Symbol for background shear
 

Can someone please help me with the symbol for background shear? To give you a bit of context, I am referring to background shear as in the following sentence taken from Hussian et al.:

Vorticity magnitude has been widely used to educe CS and represent vortex cores. However, this approach, though fairly successful in the free shear flows investigated so far, may not always be satisfactory since vorticity magnitude does not identify vortex cores in a shear flow, especially if the background shear is comparable to the vorticity magnitude within the vortex