1. Robber Storms
Does Convection South of the Baroclinic Zone Reduce Cyclone
Precipitation Production?
Bill Borghoff# and Paul J. Roebber
Atmospheric Science Group
University of Wisconsin at Milwaukee
14th Cyclone Workshop
Hotel Mont Gabriel
Sainte Adele, Québec
21-26 September 2008
# Also NOAA/NWS Sullivan, Wisconsin

2. Robber Storms
Does Convection South of the Baroclinic Zone Reduce Cyclone
Precipitation Snowfall Production?
Bill Borghoff# and Paul J. Roebber
Atmospheric Science Group
University of Wisconsin at Milwaukee
14th Cyclone Workshop
Hotel Mont Gabriel
Sainte Adele, Québec
21-26 September 2008
# Also NOAA/NWS Sullivan, Wisconsin

3. THE ARGUMENT
Forecasters say that whenever such convection breaks out, there
is likely to be less snowfall.
They argue that moisture otherwise available for snow is being
used by the convection.

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11. The Questions

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16. Submitting a conference abstract on research that
Dave Schultz Survey Q#11
Submitting a conference abstract on research that
has not been started yet is acceptable. TRUE FALSE

17. Submitting a conference abstract on research that
Schultz Survey
Submitting a conference abstract on research that
has not been started yet is acceptable. TRUE FALSE

18. THE QUESTIONS
1) IS there systematically less snowfall observed in such systems?
2) If so, IS this the result of a moisture deficit?

19. WARNING: These activities are not Federally sanctioned …

20. METHOD
1) Assemble a database of winter storms in the midwestern US
(with and without convection) for Nov-Mar Must
have ≥ 6” snow.
2) Record snow amounts (areal average), max 700 hPa vertical
motion and precipitable water in <0°C precip. shield (6-h from
Reanalysis).
3) Diagnose “convection” from radar reflectivity (ZR ≥ 40 dBZ).
This will be cross-checked with CG lightning reports (not yet).
4) Use a bootstrap sampling procedure to compare otherwise
“like” storms with and without convection.

21. METHODS II
5) How to define “like” storms? We use storm average, max and
min vertical motion over the lifetime of the storm and define
a similarity score s
-0.32 v.s (avg)
Note: for s ≤ 0.17 Pa s v.s (max)
-0.15 v.s (min)
Don’t include PW since Precip ~ *PW
6) Compute ∆SNOW = snow (non convective) - snow (convective)
for all “like” pairs from sample of 84 events.

22. RESULTS
Null hypothesis: “Like” storms as defined by s have an even
distribution of ∆SNOW about zero.
We use the distribution-free “sign test” to evaluate…
We reject the null hypothesis (p<0.0001). The distribution skews
strongly positive so we conclude that “like” storms DO produce
more snowfall (+1-3” across sample) when convection is absent.
These results are NOT sensitive to threshold for s.

23. Does Convection South of the Baroclinic Zone Reduce Cyclone Snowfall Production?

24. Surprisingly (to me), YES!
Does Convection South of the Baroclinic Zone Reduce Cyclone Snowfall Production?
Surprisingly (to me), YES!

25. But wait a minute …

26. Possible explanations …
Moisture robbing?
implication is that PW should be higher in the snow region in non-convective cases since moisture is being removed in convective cases.
There are significant differences in PW between the non-convective and convective samples. But it is higher in convective cases!
Duration differences?
If PW is higher in the snow region in convective cases and  is the same, how is more snow produced in the non-convective cases? Is the duration longer for these storms?
No significant difference between two samples.

27. Possible explanations (continued) …
Sub-synoptic precipitation (e.g. bands)?
Diagnosed using radar (≥30 dBZ). More than 75% of storms.
No significant difference between two samples.
More melted equivalent precipitation/higher snow density?
Could produce less snow despite higher PW.
Common large-scale influence?
Suppose WAA is helping to prime the environment for convection and is also producing lift for snow, while non-convective storms have lift from other processes (e.g. CVA).
Could explain higher PW in snow zone in convective cases.

28. Other work … conclusions to follow
• Detailed diagnostics on “like” storm pairs
• Other measures of storm similarity (e.g. other variables, multidimensional cluster techniques)?
• Possible numerical model experiments (if motivated by diagnostic results)?