Quantifying the effects of LH is difficult because of the relatively small spatial scales of moist processes, which can only be accurately represented with high model resolution (Willison et al 2013).…”

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

“…Two special categories of cyclones with an important contribution from low-level latent heating are type C cyclones (Deveson et al 2002;Plant et al 2003;Ahmadi-Givi et al 2004) and diabatic Rossby waves (DRWs; Parker and Thorpe 1995;Wernli 2011, 2013). Type C cyclones are characterized by an interaction between an upper-level dry and a diabatically generated low-level PV anomaly, while the surface thermal anomaly plays a minor role for development.…”

“…At upper levels diabatic PV destruction leads to low PV values in the WCB outflow. These produce significant negative PV anomalies in the tropopause region, which can interact with the extratropical waveguide and thereby substantially influence the downstream flow (e.g., Wernli 1997;Pomroy and Thorpe 2000;Massacand et al 2001;Ahmadi-Givi et al 2004;Grams et al 2011;Methven 2015).…”

Quantifying the climatological relationship between extratropical cyclone intensity and atmospheric precursors

“…This PV anomaly enhanced the eastward propagation of the surface wave and slowed the propagation of the upper-level wave thus keeping the upper-level PV wave coupled to the low-level surface wave. Diabatically generated midlevel positive PV anomalies (where midlevel is here defined as approximately between 800 hPa and 500 hPa) can also be associated with enhanced upper-level divergence which expands the downstream ridge leading to the formation of a narrow and deep tropopause fold [Posselt and Martin, 2004;Ahmadi-Givi et al, 2006]. Statospheric PV anomalies can then merge with the diabatically produced midlevel PV features to form a vertically aligned tower of PV.…”