The Contribution of Radiative Cooling to the Formation of Cold-Core Anticyclones

Abstract

Cold-core anticyclones are a dominant feature of the circulation in the high latitudes during the cold half of the year. This paper focuses on how the radiative cooling associated with the formation of continental polar air masses contributes to the anticyclogenesis. The processes occurring in cold-core anticyclones are investigated with a nonlinear axisymmetric numerical model. The model experiments employ three different types of radiative cooling parameterization: 1) externally specified radiative cooling rates; 2) internally determined radiative cooling rates with all condensed water assumed to fall out immediately, not affecting the radiative transfer (after Wexler); and 3) fully interactive calculation, whereby the radiative effects of the condensate that forms in the cooling air are included (Curry). The following physical processes are considered in the fully interactive calculation: 1) condensation of water vapor and freezing of liquid water; 2) radiative transfer from water vapor, CO2, liquid water drops, and ice crystals; 3) gravitational fallout flux of water drops and ice crystals; and 4) surface enthalpy flux, including the heat received at the surface from the underlying snow/ice. The model results show that after 5 days of integration, the central surface pressure increase is 7 mb for Wexler's cooling mechanism, and 10 mb for Curry's cooling mechanism. A positive feedback loop is shown to exist between the formation of condensate in the cooling air and anticyclogenesis; radiative cooling from condensate enhances anticyclogenesis; and the large-scale meridional circulation associated with anticyclone replenishes the moisture in the layer of condensate, thus enhancing the radiative cooling. Results from the experiments employing externally specified radiative cooling rates show that the central surface pressure increase is largest for (i) increased amount of cooling; (ii) increased horizontal extent of the cooling; (iii) decreased vertical extent of the cooling; (iv) decreased friction; and (v) increased latitude.