A Diagnosis of Unbalanced Flow in Upper Levels during the AYE-SESAME I Period

Abstract

Terms of the balance equation were calculated at 300 mb to diagnose unbalanced flow in the upper troposphere both prior to and during a period of strong convection which took place during the AVE-SESAME I period. The sum of the balance equation terms displayed large imbalances(>50?10?10s?2)over the Red River Valley as early as 2100 UTC. This region grew in magnitude, expanding over Oklahoma during the next 3?6 hours. The vorticity and Laplacian terms in the balance equation dominated this imbalance. An examination of ageostrophic, geostrophic and actual 300 mb winds at 2100 UTC revealed that the ageostrophic winds over Oklahoma were directed towards lower geopotential heights, indicating that the flow was neither in geostrophic balance nor merely responding to curved flow as described by the gradient wind equation. Such imbalance led to substantial increases in divergence and midlevel upward vertical motion. The remaining terms of the divergence equation were computed and summed. These terms partially compensated for the strong divergence tendencies created from the balance equation. In this way, the divergence and vertical motion terms of the divergence equation checked the growth of divergence in upper levels. Finally, an error assessment was conducted on the terms of the divergence equation. Although balance equation terms are susceptible to substantial error due to random errors in wind and height data, the patterns of these terms are more reliable, thereby permitting the conclusions of this case study.