Aerial Observations of the Yugoslavian Bora

Ronald B. Smith Yale University, New Haven, CT 06511

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Abstract

The first aircraft observations of the bora in Yugoslavia were accomplished during the ALPEX project in 1982. Data from all five ALPEX bora flights have been analyzed in a comparative study of bora structure. Although the bora varies considerably in depth, in the strength of the incoming low level flow, and in the direction of the winds aloft, several common features are evident. These include: upstream descent and acceleration beginning where the mountains rise; an approximate coincidence between the depth of the uppermost descending streamline and the wind reversal level upstream (when a reversal exists); a decoupling of the flow aloft associated with a splitting of the inversion and the formation of a thick mixed layer downstream; a narrow region of intense turbulence and an ascending jet just downstream of the plunging bora. The bora structure is similar in many respects to the Boulder windstorm. Internal hydraulic theory, taking into account the decoupling effect of the intermediate layer, appears to describe both phenomena.

Abstract

The first aircraft observations of the bora in Yugoslavia were accomplished during the ALPEX project in 1982. Data from all five ALPEX bora flights have been analyzed in a comparative study of bora structure. Although the bora varies considerably in depth, in the strength of the incoming low level flow, and in the direction of the winds aloft, several common features are evident. These include: upstream descent and acceleration beginning where the mountains rise; an approximate coincidence between the depth of the uppermost descending streamline and the wind reversal level upstream (when a reversal exists); a decoupling of the flow aloft associated with a splitting of the inversion and the formation of a thick mixed layer downstream; a narrow region of intense turbulence and an ascending jet just downstream of the plunging bora. The bora structure is similar in many respects to the Boulder windstorm. Internal hydraulic theory, taking into account the decoupling effect of the intermediate layer, appears to describe both phenomena.

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