Expected Impact of Global Warming on Shear Patterns Favorable for Supercells and Supercell Thunderstorms




   We know that the propensity for thunderstorms to rotate is related to the vertical shear in the environment within which such thunderstorms develop.  The vertical shear is related to how strong the jet stream, say at 500 mb, is relative to the surface winds.


Meteorologists and climate scientists studying the impact of global warming predict that most of the warming will take place at higher latitudes while the temperatures in the equatorial regions would change the least.  This will result in a weaker equator-to-pole temperature gradient than exists today.


The strength of the polar jet stream is related to the height gradient found in middle latitudes, with higher 500 mb heights to the south and lower middle latitude heights to the north, and the slope of the isobaric surfaces in the middle latitudes.  If the temperatures in the equatorial regions stay the same, while temperatures in the higher latitudes increase, this height gradient and isobaric slopes will get weaker, and so will the average strength of the polar jet stream.


Vertical wind shear in the middle latitudes is related to the (generally) increase in wind speeds with height found in that region.   Since the winds at the ground are always weak, a decrease in the 500 mb wind speed (weaker jet stream) will result in a decrease in vertical wind shear. However, though this may eliminate some of the high deep layer shear supercell events we currently experience, the background shear is expected to remain favorable for organized severe storms, including multicells and supercells. With the expected added effect of increased buoyancy, severe storms can then be expected to increase.

Fig. 1:  Schematic cross-section of the Troposphere at the time of the winter solstice in the Northern Hemisphere.  This is taken from Lab 10.


There is an interesting twist, pun intended, to all of this. In a pattern of lower zonal wind speeds, the meridionality (the tendency for larger wavelength, larger amplitude disturbances in the middle and upper troposphere) is expected to increase. Such waves are slower moving, but such waves also have an interesting pattern in the height contour field.



Fig. 2: A typical 500 mb map during a so-called "Low Index" (low zonal index).

Fig. 2 shows a forecast 500 mb height pattern typical of a highly meridional flow. Such patterns can occur during the core of the winter due to the evolution of temperature advection patterns, and are termed "Low Zonal Index" patterns. But they become more typical during times of relaxed pole-equator temperature differences. This type of pattern is thus expected to become more frequent during the 21st century.


Notice the generally low wind speeds over most of the map. But notice also the higher wind speeds, generally >45 knots around the bases of the troughs into their eastern portions. Under the eastern portion of such troughs, veritcal wind shear will be in favorable ranges for not only supercells, if thunderstorms develop in that environment, but tornadic supercells (please remember that the key in understanding tornadic supercells also includes low level wind shear, which is impossible to anticipate in the highly idealized foreshadowings made for the 21st century).