The Effect of Jet-Streak Curvature on Kinematic Fields

Abstract

A simple two-layer primitive equation (PE) model is used to study the effect of curvature on jet-streak kinematics, specifically vertical motion. Three types of vertical motion are studied: kinematic (PE) vertical motion, quasigeostrophic (QG) vertical motion, and vertical motion associated with an unbalanced component of the flow, partially due to inertial-gravity waves (IGW). The latter vertical motion is computed as the difference between the PE vertical motion and ?balanced? vertical motion derived from Krishnamurti's balanced omega equation. In addition, the upper-level ageostrophic flow is discussed as it relates to the patterns of divergence associated with various jet curvatures. The PE model was run out to 12 h to we how straight-line (SL), cyclonic (CY), and anticyclonic (AC) curvature affects jet-streak kinematics. At the initial time, a two-cell pattern of vertical motion was found for the CY and AC jet streaks as opposed to the four-cell pattern associated with the SL jet streak. Also, the vertical-motion centers for the AC and CY jet streaks were aligned more along the jet axis than across it, in contrast with the SL case. Quasigeostrophic vertical motion for the SL and CY jet cases agreed well with the PE vertical motion but were much weaker than the PE vertical motion for the AC case. Results for 12 h into the model run showed that unbalanced or IGW vertical motions were strongest for the CY case where they were equal to about one-half of the PE vertical motions. A comparison of QG vertical motion with balanced vertical motion illustrates the effect of curvature most dramatically. For cyclonic curvature, QG vertical motions are 50% stronger than the balanced vertical motions, while for anticyclonic curvature, they are 50% weaker than the balanced vertical motion. For the SL and AC cases, unbalanced vertical motions were smaller but were still a significant part of the total PE vertical motion. Thus, the greatest mutual adjustment between the mass and momentum fields occurs with cyclonically curved jet streaks. A comparison of vertical motions from runs in which the curvature was varied revealed that the magnitude of the vertical motion is strongest with cyclonic jet streaks, more modest with anticyclonic jet streaks, and weakest with straight-line jet streaks. This is reflected in the maximum Rossby numbers for these cases that were 1.0, 0.40, and 0.12, respectively.