New Evidence for Improving Omega Estimation by Explicitly Considering Horizontal Divergence

It is well known that the quasi-geostrophic (QG) omega equation with only two contributors respectively associated with vorticity advection (VA) and temperature advection is derived for midlatitude synoptic-scale systems only. Based on reliable reanalysis data, new evidence revealed by cyclonic and anticyclonic cases indicates that forecasters might sometimes experience problems by paying too much attention to the 500-hPa VA when estimating vertical motions not only in subtropical systems but also in systems meeting all the assumptions of the QG omega equation. Our investigations also showed that explicitly considering the vertical profiles of horizontal divergence could allow for better interpretation of vertical motions and weather in these real cases, suggesting that this equation might not be sufficient due to the presence of only two horizontal-divergence-related (HDR) mechanisms and the absence of other HDR mechanisms, e.g., frictional force, mountain barriers, diabatic/adiabatic processes, and acceleration/deceleration of air flows.