ON UNDERSTANDING HEIGHT TENDENCY

Top-down height tendency reasoning is explained and examined. This app roach uses the assumption of a stratospheric level of insignificant dy namics (LID)-where height and pressure tendencies are considered negli gible-to simplify the understanding of cyclone-scale hydrostatic heigh t (pressure) tendency in the troposphere. Quasigeostrophic analytic mo del results confirm the existence of such a LID for scales less than a pproximately 5000 km. An examination of a height tendency equation wit h the LID assumption shows that there must be net integrated local war ming (cooling) between the LID and any level below the LID where heigh ts are falling (rising). The local temperature tendency, which from th e thermodynamic equation results from advection, diabatic heating, and the product of vertical motion and static stability, reflects the com bined actions of all thermodynamic and dynamic processes that together promote hydrostatic height change in isobaric coordinates. In particu lar, the important dynamic effects of mass-diverging secondary circula tions are implicitly contained in the local temperature tendency. New observational evidence and analytic model simulations supporting the t op-down approach for understanding height tendency are also provided. The analytic model simulations show that isolated layers of equivalent diabatic heating and temperature advection do not produce equivalent dynamic responses in the vertical-motion field and height tendency fie lds. This result is used to explain observations that temperature adve ctions in the upper troposphere/lower stratosphere are associated with larger lower-tropospheric height tendencies than equivalent temperatu re advections in the lower troposphere.