An easterly tip jet off Cape Farewell, Greenland. II: Simulations and dynamics

Abstract

AbstractAn easterly tip jet that occurred on 21 February 2007 off Cape Farewell, Greenland, is examined. In Part I of this article aircraft observations were described. Now, in Part II, numerical simulations and an analysis of the dynamical forcing mechanisms are presented. The simulations make use of a limited‐area 12 km resolution configuration of the Met Office's Unified Model. Sea‐surface temperatures and sea‐ice concentrations have been replaced using the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) product, addressing a boundary‐layer temperature bias, while roughness lengths over sea ice have been updated, addressing a wind‐speed bias. These modifications ensured a reasonably accurate simulation: generally within 1–2 K and 2–3 m s−1 when compared with dropsonde observations. A momentum‐budget analysis along a curved locus through the core of the jet has been derived. Off southeast Greenland, the easterly tip jet was in cross‐jet geostrophic balance, but was being accelerated downstream by an along‐jet pressure gradient. Over the curved part of the locus, as the jet rounded Cape Farewell, a cross‐jet residual suggests that the jet was unbalanced at the height of the jet core. This residual decreases with height so that an approximate gradient wind balance applies in the upper part of the jet. The anticyclonic curvature, characteristic of easterly tip jets, was caused by a dramatic decrease in the cross‐jet pressure‐gradient force at the end of the barrier, after which the jet aligned with the synoptic‐scale isobars and returned to approximate geostrophic balance. The momentum budget is shown to be robust and applicable to other cases. Copyright © 2009 Royal Meteorological Society