Phase‐locking of a rapidly developing extratropical cyclone by Greenland's orography

Abstract

AbstractWe present HIRLAM simulations of a deep extratropical cyclone that developed off the southeast coast of Greenland on 2–3 March 2007. The purpose of the simulations is to understand the role of orographic forcing for the cyclone evolution, relating the results to previous model studies. The cyclone evolution was preceded by a powerful cold air outbreak over Greenland, starting on 27 February, manifested by a southward movement of an upper‐level potential vorticity (PV) anomaly from 80°N to 60°N. In addition to a CONTROL run, starting at 0000 UTC 2 March, which captures the main features of the cyclone evolution quite well, we have carried out simulations in which Greenland's orography was removed (NOGREEN), as well as simulations with different starting times. In the NOGREEN simulation starting at 0000 UTC 2 March, the cyclone deepens more rapidly than in CONTROL, due to a stronger cold advection on the rear side, leading to a more rapid baroclinic energy conversion. Furthermore, the cyclone position is shifted northward by 500 km, compared to the CONTROL run. A very different result is found in the NOGREEN simulations that were started 24–36 hours earlier, as the cyclone off Greenland's southeast coast is now displaced eastwards by hundreds of km, and more so as the run starts earlier. The results indicate a phase‐locking by Greenland of a transient PV anomaly, indicating a mechanism for understanding cyclogenesis in this area. Without Greenland's orography, the PV anomaly is unconstrained, and the curvature of its southward trajectory is larger. Copyright © 2009 Royal Meteorological Society