Reexamination of the Structure of the Ageostrophic Wind in Baroclinic Waves

Abstract

This paper reexamines the structure of ageostrophic winds in a baroclinic wave presented by Lim, Holton, and Wallace. It is found that the nonzero growth rate is indispensible to the compete explanation of the observed structure of ageostrophic winds. For the unstable mode the isallobaric wind shifts slightly westward by φ = tan?1(σi/σr) from the state of the neutral mode both at the upper and lower level. This makes the convergence and divergence patterns shift eastward at the upper level and westward at the lower level, which comes closer to reality where the vertical motion is nearly upright. The cancellation between the isallobaric wind and the advective part of ageostrophic wind for unstable mode differs from place to place so that zonal asymmetry with respect to the center of each high pressure and low pressure region appears, which results in a dramatic change from the results of the neutral mode. At the upper level the orientation is mainly zonal. But the magnitude of the zonal component of the ageostrophic wind is stronger on the right side both of the ridge and the trough. On the left side of the ridge, the ageostrophic winds that blow up the pressure gradient are weaker than those that blow down the pressure gradient on the right-hand side. The same is true for the trough region. At the lower level, on the other hand, the orientation of the ageostrophic wind is mainly meridional. But there are additional components directed down the pressure gradient nearly everywhere. This results in a slight modification of the lower-level orientation of the ageostrophic wind, in which zonal components enter. The overall feature is that the ageostrophic wind that blows down the pressure gradient overwhelms the counterpart. The implication of this feature is that this is another indication that the mode is indeed baroclinically unstable. The above features are also verified in the numerical model framework.