Reply to “Comments on Dry-Season Precipitation in Tropical West Africa and its Relation to Forcing from the Extratropics” by T. Spengler and J. Egger

Abstract

Hydrostatic tendency equations of pressure and geopotential have been used in various forms since the beginning of the twentieth century. In contrast to classical mass flux divergence formulations, all forms that involve vertical integrals of temperature tendencies contain an additional term related to geopotential tendency at the upper limit of the integral. In many previous studies, including a recent work by two of the authors on a case of unusual wintertime precipitation in tropical West Africa, it has been assumed that there exists a pressure level in the stratosphere (usually 100 or 50 hPa) where these tendencies become negligible. This assumption implies a direct relation between net column heating and surface pressure fall. Prompted by a critique of Spengler and Egger, the validity of the concept of a stratospheric level of insignificant dynamics is tested here for the previously studied case on the basis of operational analyses from the ECMWF. At least for low latitudes, significant tendencies with some spatial and temporal variations are found up to 10 hPa, which renders a general neglect of this term on a fixed pressure level problematic. These results call for a more detailed investigation of the dynamical causes of the analyzed tendencies, a reevaluation of some previous work, and a more careful design of future studies on this subject.