Talk Abstract: TBA
The weak temperature gradient (WTG) approximation is extended to the basic equations on a rotating plane. The circulation is decomposed into a diabatic component that satisfies WTG balance exactly and a deviation from this balance. Scale analysis of the decomposed basic equations reveals a spectrum of motions, including unbalanced inertio-gravity waves and several systems that are in approximate WTG balance. The balanced systems include equatorial moisture modes with features reminiscent of the MJO, off-equatorial moisture modes that resemble tropical depression disturbances, "mixed systems" in which temperature and moisture play comparable roles in their thermodynamics, and moist quasi-geostrophic motions. In the balanced systems the deviation from WTG balance is quasi nondivergent, in nonlinear balance, and evolves in accordance with the vorticity equation. The evolution of the strictly-balanced WTG circulation is in turn described by the divergence equation. Together, the vorticity and divergence evolve as a result of horizontal forcing by the strict WTG flow and its deviation, implying that motion under the WTG approximation is restricted to evolve at a given pressure level even when diabatically-driven vertical motions are present. The vorticity and divergence equations form a closed system of equations when the irrotational circulation is in WTG balance and the nondivergent circulation is in nonlinear balance. The resulting “WTG equations” may elucidate how interactions between diabatic processes and the horizontal circulation shape slowly-evolving tropical motions
All seminars this quarter will be in a hybrid format. Recording available upon request.