|Comments on “The climatology of Brewer-Dobson circulation and the contribution of gravity waves” by Sato and Hirano|
The reviewer appreciates the authors’ efforts on improving the manuscript during several revision processes. The reviewer could understand better in the current manuscript what authors want to say. Nevertheless, there are some points, mainly underlain assumptions of the manuscript, which are still unacceptable to the reviewer. Considering the impacts of the current results to potential readers, the reviewer would like for the following points to be clarified before the manuscript is accepted to ACP.
1. In the TEM equation (1), the EDP represents forcing by the all “resolved waves” which includes both planetary waves and resolved GWs (mainly inertial GWs”: IGWs). This is not only for high resolution GCMs, but the ones used in the current study with horizontal resolution of about 1 degree in latitude/longitude. The only difference is that horizontal scales of IGWs resolvable can be smaller for high resolution GCMs. That is, the EPD calculated in the present study should not be solely from Rossby waves. Accordingly, GWF should be from the “parameterized subgrid-scale” GWs, which cannot be resolved from data grids. Theoretically, sources of GWs for any GWD parameterization should be less than the model grid spacing, as done in orographic GWD and convective GWD schemes. Therefore, if model resolution is relatively coarse (that is, resolved GWs are less abundant), the parameterized subgrid-scale GWF should be larger (based mostly on tuning), in general, in order for the momentum balance.
If authors really want to put all scales of GWs, including both resolved GWs and parameterized GWs, to the potential GWF, the EDP should be calculated separately based on the zonal wavenumber (e.g., say k < 20 are Rossby waves and k > 20 are resolved GWs, where k is the zonal wavenumber), and then sum of the EPD by the resolved GWs and parameterized GWD can be a total GWF.
2. In the present study, GWF is estimated from the residual of the TEM equation, which is similar to some previous studies before the parameterized GWD output was provided from the reanalysis data sets. The residual term is similar to the sum of the parameterized GWD and assimilation increment. The assimilation increment is simply a model error against observation, and there is no way to isolate individual process to induce the error. In order for the resolved grid values of wind and temperature to be similar among the reanalysis data sets, as in the current study represented by EPD comparison, there might be a significant assimilation increment for each data sets (as shown from MERRA and MERRA-2 in Fig. 15), which should be different from each data sets that were produced using different GCM models. Note again that the assimilation increment cannot be solely by the uncertainty of the GWD parameterization of the model. Therefore, the residual of the TEM equation cannot be considered as potential GWF.
3. The GWD output from each reanalysis data set, represented by pGW forcing in the current manuscript, is purely model output, because there are no observational data to assimilate it. In current GCMs, parameterized GW momentum flux is significantly overestimated in the middle atmosphere, as shown from Geller et al. (2013, JCL) in the stratosphere (at z = 40 km, Figs. 4 & 5 of Geller et al., 2013). Considering this situation, contribution of GWs to the BDC suggested from the current study, which is estimated by the residual of the TEM equation (pGW forcing + assimilation increment), is highly overestimated.