Precipitation Banding in Idealized Baroclinic Waves

Abstract

Moist idealized baroclinic-wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet. For a frictionless lower boundary, and with no latent-heat release or surface heat and moisture fluxes, warm advection is strong and a bent-back warm front forms. Although a narrow vertical-velocity maximum forms within the area of synoptic-scale ascent near the triple point, only a wide warm-frontal band forms. As surface roughness length increases between simulations to that of an ocean then a land surface, warm advection is reduced and the cold front becomes stronger relative to the warm front. A separate narrow rainband forms along the cold front, which is more intense and farther removed from the wide warm-frontal band when roughness length is greater. In the simulation with roughness length appropriate to the ocean, after the narrow band decays, the precipitation becomes oriented along the warm conveyor belt in the warm sector. When latent-heat release is included, this warm-sector precipitation evolves into multiple bands, which eventually weaken with the cyclone. When surface heat and moisture fluxes are included, the ascent at the surface cold front stays strong and a well-defined cold front of the anafront variety persists through this mature stage. The surface precipitation remains in a single intense band along and ahead of the cold front. Therefore, strong surface heat and moisture fluxes inhibit multiple bands, but a simulation with lower sea surface temperature (SST) more closely resembles the simulation without surface heat and moisture fluxes, demonstrating that the detailed structure and evolution of precipitation banding is sensitive to SST.