|Comments on: Study of a Prototypical Convective boundary layer observed during BLLAST: contributions by large-scale forcings.|
The authors have produced an interesting study, wherein they derived the subsidence by modifying the “prototypical” CBL growth equation to allow for subsidence. By forcing the idealized PBL to follow the observed growth, they are able to reproduce a reasonable-looking subsidence time series, which is similar to that produced in the ECMWF simulation for the day. They point out that the subsidence reaches a maximum in the early afternoon, and is similar to subsidence from the ECMWF model.
I think the paper merits publication. However, there are some points of concern, some easily fixed, some more difficult to fix that might have to be recognized by the authors as shortcomings.
First, I do not understand why subsidence isn’t part of the description of a “prototypical” boundary layer. I learned the fair-weather convective boundary layers grew into subsiding air; with their depth (for dry air) a balance between subsidence form above and buoyant convection from below. The marine boundary layer, for example reaches an equilibrium height that balances between the subsidence from above and the buoyancy fluxes from below.
Is there a standard way to iterate to back out the subsidence and advection? It would be useful to see exactly how this is done. Accounting for subsidence alone seems kind of trivial, but including advection seems more complicated. If this has been done elsewhere, a reference would be useful.
What the paper does, though, is managed to back out a subsidence that is strongly related to the time of day. One wonders, in this terrain, if this isn’t related to mountain-valley circulations – a natural place to look, as another reviewer pointed out, is in David Whiteman’s book (Mountain Meteorology, Oxford University Press, 2000).
More worrisome is the variety of land-use in the area, while the measurements used only represent a subset of the land cover. Is it possible to either include measurements over all the types of land cover, or for the authors to describe what sort of biases would be expected by not including the unrepresented land cover?
More specific comments
L40. Does the PBL really reach an equilibrium state in the late afternoon? I find this hard to believe, but then I am not sure of the definition of “late afternoon.” From the point of view of turbulence, such an equilibrium is often reached much earlier. And the depth of the PBL depends on the offsetting effects of surface fluxes and subsidence, which can lead to max PBL depths any time during the afternoon (with earlier peak PBL depths when there is stronger subsidence).
L140. You should define what “favorable weather” means. What determines an IOP during BLLAST?
Section 3, L150. Is it really “large” scale or mesoscale to synoptic scale?
L186. “larger” scale forcings? The discussion does include “mesoscale,” so it’s not really “large” scale, but rather “larger” (than turbulence) scale.
L178, item 4. What about wind structure?
Labeling of Figure 2a and 2b not consistent with text.
Figure 3. It looks like the wind shear in the second sounding is at a low enough level to increase the entrainment buoyancy flux to magnitudes greater than -0.2 times the surface values. It might be useful to plot U and V (rather than speed and direction) and compare the shears to what Conzemius and Fedorovich consider important. If the shears are important, you could look at the impact of increasing the entrainment buoyancy flux magnitude and seeing how this affects your subsidence estimates.
Figure 4. I can’t see the labels on some of the land use types. Are the seven surface flux stations the only ones, or just the ones used? (Some of the text suggested there might be measurements over other land-use types)
Paragraph including L290. This summary of the impact of surface heterogeneity is confusing and not very comprehensive. There are two types of heterogeneity: land use and terrain. The impacts of land use were first explored by Richard Anthes (1984, J. Appl. Meteor; Yan and Anthes, Mon. Wea. Rev, 1988, both of which probably have earlier references), Moti Segal, (Segal. et al., Mon. Wea. Rev., 1989 and other articles), and Roger Pielke, Sr. (e.g. Pielke et al., Mon. Wea. Rev. 1999 and references therein). The impact of terrain in mountainous areas has been treated by Whiteman and many others; the impact of much more subtle terrain variation has been dealt with extensively, starting with Walko et al. (1991, Boundary-Layer Meteorology) and Krettenauer and Schumann (Meteorol. Atmos. Phys. V. 41, 1989). Mesoscale circulations from a combination of land use and even modest terrain are documented in LeMone et al. (2002, Boundary-Layer Meteorology) at the 60-km scale, and on much smaller scale by Grossman et al. (J. Geophys. Res. Atmospheres 2004). The mentioned papers don’t all need to be cited – there are so many observational and modeling studies on flow around mountains that the latter references would probably be more useful.
Section 4.2.1. I think it would be helpful to the reader to see more discussion and a simple set of equations, which describes how the entrainment and subsidence are backed out.
P. 9 top right. L 593. Should be “buoyancy flux” (more precise).
p. 9, last paragraph of Section 6.0. It’s common to have a secondary maximum of near-surface mixing ratio around late afternoon as well as early morning. It results from surface moisture flux not being offset by mixing from above.
Section 6.1. Two comments. Using the virtual potential temperature rather than potential temperature would make the sounding slightly more near neutral. Also, the balloon could be traveling from a cooler area to a warmer area as it rises through the boundary layer, either due to traveling to above a warmer surface, or simply entering a warm updraft.
L812. “The” TKE (letter missing)
L830. TKE more directly related to virtual-temperature flux, but this doesn’t invalidate this statement.
L840 and elsewhere. While advection is mentioned, it is only discussed briefly. How do the advection values compare to ECMWF. Is advection constant, as implied in Table 1, or does it vary with time? How important is horizontal advection relative to subsidence?
In Fig. 7, is the “prescribed” subsidence based on the derived subsidence as implied in L559?