|This paper extends earlier analysis of the tape recorder signal in water vapor by including the seasonal cycle and focusing primarily on the lowermost part of the signal near 80 hPa. If I were interested in further research on this topic I would certainly want to consult this paper to build on the authors’ results. I regard their findings as a partial improvement, perhaps half-way to where we need to go to understand fully the implications of water vapor behavior for mean vertical motion, vertical diffusion, and lateral in-mixing. The MLS data are inferior to HALOE for this purpose, the model is, well, just a model, and ERA-I is excessively diffusive, perhaps worthless of this purpose. I would probably rely on extending the HALOE analysis (Dunkerton, 2001 JAS) with the precision (for lowermost levels) illustrated by the authors.|
Two issues are overlooked that otherwise limit my appreciation for this work. First, it is now well established that the moist part of the tape recorder is driven by vertical, then lateral, transport of water vapor injected by overshooting convection in the Asian summer monsoon (Gettelman et al.,, 2004 JGR, et seq.). It is surprising if, in fact, no one has repeated Mote et al. with time-varying side boundary conditions to mimic this effect. Second, although the authors seem to casually ascribe vertical diffusion to breaking gravity waves, it is well-known that such waves (if undergoing local convective instability in their phase of overturning) are not effective in mixing heat and constituents vertically (Coy et al., 1988 JAS). Inertia-gravity waves may undergo shear instability at large amplitude, altering this result possibly in a significant way (Dunkerton, 1985 JAS, et seq.).
Meanwhile, what is the role of overshooting convection in penetrating local theta surfaces? The main conclusion emerging from our work and subsequent studies of (de)hyrdration by overshooting convection is that ambient relative humidity determines the outcome: if high, leading to freeze-drying, if low, leading to hydration. While their latest additions to the text address dehydration, nothing is said to address hydration. In summary, while there are many reasons to publish this work, it will be a stepping stone, not close to any final answer, on a most intriguing problem. Publication is recommended.
Minor suggestion: be sure to emphasize that only the lowermost part of the signal is analyzed. Whether your analysis benefits higher levels, with respect to annual & QBO influences on parameters, remains to be determined. I did some forward modeling prior to Mote et al. to ensure that vertical diffusivity must decrease rapidly with height above 80 hPa. Indeed, it must, otherwise the signal is too wide and decays much too fast. Frankly, if you have HALOE results not shown, I would add them prior to publication.