Abstract
AbstractMoist 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.
Highlights
Heavy precipitation within extratropical cyclones is often organized into mesoscale bands (e.g., Houze et al 1976; Hobbs 1978; Houze and Hobbs 1982; Browning 1986, 1990, 2005; Browning et al 1997; Browning and Roberts 1999). Houze and Hobbs (1982) classified precipitation bands by where they occur in extratropical cyclones, producing a conceptual model featuring six different types of precipitation bands: warm frontal, warm sector, wide cold frontal, narrow cold frontal, prefrontal cold surge, and postfrontal
Moist idealized baroclinic waves were simulated at 20-km grid spacing and the evolution of the associated precipitation bands documented
The new elements are the focus on the formation and structure of precipitation bands in a three-dimensional model, and the investigation of sensitivity to a greater variety of physical processes
Summary
Heavy precipitation within extratropical cyclones is often organized into mesoscale bands (e.g., Houze et al 1976; Hobbs 1978; Houze and Hobbs 1982; Browning 1986, 1990, 2005; Browning et al 1997; Browning and Roberts 1999). Houze and Hobbs (1982) classified precipitation bands by where they occur in extratropical cyclones, producing a conceptual model featuring six different types of precipitation bands: warm frontal, warm sector, wide cold frontal, narrow cold frontal, prefrontal cold surge, and postfrontal. Even in two-dimensional (e.g., Knight and Hobbs 1988; Bénard et al 1992a; Xu 1992; Pizzamei et al 2005) and three-dimensional (e.g., Zhang and Cho 1995; Gray and Dacre 2008) idealized model simulations, fronts with an initial single maximum of vertical velocity may develop multiple maxima of vertical velocity over time These observations and modeling results motivate the questions of what causes precipitation bands in cyclones and what determines whether they are singly or multiply banded. The purpose of this paper is to examine the formation and evolution of banded precipitation within idealized baroclinic waves to better understand their sensitivity to diabatic processes, surface friction, the. Release of latent heat, and surface heat and moisture fluxes To address these issues, a three-dimensional primitive equation model is used to simulate precipitation bands within an idealized baroclinic wave.
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