Beyond the Rigid Lid: Baroclinic Modes in a Structured Atmosphere

Romps, D., Lawrence Berkeley National Laboratory

General Circulation and Single Column Models/Parameterizations

Convective Processes

Edman J and D Romps. 2017. "Beyond the rigid lid: Baroclinic modes in a structured atmosphere." Journal of the Atmospheric Sciences, 74(11), 10.1175/JAS-D-17-0140.1.


The buoyancy one hour after a first-baroclinic heating is applied to the troposphere at x=0 in a 2D Boussinesq fluid with (left) no change in stratification at 15 km and (right) a 2.5-fold increase in stratification at 15 km.


The buoyancy one hour after a first-baroclinic heating is applied to the troposphere at x=0 in a 2D Boussinesq fluid with (left) no change in stratification at 15 km and (right) a 2.5-fold increase in stratification at 15 km.

Science

In the rigid-lid approximation to the troposphere’s upper boundary, heating generated by a storm leads to pulses of buoyancy that whip around the troposphere forever in the absence of any dissipative processes. In reality, these buoyancy pulses melt away by the propagation of gravity waves up through the troposphere’s upper boundary. This study calculates the analytical solution for this process in the case of a 2D atmosphere.

Impact

This study derives the Green's function for buoyancy in an atmosphere where the tropopause is modeled as a transition to a higher, but still finite, stratification. The resulting solutions show how the pulses from tropospheric heating by a storm melt away in time.

Summary

What happens when a storm heats the troposphere in a 2D atmosphere? A simple model of the resulting buoyancy can be found by assuming that the tropopause is a rigid lid. In that model, the buoyancy splits into two pulses that travel forever outward from the location of the heating. What is unrealistic about this picture is that those pulses never dissipate: they travel unaltered forever. This artificial behavior is a consequence of assuming a rigid lid at the tropopause.

In a more realistic atmosphere, the tropopause is not a rigid lid, but instead marks a transition between the stratification of the troposphere and the higher stratification of the stratosphere. In this paper, we derive the first Green's function for buoyancy in such an atmosphere. The resulting solutions show how the pulses melt away in time.