Bulk Richardson number

Below is the formula for the BRN. Where g is gravitational acceleration, T_v is absolute virtual temperature, Δθ_v is the virtual potential temperature difference across a layer of thickness Δz (vertical depth), and ΔU and ΔV are the changes in horizontal wind components across that same layer.[1]

${\displaystyle R_{B}={\frac {(g/T_{v})\Delta \theta _{v}\Delta z}{(\Delta U)^{2}+(\Delta V)^{2}}}}$

High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 50 suggest environmental conditions favorable for supercell development.[3]

In the limit of layer thickness becoming small, the Bulk Richardson number approaches the Gradient Richardson number, for which a critical Richardson number is roughly Ri_c= 0.25. Numbers less than this critical value are dynamically unstable and likely to become or remain turbulent.[1]

The critical value of 0.25 applies only for local gradients, not for finite differences across thick layers. The thicker the layer is the more likely we are to average out large gradients that occur within small sub-regions of the layer of interest. This results in uncertainty of our prediction of the occurrence of turbulence, and now one must use an artificially large value of the critical Richardson number to give reasonable results using our smoothed gradients. This means that the thinner the layer, the closer the value to the theory.[2]

• Monin–Obukhov length
• Richardson number
• Atmospheric dynamics
• Atmospheric thermodynamics

• Help - Bulk Richardson Number - NOAA Storm Prediction Center
• Thompson, Richard L.; Edwards, Roger; Hart, John A.; Elmore, Kimberly L.; Markowski, Paul (December 2003). "Close Proximity Soundings within Supercell Environments Obtained from the Rapid Update Cycle" (PDF). Weather and Forecasting. American Meteorological Society. 18 (6): 1243–1261. doi:10.1175/1520-0434(2003)018<1243:CPSWSE>2.0.CO;2.