Central and Eastern United States Surface Pressure Variations
Derived from the USArray Network
Alexander A. Jacques, John D. Horel, Erik T. Crosman and Frank L. Vernon
Submitted to Monthly Weather Review
Author’s Response to Reviewer #2 Recommendations
This is a relatively short, concise article that reports on findings from a unique set of highresolution pressure measurements over the eastern two-thirds of the United States over a 4-year
period. The paper is well-written, the methodology sound, and the results, while not
revolutionary, confirm, quantify, and integrate much of what is already understood about
surface pressure fluctuations in the United States across a broad range of spatial and temporal
scales. The findings have limitations, though, with respect to providing a reliable climatology
owing to the short duration and shifting areal coverage of the data set, as the authors
appropriately acknowledge. However, it is a nice first step to providing a climatology using a
unique data set, and the paper is therefore worthy of publication.
While my comments are deemed minor, I have several suggestions the authors may want to
consider in their revision of the paper.
1. A solid argument is given for filtering in the three temporal scales – synoptic, sub-synoptic,
and mesoscale – and the authors properly acknowledge that the sub-synoptic category partially
overlaps the other two. They indicate that this category includes cold fronts and larger
convective complexes. However, the overlap with mesoscale might be a bit greater than the
authors acknowledge, particularly when viewing Figs. 23c and 25c (the JJA pressure signature
maps for sub-synoptic and mesoscale). There is clearly considerable overlap of the maxima in
the Central Plains in these two plots. Is the distinction between the phenomena contributing to
the two maxima simply the size and duration of convective complexes, or are there other factors
at play? Also, I wonder if these maxima are in part related to the corridors of MCS passage
reported in Carbone et al. (2002) and subsequent papers.
The authors agree that there is prominent overlap between the maxima in the meso-filtered
and sub-synoptic filtered signatures, and that identical events are likely being sampled by
both individual filtered datasets. On line 81 in the text the pressure phenomena are
described to occur on “overlapping temporal scales.” In addition, a discussion in the text on
lines 213-216 discusses the ambiguity when interpreting the subsynoptic filter (4 to 30 h) due to
the aforementioned overlap. There are likely several factors at play in addition to duration
and size. These higher frequencies do appear to be in a similar region that experiences
episodic sequences of organized convection as described by Carbone et al. (2002), which
has been added to the discussion. As an example, the timing of these sequences passing
over an individual station (e.g., 6 hours apart between gust front pressure perturbations)
can produce a signal on the sub-synoptic filtered time series. With a wide variety of
phenomena affecting the sub-synoptic scale filtered time series, concluding specific
phenomena responsible for specific fluctuations proved difficult without examining specific
cases.
2. On the same point, the interpretation of the maximum over the Ohio Valley for the subsynoptic category is not completely clear. If it is related to cold fronts, is the argument made that
there are more cold fronts (or stronger pressure anomalies associated with them) in the Ohio
Valley than other areas? Or is there another interpretation of this finding?
In a similar manner, the authors acknowledge that it is likely a diverse set of phenomena
producing the stronger pressure variations seen on the sub-synoptic scale. Examination of
a few of the larger magnitude events during the winter seasons over the Ohio Valley using
the web tools revealed that several events at stations near the maximum appeared to be
associated with some sort of cold frontal passage, so cold fronts were provided as an
example of wintertime phenomena in the ubiquitous results. However, as another example,
very progressive synoptic scale systems could also influence the sub-synoptic signal due to
an upper temporal limit of 30 hours. Due to the broad diversity of phenomena
encompassed in the sub-synoptic filtered time series, it is difficult to conclude without
extensive analysis whether the maxima in the Ohio Valley is strictly due to either more
prevalent or more potent frontal passages.
3. Two case studies are provided for meso-filtered events, and one for a synoptic event.
Considering that there are three categories, why did you not present one example of each?
Sub-synoptic case studies were considered for several phenomena including frontal
passages, progressive synoptic systems, and episodic convection. The authors concluded
that while sub-synoptic signals were produced, it was difficult to identify an event as truly
“sub-synoptic” due to the strong signal still simultaneously prominent in either the meso or
synoptic filtered data. An example that would be unambiguously sub-synoptic-- diurnal
and semidiurnal tides—was considered as a case study, but not presented given the smaller
audience that would be interested in diurnal tides compared to convection, and the fact
that algorithmic treatment different from the sub-synoptic filter itself to truly isolate
magnitude and phase of each cycle was required. As seen with the overlapping regions of
frequencies, the sub-synoptic produces a fair amount of ambiguity. The meso and synoptic
are distinctly seperate from each other and appear to capture events for their associated
phenomena well within the filtering, as demonstrated by the case studies. . For these
reasons, the authors concluded that providing two examples of contrasting mesoscale
phenomena appeared to be appropriate.
4. The first case presented (Figs. 7-9) is described as a gravity wave event. I do not dispute that
interpretation, but there is likely more at play in this case related to the dynamics of the
precipitation system that occurred. The large pressure falls and strong easterly winds were at
the trailing edge of the stratiform precipitation region, much like the results for the highamplitude MGW case reported in the paper you refer to by Ruppert and Bosart (2014). Their
article points out the importance of the precipitation system in also contributing to the large
pressure falls since the location of those falls occur exactly where one would expect in a wake
low hugging the back edge of a trailing stratiform precipitation system.
The authors have expanded the first case presented by discussing the possible evolution to
a mesohigh-wake low couplet from the gravity wave interacting with precipitation, as
referred to by Ruppert and Bosart (2014).
Typos, other minor points:
Abstract, line 43: shifted
Abstract, lines 43-45: strong used three times in one sentence
Line 178: With regard to tossing out pressure changes exceeding 2 hPa s-1, is it possible that
any of the sensors experienced a tornado or dust devil passage? Line 264: Ruppert misspelled
Line 339 and several other places: furthest should be farthest
Line 460: shifts should be shift
The minor revisions and typos listed here have been modified based on recommendations.
Regarding the removal of 2 hPa s-1 or greater pressure changes, it certainly is possible that
these large pressure rates could be the cause of a tornado or dust devil passage. Initial
inspection of these occurrences coincided with missing data periods, which caused concerns
with the initial filtering attempts. Thus, a cautious approach was taken where short
discrete pressure fluctuations were considered suspect. Observations were only flagged
rather than removed; leaving the original observations intact for future study should any
events be discovered and become of interest.