Extratropical Lapse Rates in Very Hot Climates

The interplay between convective processes and the stabilizing effects of largescale systems remains debated, especially for warmer climates. We study sets of simulations of past and present climates in which carbon dioxide (CO2) concentrations vary over a wide range: from pre-industrial era levels of 280 ppm to an exceedingly high value of 8960 ppm. This allows us to assess the importance of convective processes relative to middle latitude thermal stratification and changes to the general circulation in progressively warmer climate states.

As a tool to assess the stability of the atmosphere, we calculate a thermodynamic variable called saturation potential vorticity (P*), which has the property of being identically zero wherever lapse rates are neutral with respect to moist convection, and large where lapse rates are stable. Using P* allows us to examine convective motions arising from both gravitational accelerations as well as symmetric instabilities.

We assess the ability of current models (of varying spectral resolutions) in resolving conditions unstable to slantwise convection. We show that the coarse resolution reanalysis data captures instances of observed slantwise convection. Our results show examples of vertically stable lapse rates, while being conditionally unstable along slanted angular momentum surfaces. This suggests the need for model parameterizations of convection to include instabilities arising from symmetric displacements.

Tropical regions are neutral with respect to moist convection while higher latitudes most often have stable lapse rates, especially during the winter months. In the warmer climate simulations, the frequency of convectively neutral air masses increases in both middle latitude and polar regions, especially during the summer months. These simulations also show expansion of the Hadley cell and shifting of middle latitude storm tracks. Using Maximum Potential Intensity (MPI) as a tool to assess the upper bound of hurricane strength, we show sustainment of intense tropical cyclones in regions they cannot in our present climate.