Bulk Richardson Number (BRN): proportional to CAPE and inversely proportional to shear

 

Very high BRN values: the shear is too weak to stop the outflow pool of cold (downdraught) air moving quickly away from the parent updraught: new cells may form as the gravity current propagates downstream, but usually well away from the parent - and distinct from it. Also, as the updraught is quasi-zero sheared vertically, the rain shaft falls into a saturated environment (little or no evaporation possible), and there is no potential for a substantial downdraught.

 

Mderate BRN values: the shear element (especially speed shear) now plays a crucial part in skewing the updraught, tilting the growing cloudy environment, allowing some or all of the precipitation shaft to fall into unsaturated air - evaporative cooling (plus precipitation drag) will generate a cold downdraught - the drier the air, the greater the potential for accelerating cold/gravity current flow. As the downdraught hits the surface, it splays out, meeting environmental inflow and generating new cells away from (but close by) the parent cell - this is the basis of the multi-cell thunderstorm.

 

Low BRN values (but NOT quasi-zero numbers .. see below): Supercell storms may occur (other factors being right). The shear is now strong (and composed of both speed and significant directional components - this latter is most important), and the new/growing cell effectively forms alongside or even within the environment of the 'parent', the whole forming a steady-state system. The situation is complex though and rotation of the developing storm must be present - this is thought to be due to advection (and significant distortion / stretching) of low-level horizontal vorticity into the updraught of the storm environment, but knowledge here is still incomplete, though growing. (It is here that Helicity comes into play .... see elsewhere).

 

Very low BRN values: the shear is too strong against very weak CAPE: the developing convective (cloudy) towers are ripped apart and are generally too ill-organised for persistent self-sustaining Cb development. However, note that within the environment of a tropical storm, a low BRN may be associated with organised convective 'streets'.