The stability of the midlatitudes is higher than neutral, and the obvious question is why? Early hypothesis primarily focused on dynamical heating and other dry processes to try to simultaneously explain both the height of the tropopause and the atmospheric lapse rate. With the aid of high-quality reanalysis data, recent studies have largely shown that moist convection plays a significant role in the latter.

[The seasonal cycle of moist bulk stability between the surface and tropopause. Values are the differences between the monthly-mean maximum and minimum. There is a large zonal variation in the northern hemisphere, owing to exceptionally high stabilities over land, especially the eastern halves of the continents, in winter.]

The central argument is that moist processes coupled with baroclinic disturbances increases midlatitude stability above moist-neutral. Studies have argued that this is manifest as a relationship between meridional temperature gradients and the change in equivalent potential temperature between the surface and the tropopause, where the difference between 20 and 70 degrees at 500mb is taken to be proportional to this change in potential temperature at 50 degrees (Juckes 2000).

Instead, in my studies, the gradient of equivalent potential temperature is taken at 10m and related to the change in equivalent potential temperature between 10m and the tropopause at that grid location. Near-surface gradients are more representative of convection, and convective processes tend to have their effects localized within the dimensions of reanalysis model data.

I based my calculations of tropopause pressure on the standard WMO lapse rate-based definition, using a quadratic interpolation scheme to estimate the pressure between model levels.

Calculations of the average potential temperature lapse rate are a weighted average of lapse rates on model levels, the latter obtained using finite difference methods. To estimate the geopotential height of the tropopause, I used an interpolation scheme based on the hypsometric formula.

Contrary to previous studies, it is apparant that the Southern Hemisphere seasonal cycle is better captured by theory than the Northern Hemisphere. This is due primarily to the large fraction of land in the northern hemisphere midlatitudes, which has a different and higher amplitude seasonal cycle of stability than the atmosphere over the ocean. Further, there are major zonal variations in stability in the northern hemisphere, whereas the southern hemisphere is characterized by approximate zonal symmetry.

Above and to the right is the lapse rate (per kilometer) of equivalent potential temperature from the surface to tropopause. Of particular interest is, again, the land/ocean contrast in the northern hemisphere, but also the effects of the Hadley circulation, manifest as a zonal band of high stability in the winter hemisphere.

The graphs that follow diplay theoretical versus observed values with a 1:1 line for reference. Each data point represents a monthly mean for all midlatitude (30-60 degrees) grid points in the respective hemisphere over the 1979-2009 period. All data used is from MERRA, courtesy of the Global Modeling and Assimilation Office (GMAO) and the GES DISC. Colors are (blue) DJF, (green) MAM, (red) JJA, (black) SON, with the order of markers being circle, x, cross.

[Theoretical vs. observed values for 30-60S (left) and 30-60N (right). The top and bottom are the 400hPa to 10m and tropopause to 10m bulk moist stabilities. Note that there are severe departures from the 1:1 line in the N.H. for stability up to the tropopause, likely owing to the very low stability over land in the summer. In general, the seasonal cycle in the N.H. is not well-classified by either theory.]

Below is a plot of Northern Hemisphere theory analysis. On the left is the moist (equivalent) bulk stability, on the right the dry bulk stability. Again, the upper plots are stability up to 400hPa and the lower up to the tropopause. While neither theory can properly capture the seasonal cycle, it is clear that moisture absolutely must be taken into account, as the dry theory grossly overestimates winter stability and underestimates summer stability.

Below is the same plot for the Southern Hemisphere. Dry theory performs significantly better here, though the moist theory for stability up to the tropopause remains the most accurate. The dry theory up to 400hPa overestimates summer and underestimates winter stability, whereas the moist theory up to the tropopause has significantly less seasonal bias.

Full citation:

Frierson, D. M. W., and N. A. Davis. The seasonal cycle of midlatitude static stability over land and ocean in global reanalyses. Geophys. Res. Lett., 38, L13803, doi:10.1029/2011GL047747, 2011.

The official journal link can be found here.

A PDF download of the full paper can be found here.

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