Three-Dimensional Computations of Equivalent Potential Vorticity

Abstract

Moore and Lambert showed how a quantity called equivalent potential vorticity (EPV) can provide quantitative values to assess conditional symmetric instability (CSI), also known as slantwise instability. Expanding the EPV equation into three dimensions, the equation becomes a function of the geostrophic wind shell, the horizontal equivalent potential temperature gradient, the absolute geostrophic vorticity, and the vertical equivalent potential temperature gradient, all of which are easily computed from gridded data. The equation reduces further by recognizing that the geographic wind shear is a function of the horizontal equivalent potential temperature gradient in a saturated environment. This reduced equation is difficult to evaluate quantitatively because of its dependence on the local value of the moist adiabatic lapse rate. Nevertheless, it shows that horizontal temperature gradients will always act to promote slantwise convection. EPV is a mixture of upright potential instability and slantwise instability. While this is obviously a drawback if one were attempting to assess CSI separately from VPI by combining both instabilities, EPV becomes an all-purpose convection diagnostic tool. EPV is computed from numerical model grids at the National Severe Storms Forecast Center.