Reexamination of the structure of the ageostrophic wind in baroclinic waves

This paper reexamines the structure of ageostrophic winds in a baroclinic w ave presented by Lim, Holton, and Wallace. It is found that the nonzero gro wth rate is indispensible to the compete explanation of the observed struct ure of ageostrophic winds. For the unstable mode the isallobaric wind shift s slightly westward by phi = tan(-1)(sigma(t)/sigma(r)) from the state of t he neutral mode both at the upper and lower level. This makes the convergen ce and divergence patterns shift eastward at the upper level and westward a t 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 age ostrophic 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 th e right side both of the ridge and the trough. On the left side of the ridg e, the ageostrophic winds that blow up the pressure gradient are weaker tha n those that blow down the pressure gradient on the right-hand side. The sa me is true for the trough region. At the lower level, on the other hand, th e 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 t he ageostrophic wind, in which zonal components enter. The overall feature is that the ageostrophic wind that blows down the pressure gradient overwhe lms the counterpart, The implication of this feature is that this is anothe r indication that the mode is indeed baroclinically unstable. The above fea tures are also verified in the numerical model framework.