Diagnostic Study of Warm-Core Anticyclones Associated with a Blocking Circulation Pattern.
The first integrated treatment of the mass, absolute angular momentum, and kinetic energy budgets of the central portion of a blocking anticyclone is presented in this thesis. Budgets are evaluated for the period from 12 GMT 17 April 1971 through 12 GMT 1 May 1971 during which a blocking pattern of circulation persisted over Canada. Quasi -Lagrangian diagnostics using isentropic coordinates are evaluated for a cylindrical budget volume that is centered over the surface position of the anticyclone. The "classical" picture of inward lateral mass transport in the upper troposphere, sinking motion, and outward lateral mass transport in the lower troposphere is confirmed. The mass-weighted and area-averaged heating rate, calculated as a residual in the mass budget, indicates a cooling rate of up to 1-2 K day('-1) throughout the troposphere. An independent direct evaluation of the heating rate yields a similar value. In the upper troposphere, the eddy mode of lateral absolute angular momentum transport advects anticyclonic relative angular momentum into the budget volume. This mode contributes to maintaining the anticyclonic relative angular momentum while the friction torque in the lower troposphere removes anticyclonic relative angular momentum. The eddy mode of transport is associated with an "inverted S-shaped" flow pattern in the upper troposphere, i.e., anticyclonic curvature in the northwestern part of the volume and cyclonic curvature in the southeastern part. Considering the thermal wind relation, this flow pattern is associated with the vertical tilt and baroclinic structure of the anticyclone. The demonstrated importance of asymmetric features of the anticyclonic circulation represents a departure from the oversimplified "classical" view of the symmetric, warmcore anticyclone. The time-averaged kinetic energy budget for the upper troposphere reveals an inward lateral transport of kinetic energy and a destruction of kinetic energy due to cross-isobar flow. This combination of advection and destruction of kinetic energy is expected in the exit region of a jet stream, and the time-averaged flow pattern indicates that the right-front quadrant of a jet core is situated within the budget volume. A small positive generation of available potential energy is associated with the net cooling in the budget volume. This implies that there is more cooling at relatively low pressure and less cooling at relatively high pressure on isentropic surfaces. This generation of available, potential energy is of comparable magnitude or smaller than the frictional dissipation of kinetic energy. Additional studies are needed to confirm the importance of the role of the asymmetric structure of the warm-core anticyclone as well as the importance of the role of jet streams on the edges of the anticyclones. The role of the asymmetric structure and of the jet streams could have important implications for assessing failures of prediction models to successfully simulate growth, stability, and dissipation of blocks.
- Pub Date:
- Physics: Atmospheric Science