TRAJECTORIES

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What are the differences between isobaric, isentropic, and kinematic trajectories?

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Most calculation methods use observed or model analyzed winds to compute the horizontal advection component and usually one of three assumptions to compute the vertical component of the trajectory. These three assumptions are:

• the trajectory remains on a surface of constant pressure (isobaric),
• the trajectory follows a surface of constant potential temperature (isentropic),
• or the trajectory moves with the vertical velocity wind fields (kinematic) generated by a diagnostic or prognostic meteorological model.

Transport is more difficult to model in the boundary layer (BL) than in the upper troposphere or stratosphere due to significant amounts of moisture and the diabatic factors driving the growth and dissipation of the BL itself. One assumption of the kinematic approach is that the three-dimensional velocity fields generated by the meteorological model contain contributions from all the adiabatic and diabatic components. Many of the citations using dry isentropic approaches frequently are preceded by qualifying statements regarding the exclusion of situations that have large diabatic components: convective boundary layers, cloud coverage, and areas of precipitation. One may want to use the isobaric assumption when comparing wind fields, or an isentropic assumption to resolve vertical motions that may not be evident when using only coarse spatial and temporal resolution meteorological fields. As can be seen in the examples below for the same case as shown previously, unless conditions are very non-adiabatic, most approaches will yield similar results.

Isobaric trajectory	Isentropic trajectory