ertel_potential_vorticity

The Ertel potential vorticity is the scalar product of the atmospheric absolute vorticity vector and the gradient of potential temperature. It is a conserved quantity in the absence of friction and heat sources [AMS Glossary, http://glossary.ametsoc.org/wiki/Ertel_potential_vorticity]. A frequently used simplification of the general Ertel potential vorticity considers the Earth rotation vector to have only a vertical component. Then, only the vertical contribution of the scalar product is calculated. It is strongly recommended that a variable with this standard name should have the attribute units_metadata="temperature: difference", meaning that it refers to temperature differences and implying that the origin of the temperature scale is irrelevant, because it is essential to know whether a temperature is on-scale or a difference in order to convert the units correctly (cf. https://cfconventions.org/cf-conventions/cf-conventions.html#temperature-units). rdf:langString