Nonhydrostatic Simulation of Frontogenesis in a Moist Atmosphere. Part III: Thermal Wind Imbalance and Rainbands

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  • 1 Centre National de Recherches Météorologiques (Météo-France and CNRS), Toulouse, France
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Abstract

The dynamical mechanisms contributing to the cross-front ageostrophic circulation are identified in high-resolution (40 to 5 km) nonhydrostatic simulations of moist frontogenesis.

In a first step, the importance of the alongfront ageostrophic circulation is assessed. The structure of the intense thermal wind imbalance (TWI) occurring in the vicinity of the surface cold front is diagnosed and explained using a budget of the alongfront vorticity η. It allows one to propose a new balance in terms of the steadiness of the η field in the system moving framework. The TWI is thus found nearly equal to the total cross-front η transport by resolved and subgrid scales. It is shown that, first, the deviation of the prefrontal air toward the front, enhanced by the surface friction and cloud diabatic processes, allows generation of a layer of positive η near at the top of the PBL. Second, the frontal lifting of this η structure is responsible for the basic structure of the TWI.

In a second step, a general form of the Sawyer–Eliassen (SE) diagnostic equation is used, including diabatic effects as well as effects of thermal wind imbalances (or “ageostrophic residue”). This latter effect is evaluated using the steadiness balance, which is confirmed by a budget diagnosis. The solution of this SE equation provides an accurate diagnostic of the causes of the secondary circulation, both qualitatively and quantitatively, down to small scales.

Finally, the SE equation is used to explain the formation and localization of rainbands in regions of effective symmetric stability. In particular, it is shown that the “ageostrophic residue” plays a crucial role on the behavior of the bands. It explains about 25% and 60% of the intensity of the warm sector-wide rainband and of the narrow cold-frontal rainband, respectively, for a case with intense surface friction.

Abstract

The dynamical mechanisms contributing to the cross-front ageostrophic circulation are identified in high-resolution (40 to 5 km) nonhydrostatic simulations of moist frontogenesis.

In a first step, the importance of the alongfront ageostrophic circulation is assessed. The structure of the intense thermal wind imbalance (TWI) occurring in the vicinity of the surface cold front is diagnosed and explained using a budget of the alongfront vorticity η. It allows one to propose a new balance in terms of the steadiness of the η field in the system moving framework. The TWI is thus found nearly equal to the total cross-front η transport by resolved and subgrid scales. It is shown that, first, the deviation of the prefrontal air toward the front, enhanced by the surface friction and cloud diabatic processes, allows generation of a layer of positive η near at the top of the PBL. Second, the frontal lifting of this η structure is responsible for the basic structure of the TWI.

In a second step, a general form of the Sawyer–Eliassen (SE) diagnostic equation is used, including diabatic effects as well as effects of thermal wind imbalances (or “ageostrophic residue”). This latter effect is evaluated using the steadiness balance, which is confirmed by a budget diagnosis. The solution of this SE equation provides an accurate diagnostic of the causes of the secondary circulation, both qualitatively and quantitatively, down to small scales.

Finally, the SE equation is used to explain the formation and localization of rainbands in regions of effective symmetric stability. In particular, it is shown that the “ageostrophic residue” plays a crucial role on the behavior of the bands. It explains about 25% and 60% of the intensity of the warm sector-wide rainband and of the narrow cold-frontal rainband, respectively, for a case with intense surface friction.

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