EFFECTS OF SURFACE DRAG ON FRONTS WITHIN NUMERICALLY SIMULATED BAROCLINIC WAVES

A comparative analysis of simulations of baroclinic waves with and wit hout surface drag is presented, with particular reference to surface f eatures. As in recent studies, the present simulations show that, comp ared to simulations with no drag, those with surface drag are less inc lined to develop a secluded warm sector, and that drag weakens the war m front while the cold front remains strong. The authors demonstrate t hat analogous effects occur when Ekman pumping is used in nonlinear qu asigeostrophic numerical simulations of unstable baroclinic waves in a channel. However, since the quasigeostrophic model produces symmetric highs and lows in the unstable baroclinic wave, the cold and warm fro nts are therefore also symmetric and hence equally affected by the Ekm an pumping. The different effect that friction has on the warm front w ith respect to the cold front in the primitive-equation simulations is fundamentally related to the tendency for the lows to be strong and n arrow and the highs weak and broad, and for the warm front to form jus t north of, and extend eastward from, the low, while the cold front ex tends between the high and the low. The authors' thesis is that the Ek man pumping associated with the low, at the location where the warm fr ont would form in the absence of surface friction, acts to resist the formation of the warm front, while the cold front, positioned between the high and the low where Ekman pumping associated with the baroclini c wave is weak, is :therefore relatively unaffected. Given the weaknes s of Ekman pumping associated with the baroclinic wave in the vicinity of the incipient cold front, the present simulations indicate that co ld frontogenesis occurs in the drag case in much the same way as in th e no-drag case. Present analysis shows that the horizontal advection c reating the cold front is a combination of geostrophic and ageostrophi c effects. A portion of the ageostrophic frontogenesis is a response t o geostrophic frontogenesis, as in the case without surface drag; howe ver with surface rag: a significant portion of the cross-front ageostr ophic flow is due to the Ekman layer associated with the front itself.