*Q̇*is the heating rate andis the vertical velocity in pressure coordinates.

*T*is replaced by

_{υ}*T*(see however rain and evaporation equation below). Equation (1) appears to be formally correct and seems to imply that heat sources and vertical motions affect the hydrostatic surface pressure instantaneously. This contradicts the tendency equation for the hydrostatic surface pressure at

*z*= 0 which, following from (2) assuming ∂

*p*/∂

*t*|

_{∞}= 0, yieldsaccording to which it is only the mass convergence aloft that has an effect on the hydrostatic surface pressure tendency.

*z*→ ∞), which is not justified (see below).

Second, even though (1) appears to be an equation for the hydrostatic pressure tendency it should be noted that *ω*, which contains a local pressure tendency as well, appears on the right-hand side of (1). Thus, the tendency equation in KF08 is implicit, since a local pressure tendency is found on both sides of (1).

One could argue that *ω* is a generalized velocity in the pressure system and that, therefore, *ω* does not contain information on the pressure tendency at a certain location. However, the integration of (1) over *z* requires just this information. In particular, one has to know *ω*, thus the pressure tendency, at the surface, before calculating the tendency following (1).

*z*system, is to use the first law of thermodynamics as a prognostic equation for pressure:where

*γ*=

*c*/

_{p}*c*.

_{υ}Equation (5) demonstrates immediately that the assumption in (4) is invalid but even more importantly, (5) is the prognostic equation for the nonhydrostatic pressure that can, for example, be used to predict the nonhydrostatic surface pressure.

*w*, the vertical velocity, from (5) yieldingwhich can be combined with the hydrostatic pressure tendency in (3) (assuming that ∂

*p*/∂

*t*|

_{∞}= 0), where

*p*now refers to the hydrostatic pressure, in order to obtain the Richardson equation (Richardson 1922),which is needed to compute

*w*in hydrostatic flow by integration over

*z*.

In other words, the hydrostatic pressure tendency in (3) is the only equation that exists to compute the hydrostatic surface pressure tendency. If there were another equation we could immediately derive a diagnostic equation that would deviate from the Richardson equation, which to our knowledge is not possible.

It follows that synoptic advection, vertical motion, and diabatic heating do not play the assigned roles in causing a surface pressure tendency as proposed by KF08. Accordingly the interpretation of their Figs. 9, 12, and 13 needs adjustment.

*P*is precipitation rate and

*E*the evaporation rate (kg s

^{−1}m

^{−2}) at the surface.

## Acknowledgments

We thank P. Knippertz and A. Fink for some clarifying remarks on an earlier version of this comment.

## REFERENCES

Knippertz, P., , and A. H. Fink, 2008: Dry-season precipitation in tropical West Africa and its relation to forcing from the extratropics.

,*Mon. Wea. Rev.***136****,**3579–3596.Kong, K-Y., 2006: Understanding the genesis of Hurricane Vince through the surface pressure tendency equation. Preprints,

*27th Conf. on Hurricanes and Tropical Meteorology,*Monterey, CA, Amer. Meteor. Soc., 9B.4. [Available online at http://ams.confex.com/ams/pdfpaper/108938.pdf].Richardson, L. F., 1922:

*Weather Prediction by Numerical Process*. 1st ed. Cambridge University Press, 236 pp.