On the Structure of an Intense Upper Cyclone (Part I)

Abstract

The present paper describes a part of the results of analysis of an intense upper cyclone, which developed over the North America. The thermal structure and the distribution of kinematical properties of the cyclone at its mature stage of development are discussed. The results are compared with analyses of upper cyclones by other investigators.
Similar to many other cases, there was a funnel-like boundary separating upper stable and lower relatively unstable air. However, in this case, only a portion of this boundary may be regarded as tropopause. It is pointed out that the thermal structure, in the upper troposphere, of such a vortex is considerably influenced by its location with respect to the subtropical jetstream. We may consider that a complete funnel-like tropopause will be found in a W-E cross-section of an upper cyclone situated far north of the subtropical jetstream, whereas such a tropopause will not be found within an upper cyclone south of the subtropical jetstream. In the present case, the cold-dome boundary was not well defined near the vortex center and the north sector, over these regions there was no significant stable layer below tropopause. On the contrary, there were a number of inversions and stable layers to the south through the west of the center. Most of these are considered to be the subsidence inversions.
Computations revealed a possibility of the reversal of vertical motion, due to very strong divergence in this region, within a thin layer near the funnel-like boundary. It is considered that this high-level strong divergence (or convergence) is related to formation (dissipation) and propagation of the tropopause. It is pointed out that the vertical motion field associated with the upper cyclone may not be described by simple models such as indirect circulation or direct circulation. Instead, actual vertical motion associated with a mature upper cyclone will be consistent with a fact that the system moves changing slightly its characteristic thermal structure, viz., a dome of cold air in the lower troposphere and a funnel-like warm air in the lower stratosphere. A simplified vertical circulation model for an eastward moving upper cyclone is given in Fig. 19. General appearance of vertical circulation in the W-E cross-section is similar to that obtained by Newton and Palmén (1963) for a very deep trough. Unlike to the earlier outoff cyclone models, a band of ascending motion was evident to the north of the center of the upper cyclone of the present case. It is considered that the difference occurs because the earlier models refer to the formation stage of a cutoff low, whereas the present paper deals with the upper cyclone nearly at the steady state.