The role of an upstream cyclone and corresponding synoptic/planetary-scale interactions during the development of a blocking anticyclone

Chih-Hua Tsou, Purdue University

Abstract

Using fields derived from GLA analyses (4$\sp\circ$ lat $\times$ 5$\sp\circ$ lon) of the FGGE SOP-I data set, the general behavior of an Atlantic blocking anticyclone is diagnosed using the extended height tendency equation and several energy quantities. In addition, comparison between quasigeostrophic (QG) and extended height tendencies are made. To further analyze the relative importance of planetary-scale, synoptic-scale, and scale- interaction forcing of this block, height tendencies and energy quantities are also calculated from a scale-partitioned form. The scale partitioning is accomplished using the Barnes objective analysis scheme. Results indicate that vorticity advection was the primary forcing mechanism during the block development. Much of the vorticity advection is attributed to the northward advection of negative relative vorticity east of a jet streak that formed between the cyclone and anticyclone. Further, height rises associated with this advection occurred as the ridge downstream from the exploding cyclone became stationary. In fact, the strong northward advection of relative vorticity is primarily due to the advection of synoptic-scale vorticity by the planetary-scale and synoptic-scale winds. This indicates the importance of the proper phasing of the synoptic-scale vorticity with the winds of both scales. Lack of proper phasing between vorticity and wind fields, weaker winds, and weaker flow across the vorticity contours caused the QG model to seriously underestimate the northward amplification of the ridge as the ridge became stationary. It is possible that the QG model would not have been able to develop a block at all. Thus, ageostrophic vorticity advection was important for the development of the block. Interestingly, despite pronounced northward warm air advection, the direct forcing of the block by thermal advection was relatively small. Rather, the thermal forcing was strongest in the upstream cyclone, which in view of the subsequent role of scale interactions in the block development suggest an indirect role for thermal advection. Energetics results suggest that baroclinic energy conversion by the synoptic-scale waves maintained the eddy kinetic energy of the total wave system. Some of this eddy kinetic energy was converted to zonal flow by barotropic energy conversion (CK).

Degree

Ph.D.

Advisors

Smith, Purdue University.

Subject Area

Atmosphere

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