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Formation and Evolution of Frontal Rainbands and Geostrophic Potential Vorticity Anomalies

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  • 1 NOAA/ERL, National Severe Storms Laboratory, Norman, Oklahoma and CIMMS, University of Oklahoma/NOAA, Norman, Oklahoma
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Abstract

A viscous semigeostrophic model is developed and used to study the formation and evolution of frontal rainbands in association with the dry and moist geostrophic potential vorticity (GPV) anomalies. The numerical results show that when moist GPV (MGPV) becomes negative in the saturated region (but the flow is still stable to viscous symmetric perturbations), banded substructures can be generated internally by a positive feedback between the moist circulation bands and geostrophic forcing anomalies in association with the generation of mesoscale GPV anomalies. In addition to the previous diagnostic results for idealized forcings, the new aspect here is that the large-scale moist ascent evolves into much finer multiple moist bands as soon as the positive feedback begins to generate banded substructures in the forcing and GPV fields. When MGPV is positive, multiple rainbands can only be generated externally by preexisting GPV or MGPV anomalies. These rainbands can be self-maintained by a weak feedback between the vertical motion and warming anomalies that operates in a partially saturated layer between the maximum and minimum levels of the undulated cloud-base boundary in association with the preexisting GPV or MGPV anomalies. The bands are seen as weak cores of upward motion surrounded by the large-scale moist ascent, rather than separated by mesoscale dry subsidences as in the case of negative MGPV.

As the negative MGPV area diminishes (mainly due to the boundary MGPV flux) and the GPV anomalies are lifted into the saturated region, the later evolution of the bands is largely controlled by the Lagrangian advection and eddy dissipation. As the geostrophic confluence flow squeezes (stretches) the bands toward (along) the front, the fine structures of GPV anomalies are smoothed by eddy viscosity, and multibands gradually “merge” into a larger single band of moist ascent. Rainbands produce not only horizontal-mean positive (negative) GPV anomalies in the lower (upper) levels but also significant mesoscale GPV anomalies in the horizontal. Boundary-layer processes can produce either positive or negative GPV flux, depending on the boundary conditions. In general, positive (negative) GPV flux is produced when warm (cold) air moves over a cold (warm) surface. The complex and yet somewhat subtle feature of GPV flux near the surface front is discussed in detail.

Abstract

A viscous semigeostrophic model is developed and used to study the formation and evolution of frontal rainbands in association with the dry and moist geostrophic potential vorticity (GPV) anomalies. The numerical results show that when moist GPV (MGPV) becomes negative in the saturated region (but the flow is still stable to viscous symmetric perturbations), banded substructures can be generated internally by a positive feedback between the moist circulation bands and geostrophic forcing anomalies in association with the generation of mesoscale GPV anomalies. In addition to the previous diagnostic results for idealized forcings, the new aspect here is that the large-scale moist ascent evolves into much finer multiple moist bands as soon as the positive feedback begins to generate banded substructures in the forcing and GPV fields. When MGPV is positive, multiple rainbands can only be generated externally by preexisting GPV or MGPV anomalies. These rainbands can be self-maintained by a weak feedback between the vertical motion and warming anomalies that operates in a partially saturated layer between the maximum and minimum levels of the undulated cloud-base boundary in association with the preexisting GPV or MGPV anomalies. The bands are seen as weak cores of upward motion surrounded by the large-scale moist ascent, rather than separated by mesoscale dry subsidences as in the case of negative MGPV.

As the negative MGPV area diminishes (mainly due to the boundary MGPV flux) and the GPV anomalies are lifted into the saturated region, the later evolution of the bands is largely controlled by the Lagrangian advection and eddy dissipation. As the geostrophic confluence flow squeezes (stretches) the bands toward (along) the front, the fine structures of GPV anomalies are smoothed by eddy viscosity, and multibands gradually “merge” into a larger single band of moist ascent. Rainbands produce not only horizontal-mean positive (negative) GPV anomalies in the lower (upper) levels but also significant mesoscale GPV anomalies in the horizontal. Boundary-layer processes can produce either positive or negative GPV flux, depending on the boundary conditions. In general, positive (negative) GPV flux is produced when warm (cold) air moves over a cold (warm) surface. The complex and yet somewhat subtle feature of GPV flux near the surface front is discussed in detail.

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